Androgens - female homosexuality .
Finger length ratios 2D:4D .
Hair whorl - male homosexuality .
Handedness and fraternal birth order impact on sexual orientation .
H-Y antigen .
Neurodevelopment - sexual orientation .
Paraphilia - Males .
Pheromones - sexual orientation .
.. PubMed search ..
Sunday, May 31, 2009
Neurodevelopment - sexual orientation
The neurodevelopment of human sexual orientation.
Neurosci Biobehav Rev. 2005;29(7):1057-66. Epub 2005 Apr 25.
Rahman Q.
One of the most enduring and controversial questions in the neuroscience of sexual behaviour surrounds the mechanisms which produce sexual attraction to either males or females. Here, evidence is reviewed which supports the proposal that sexual orientation in humans may be laid down in neural circuitry during early foetal development. Behaviour genetic investigations provide strong evidence for a heritable component to male and female sexual orientation. Linkage studies are partly suggestive of X-linked loci although candidate gene studies have produced null findings. Further evidence demonstrates a role for prenatal sex hormones which may influence the development of a putative network of sexual-orientation-related neural substrates. However, hormonal effects are often inconsistent and investigations rely heavily on 'proxy markers'. A consistent fraternal birth order effect in male sexual orientation also provides support for a model of maternal immunization processes affecting prenatal sexual differentiation. The notion that non-heterosexual preferences may reflect generalized neurodevelopmental perturbations is not supported by available data. These current theories have left little room for learning models of sexual orientation. Future investigations, across the neurosciences, should focus to elucidate the fundamental neural architecture underlying the target-specific direction of human sexual orientation, and their antecedents in developmental neurobiology.
School of Psychology, University of East London, The Green, London E15 4LZ, UK. q.rahman@uel.ac.uk
Neurosci Biobehav Rev. 2005;29(7):1057-66. Epub 2005 Apr 25.
Rahman Q.
One of the most enduring and controversial questions in the neuroscience of sexual behaviour surrounds the mechanisms which produce sexual attraction to either males or females. Here, evidence is reviewed which supports the proposal that sexual orientation in humans may be laid down in neural circuitry during early foetal development. Behaviour genetic investigations provide strong evidence for a heritable component to male and female sexual orientation. Linkage studies are partly suggestive of X-linked loci although candidate gene studies have produced null findings. Further evidence demonstrates a role for prenatal sex hormones which may influence the development of a putative network of sexual-orientation-related neural substrates. However, hormonal effects are often inconsistent and investigations rely heavily on 'proxy markers'. A consistent fraternal birth order effect in male sexual orientation also provides support for a model of maternal immunization processes affecting prenatal sexual differentiation. The notion that non-heterosexual preferences may reflect generalized neurodevelopmental perturbations is not supported by available data. These current theories have left little room for learning models of sexual orientation. Future investigations, across the neurosciences, should focus to elucidate the fundamental neural architecture underlying the target-specific direction of human sexual orientation, and their antecedents in developmental neurobiology.
School of Psychology, University of East London, The Green, London E15 4LZ, UK. q.rahman@uel.ac.uk
Sunday, May 3, 2009
Pheromones - sexual orientation
Brain response to putative pheromones in lesbian women.
Proc Natl Acad Sci U S A. 2006 May 23;103(21):8269-74. Epub 2006 May 16.
Berglund H, Lindström P, Savic I.
The progesterone derivative 4,16-androstadien-3-one (AND) and the estrogen-like steroid estra-1,3,5(10),16-tetraen-3-ol (EST) are candidate compounds for human pheromones. In previous positron emission tomography studies, we found that smelling AND and EST activated regions primarily incorporating the sexually dimorphic nuclei of the anterior hypothalamus, that this activation was differentiated with respect to sex and compound, and that homosexual men processed AND congruently with heterosexual women rather than heterosexual men. These observations indicate involvement of the anterior hypothalamus in physiological processes related to sexual orientation in humans. We expand the information on this issue in the present study by performing identical positron emission tomography experiments on 12 lesbian women. In contrast to heterosexual women, lesbian women processed AND stimuli by the olfactory networks and not the anterior hypothalamus. Furthermore, when smelling EST, they partly shared activation of the anterior hypothalamus with heterosexual men. These data support our previous results about differentiated processing of pheromone-like stimuli in humans and further strengthen the notion of a coupling between hypothalamic neuronal circuits and sexual preferences.
Department of Medicine, and Stockholm Brain Institute, Karolinska University Hospital, Karolinska Institutet, 171 76 Stockholm, Sweden.
Brain response to putative pheromones in homosexual men.
Proc Natl Acad Sci U S A. 2005 May 17;102(20):7356-61. Epub 2005 May 9.
Savic I, Berglund H, Lindström P.
The testosterone derivative 4,16-androstadien-3-one (AND) and the estrogen-like steroid estra-1,3,5(10),16-tetraen-3-ol (EST) are candidate compounds for human pheromones. AND is detected primarily in male sweat, whereas EST has been found in female urine. In a previous positron emission tomography study, we found that smelling AND and EST activated regions covering sexually dimorphic nuclei of the anterior hypothalamus, and that this activation was differentiated with respect to sex and compound. In the present study, the pattern of activation induced by AND and EST was compared among homosexual men, heterosexual men, and heterosexual women. In contrast to heterosexual men, and in congruence with heterosexual women, homosexual men displayed hypothalamic activation in response to AND. Maximal activation was observed in the medial preoptic area/anterior hypothalamus, which, according to animal studies, is highly involved in sexual behavior. As opposed to putative pheromones, common odors were processed similarly in all three groups of subjects and engaged only the olfactory brain (amygdala, piriform, orbitofrontal, and insular cortex). These findings show that our brain reacts differently to the two putative pheromones compared with common odors, and suggest a link between sexual orientation and hypothalamic neuronal processes.
Department of Clinical Neuroscience, Karolinska University Hospital, 171 76 Stockholm, Sweden. ivanka.savic-berglund@neuro.ki.se
Smelling of odorous sex hormone-like compounds causes sex-differentiated hypothalamic activations in humans.
Neuron. 2001 Aug 30;31(4):661-8.
Savic I, Berglund H, Gulyas B, Roland P.
The anatomical pathways for processing of odorous stimuli include the olfactory nerve projection to the olfactory bulb, the trigeminal nerve projection to somatosensory and insular cortex, and the projection from the accessory olfactory bulb to the hypothalamus. In the majority of tetrapods, the sex-specific effects of pheromones on reproductive behavior is mediated via the hypothalamic projection. However, the existence of this projection in humans has been regarded as improbable because humans lack a discernable accessory olfactory bulb. Here, we show that women smelling an androgen-like compound activate the hypothalamus, with the center of gravity in the preoptic and ventromedial nuclei. Men, in contrast, activate the hypothalamus (center of gravity in paraventricular and dorsomedial nuclei) when smelling an estrogen-like substance. This sex-dissociated hypothalamic activation suggests a potential physiological substrate for a sex-differentiated behavioral response in humans.
Division of Human Brain Research, Department of Neuroscience, Karolinska Institute, Stockholm, Sweden. ivanka.savic-berglund@neuro.ki.se
Brain imaging studies of the functional organization of human olfaction.
Neuroscientist. 2002 Jun;8(3):204-11.
Savic I.
It is believed that sensory functions are organized in a hierarchical and parallel manner. The sense of smell differs in several aspects from other senses: odors can immediately elicit emotional evocations, they are remembered after a long time, and they are difficult to label. This raises the question of whether odorous stimuli may be processed differently from the other sensory stimuli. New data from brain imaging studies suggest that this is not the case and that the specific characteristics of the sense of smell can be attributed to the engagement of limbic structures at an early stage in the signal processing.
Department of Neuroscience, Karolinska Institute, Stockholm, Sweden. ivanka.savic-berglund@neuro.ki.se
Functional magnetic resonance imaging of human olfaction.
Neuroimaging Clin N Am. 2001 May;11(2):237-50, viii.
Weismann M, Yousry I, Heuberger E, Nolte A, Ilmberger J, Kobal G, Yousry TA, Kettenmann B, Naidich TP.
Olfaction is our basic sense phylogenetically and embryologically. Little is known, however, about how the human brain encodes the quality of odors, odor-associated memories, and emotions. Olfactory information is projected from the olfactory bulb to the primary olfactory cortex, which is composed of the anterior olfactory nucleus, the olfactory tubercle, the piriform cortex, the amygdala, the periamygdaloid region, and the entorhinal cortex. From there, the primary olfactory cortex projects to secondary olfactory regions including the hippocampus, ventral striatum and pallidum, hypothalamus, thalamus, orbitofrontal cortex, agranular insular cortex, and cingulate gyrus. Functional MR studies using olfactory stimuli as paradigms show activation of many of these areas and can advance our understanding of odor perception in humans.
Department of Radiology, Medical University, Luebeck, Germany.
Imaging of brain activation by odorants in humans.
Curr Opin Neurobiol. 2002 Aug;12(4):455-61.
Savic I.
Application of positron emission tomography and magnetic resonance imaging has provided several new insights into various olfactory functions. One is that sniffing and smelling engage separate subsystems in the human olfactory cortex. Another is that perception of odorous compounds (odorants) is mediated by a set of core regions, which are partly different for pure olfactory than for olfactory plus trigeminal odorants. Depending on the task associated with odor perception, the core regions are recruited together with other circuits, in a parallel and hierarchical manner. The sense of smell seems, therefore, to be organized similarly to other sensory modalities, and the specific psychophysical characteristics of olfaction should be attributed to an early involvement of the limbic system rather than to a conceptually different mode of processing.
Department of Neuroscience, Karolinska Institute, Retziusväg 8, 17177, Stockholm, Sweden. ivanka.savic-berglund@neuro.ki.se
Brain activation during odor perception in males and females.
Neuroreport. 2001 Jul 3;12(9):2027-33.
Bengtsson S, Berglund H, Gulyas B, Cohen E, Savic I.
Several studies indicate that women outperform men in olfactory identification tasks. The psychophysical data are more divergent when it comes to gender differences at levels of odor processing which are cognitively less demanding. We therefore compared cerebral activation with H2(15)O PET in 12 females and 11 males during birhinal passive smelling of odors and odorless air. The odorous compounds (odorants) were pure olfactory, or mixed olfactory and weakly trigeminal. Using odorless air as the baseline condition, activations were found bilaterally in the amygdala, piriform and insular cortices in both sexes, irrespective of the odor. No gender difference was detected in the pattern of cerebral activation (random effect analysis SPM99, corrected p less than 0.05) or in the subjective perception of odors. Males and females seem to use similar cerebral circuits during the passive perception of odors. The reported female superiority in assessing olfactory information including odor identification is probably an effect of a difference at a cognitive, rather than perceptive level of olfactory processing.
Division of Human Brain Research, Department of Neuroscience, Huddinge University Hospital, Karolinska Institute, Berzelius v. 3, S-171 77 Stockholm, Sweden.
Proc Natl Acad Sci U S A. 2006 May 23;103(21):8269-74. Epub 2006 May 16.
Berglund H, Lindström P, Savic I.
The progesterone derivative 4,16-androstadien-3-one (AND) and the estrogen-like steroid estra-1,3,5(10),16-tetraen-3-ol (EST) are candidate compounds for human pheromones. In previous positron emission tomography studies, we found that smelling AND and EST activated regions primarily incorporating the sexually dimorphic nuclei of the anterior hypothalamus, that this activation was differentiated with respect to sex and compound, and that homosexual men processed AND congruently with heterosexual women rather than heterosexual men. These observations indicate involvement of the anterior hypothalamus in physiological processes related to sexual orientation in humans. We expand the information on this issue in the present study by performing identical positron emission tomography experiments on 12 lesbian women. In contrast to heterosexual women, lesbian women processed AND stimuli by the olfactory networks and not the anterior hypothalamus. Furthermore, when smelling EST, they partly shared activation of the anterior hypothalamus with heterosexual men. These data support our previous results about differentiated processing of pheromone-like stimuli in humans and further strengthen the notion of a coupling between hypothalamic neuronal circuits and sexual preferences.
Department of Medicine, and Stockholm Brain Institute, Karolinska University Hospital, Karolinska Institutet, 171 76 Stockholm, Sweden.
Brain response to putative pheromones in homosexual men.
Proc Natl Acad Sci U S A. 2005 May 17;102(20):7356-61. Epub 2005 May 9.
Savic I, Berglund H, Lindström P.
The testosterone derivative 4,16-androstadien-3-one (AND) and the estrogen-like steroid estra-1,3,5(10),16-tetraen-3-ol (EST) are candidate compounds for human pheromones. AND is detected primarily in male sweat, whereas EST has been found in female urine. In a previous positron emission tomography study, we found that smelling AND and EST activated regions covering sexually dimorphic nuclei of the anterior hypothalamus, and that this activation was differentiated with respect to sex and compound. In the present study, the pattern of activation induced by AND and EST was compared among homosexual men, heterosexual men, and heterosexual women. In contrast to heterosexual men, and in congruence with heterosexual women, homosexual men displayed hypothalamic activation in response to AND. Maximal activation was observed in the medial preoptic area/anterior hypothalamus, which, according to animal studies, is highly involved in sexual behavior. As opposed to putative pheromones, common odors were processed similarly in all three groups of subjects and engaged only the olfactory brain (amygdala, piriform, orbitofrontal, and insular cortex). These findings show that our brain reacts differently to the two putative pheromones compared with common odors, and suggest a link between sexual orientation and hypothalamic neuronal processes.
Department of Clinical Neuroscience, Karolinska University Hospital, 171 76 Stockholm, Sweden. ivanka.savic-berglund@neuro.ki.se
Smelling of odorous sex hormone-like compounds causes sex-differentiated hypothalamic activations in humans.
Neuron. 2001 Aug 30;31(4):661-8.
Savic I, Berglund H, Gulyas B, Roland P.
The anatomical pathways for processing of odorous stimuli include the olfactory nerve projection to the olfactory bulb, the trigeminal nerve projection to somatosensory and insular cortex, and the projection from the accessory olfactory bulb to the hypothalamus. In the majority of tetrapods, the sex-specific effects of pheromones on reproductive behavior is mediated via the hypothalamic projection. However, the existence of this projection in humans has been regarded as improbable because humans lack a discernable accessory olfactory bulb. Here, we show that women smelling an androgen-like compound activate the hypothalamus, with the center of gravity in the preoptic and ventromedial nuclei. Men, in contrast, activate the hypothalamus (center of gravity in paraventricular and dorsomedial nuclei) when smelling an estrogen-like substance. This sex-dissociated hypothalamic activation suggests a potential physiological substrate for a sex-differentiated behavioral response in humans.
Division of Human Brain Research, Department of Neuroscience, Karolinska Institute, Stockholm, Sweden. ivanka.savic-berglund@neuro.ki.se
Brain imaging studies of the functional organization of human olfaction.
Neuroscientist. 2002 Jun;8(3):204-11.
Savic I.
It is believed that sensory functions are organized in a hierarchical and parallel manner. The sense of smell differs in several aspects from other senses: odors can immediately elicit emotional evocations, they are remembered after a long time, and they are difficult to label. This raises the question of whether odorous stimuli may be processed differently from the other sensory stimuli. New data from brain imaging studies suggest that this is not the case and that the specific characteristics of the sense of smell can be attributed to the engagement of limbic structures at an early stage in the signal processing.
Department of Neuroscience, Karolinska Institute, Stockholm, Sweden. ivanka.savic-berglund@neuro.ki.se
Functional magnetic resonance imaging of human olfaction.
Neuroimaging Clin N Am. 2001 May;11(2):237-50, viii.
Weismann M, Yousry I, Heuberger E, Nolte A, Ilmberger J, Kobal G, Yousry TA, Kettenmann B, Naidich TP.
Olfaction is our basic sense phylogenetically and embryologically. Little is known, however, about how the human brain encodes the quality of odors, odor-associated memories, and emotions. Olfactory information is projected from the olfactory bulb to the primary olfactory cortex, which is composed of the anterior olfactory nucleus, the olfactory tubercle, the piriform cortex, the amygdala, the periamygdaloid region, and the entorhinal cortex. From there, the primary olfactory cortex projects to secondary olfactory regions including the hippocampus, ventral striatum and pallidum, hypothalamus, thalamus, orbitofrontal cortex, agranular insular cortex, and cingulate gyrus. Functional MR studies using olfactory stimuli as paradigms show activation of many of these areas and can advance our understanding of odor perception in humans.
Department of Radiology, Medical University, Luebeck, Germany.
Imaging of brain activation by odorants in humans.
Curr Opin Neurobiol. 2002 Aug;12(4):455-61.
Savic I.
Application of positron emission tomography and magnetic resonance imaging has provided several new insights into various olfactory functions. One is that sniffing and smelling engage separate subsystems in the human olfactory cortex. Another is that perception of odorous compounds (odorants) is mediated by a set of core regions, which are partly different for pure olfactory than for olfactory plus trigeminal odorants. Depending on the task associated with odor perception, the core regions are recruited together with other circuits, in a parallel and hierarchical manner. The sense of smell seems, therefore, to be organized similarly to other sensory modalities, and the specific psychophysical characteristics of olfaction should be attributed to an early involvement of the limbic system rather than to a conceptually different mode of processing.
Department of Neuroscience, Karolinska Institute, Retziusväg 8, 17177, Stockholm, Sweden. ivanka.savic-berglund@neuro.ki.se
Brain activation during odor perception in males and females.
Neuroreport. 2001 Jul 3;12(9):2027-33.
Bengtsson S, Berglund H, Gulyas B, Cohen E, Savic I.
Several studies indicate that women outperform men in olfactory identification tasks. The psychophysical data are more divergent when it comes to gender differences at levels of odor processing which are cognitively less demanding. We therefore compared cerebral activation with H2(15)O PET in 12 females and 11 males during birhinal passive smelling of odors and odorless air. The odorous compounds (odorants) were pure olfactory, or mixed olfactory and weakly trigeminal. Using odorless air as the baseline condition, activations were found bilaterally in the amygdala, piriform and insular cortices in both sexes, irrespective of the odor. No gender difference was detected in the pattern of cerebral activation (random effect analysis SPM99, corrected p less than 0.05) or in the subjective perception of odors. Males and females seem to use similar cerebral circuits during the passive perception of odors. The reported female superiority in assessing olfactory information including odor identification is probably an effect of a difference at a cognitive, rather than perceptive level of olfactory processing.
Division of Human Brain Research, Department of Neuroscience, Huddinge University Hospital, Karolinska Institute, Berzelius v. 3, S-171 77 Stockholm, Sweden.
Saturday, May 2, 2009
Androgens - female homosexuality
Sexual orientation in women with classical or non-classical congenital adrenal hyperplasia as a function of degree of prenatal androgen excess.
Arch Sex Behav. 2008 Feb;37(1):85-99.
Meyer-Bahlburg HF, Dolezal C, Baker SW, New MI.
46,XX individuals with classical congenital adrenal hyperplasia (CAH) due to deficiency of the enzyme, 21-hydroxylase, show variable degrees of masculinization of body and behavior due to excess adrenal androgen production. Increased bisexuality and homosexuality have also been reported. This article provides a review of existing reports of the latter and presents a new study aimed at replicating the previous findings with detailed assessments of sexual orientation on relatively large samples, and at extending the investigation to the mildest form, non-classical (NC) CAH. Also, this is the first study to relate sexual orientation to the specific molecular genotypes of CAH. In the present study, 40 salt-wasters (SW), 21 SV (simple-virilizing), 82 NC, and 24 non-CAH control women (sisters and female cousins of CAH women) were blindly administered the Sexual Behavior Assessment Schedule (SEBAS-A, 1983 ed.; H. F. L. Meyer-Bahlburg & A. A. Ehrhardt, Privately printed). Most women were heterosexual, but the rates of bisexual and homosexual orientation were increased above controls not only in women with classical CAH, but also in NC women, and correlated with the degree of prenatal androgenization. Classifying women by molecular genotypes did not further increase the correlation. Diverse aspects of sexual orientation were highly intercorrelated, and principal components analysis yielded one general factor. Bisexual/homosexual orientation was (modestly) correlated with global measures of masculinization of non-sexual behavior and predicted independently by the degree of both prenatal androgenization and masculinization of childhood behavior. We conclude that the findings support a sexual-differentiation perspective involving prenatal androgens on the development of sexual orientation.
New York State Psychiatric Institute & Department of Psychiatry, Columbia University, 1051 Riverside Drive, New York, NY 10032, USA. meyerb@childpsych.columbia.edu
Gender development in women with congenital adrenal hyperplasia as a function of disorder severity.
Arch Sex Behav. 2006 Dec;35(6):667-84. Epub 2006 Aug 11.
Meyer-Bahlburg HF, Dolezal C, Baker SW, Ehrhardt AA, New MI.
Prenatal-onset classical congenital adrenal hyperplasia (CAH) in 46,XX individuals is associated with variable masculinization/defeminization of the genitalia and of behavior, presumably both due to excess prenatal androgen production. The purpose of the current study was threefold: (1) to extend the gender-behavioral investigation to the mildest subtype of 46,XX CAH, the non-classical (NC) variant, (2) to replicate previous findings on moderate and severe variants of 46,XX CAH using a battery of diversely constructed assessment instruments, and (3) to evaluate the utility of the chosen assessment instruments for this area of work. We studied 63 women with classical CAH (42 with the salt wasting [SW] and 21 with the simple virilizing [SV] variant), 82 women with the NC variant, and 24 related non-CAH sisters and female cousins as controls (COS). NC women showed a few signs of gender shifts in the expected direction, SV women were intermediate, and SW women most severely affected. In terms of gender identity, two SW women were gender-dysphoric, and a third had changed to male in adulthood. All others identified as women. We conclude that behavioral masculinization/defeminization is pronounced in SW-CAH women, slight but still clearly demonstrable in SV women, and probable, but still in need of replication in NC women. There continues a need for improved instruments for gender assessment.
NYS Psychiatric Institute/Department of Psychiatry, Columbia University, 1051 Riverside Drive, NYSPI Unit 15, New York, New York 10032, USA. meyerb@childpsych.columbia.edu
Arch Sex Behav. 2008 Feb;37(1):85-99.
Meyer-Bahlburg HF, Dolezal C, Baker SW, New MI.
46,XX individuals with classical congenital adrenal hyperplasia (CAH) due to deficiency of the enzyme, 21-hydroxylase, show variable degrees of masculinization of body and behavior due to excess adrenal androgen production. Increased bisexuality and homosexuality have also been reported. This article provides a review of existing reports of the latter and presents a new study aimed at replicating the previous findings with detailed assessments of sexual orientation on relatively large samples, and at extending the investigation to the mildest form, non-classical (NC) CAH. Also, this is the first study to relate sexual orientation to the specific molecular genotypes of CAH. In the present study, 40 salt-wasters (SW), 21 SV (simple-virilizing), 82 NC, and 24 non-CAH control women (sisters and female cousins of CAH women) were blindly administered the Sexual Behavior Assessment Schedule (SEBAS-A, 1983 ed.; H. F. L. Meyer-Bahlburg & A. A. Ehrhardt, Privately printed). Most women were heterosexual, but the rates of bisexual and homosexual orientation were increased above controls not only in women with classical CAH, but also in NC women, and correlated with the degree of prenatal androgenization. Classifying women by molecular genotypes did not further increase the correlation. Diverse aspects of sexual orientation were highly intercorrelated, and principal components analysis yielded one general factor. Bisexual/homosexual orientation was (modestly) correlated with global measures of masculinization of non-sexual behavior and predicted independently by the degree of both prenatal androgenization and masculinization of childhood behavior. We conclude that the findings support a sexual-differentiation perspective involving prenatal androgens on the development of sexual orientation.
New York State Psychiatric Institute & Department of Psychiatry, Columbia University, 1051 Riverside Drive, New York, NY 10032, USA. meyerb@childpsych.columbia.edu
Gender development in women with congenital adrenal hyperplasia as a function of disorder severity.
Arch Sex Behav. 2006 Dec;35(6):667-84. Epub 2006 Aug 11.
Meyer-Bahlburg HF, Dolezal C, Baker SW, Ehrhardt AA, New MI.
Prenatal-onset classical congenital adrenal hyperplasia (CAH) in 46,XX individuals is associated with variable masculinization/defeminization of the genitalia and of behavior, presumably both due to excess prenatal androgen production. The purpose of the current study was threefold: (1) to extend the gender-behavioral investigation to the mildest subtype of 46,XX CAH, the non-classical (NC) variant, (2) to replicate previous findings on moderate and severe variants of 46,XX CAH using a battery of diversely constructed assessment instruments, and (3) to evaluate the utility of the chosen assessment instruments for this area of work. We studied 63 women with classical CAH (42 with the salt wasting [SW] and 21 with the simple virilizing [SV] variant), 82 women with the NC variant, and 24 related non-CAH sisters and female cousins as controls (COS). NC women showed a few signs of gender shifts in the expected direction, SV women were intermediate, and SW women most severely affected. In terms of gender identity, two SW women were gender-dysphoric, and a third had changed to male in adulthood. All others identified as women. We conclude that behavioral masculinization/defeminization is pronounced in SW-CAH women, slight but still clearly demonstrable in SV women, and probable, but still in need of replication in NC women. There continues a need for improved instruments for gender assessment.
NYS Psychiatric Institute/Department of Psychiatry, Columbia University, 1051 Riverside Drive, NYSPI Unit 15, New York, New York 10032, USA. meyerb@childpsych.columbia.edu
Friday, May 1, 2009
Hair whorl - male homosexuality
Hair whorl direction and sexual orientation in human males.
Behav Neurosci. 2009 Apr;123(2):252-6.
Rahman Q, Clarke K, Morera T.
Hair whorl direction is a somatic feature that is organized early during neurodevelopment and unlikely to be influenced by social factors. This study aimed to replicate a widely reported association by A. J. S. Klar (2003) between counterclockwise hair whorl direction and homosexuality in men, using more objective methodology. The authors took digital photographs of parietal surface hair whorls from 100 heterosexual men and 100 homosexual men who were predominantly right-handed. These images were rated for clockwise and counterclockwise direction (for which no more than 1 hair whorl was present) by 2 raters unaware of sexual orientation. The authors found no significant difference between heterosexual and homosexual men in hair whorl direction, but the authors did replicate the fraternal birth order effect (more older brothers for homosexual men). Number of older sisters was positively correlated with counterclockwise hair whorls in heterosexual men. These data were discussed in relation to prenatal factors assumed to play a role in the neurodevelopment of male homosexuality. (c) 2009 APA, all rights reserved.
Biological and Experimental Psychology Group, School of Biological Sciences, Queen Mary, University of London, United Kingdom. q.rahman@qmul.ac.uk
Behav Neurosci. 2009 Apr;123(2):252-6.
Rahman Q, Clarke K, Morera T.
Hair whorl direction is a somatic feature that is organized early during neurodevelopment and unlikely to be influenced by social factors. This study aimed to replicate a widely reported association by A. J. S. Klar (2003) between counterclockwise hair whorl direction and homosexuality in men, using more objective methodology. The authors took digital photographs of parietal surface hair whorls from 100 heterosexual men and 100 homosexual men who were predominantly right-handed. These images were rated for clockwise and counterclockwise direction (for which no more than 1 hair whorl was present) by 2 raters unaware of sexual orientation. The authors found no significant difference between heterosexual and homosexual men in hair whorl direction, but the authors did replicate the fraternal birth order effect (more older brothers for homosexual men). Number of older sisters was positively correlated with counterclockwise hair whorls in heterosexual men. These data were discussed in relation to prenatal factors assumed to play a role in the neurodevelopment of male homosexuality. (c) 2009 APA, all rights reserved.
Biological and Experimental Psychology Group, School of Biological Sciences, Queen Mary, University of London, United Kingdom. q.rahman@qmul.ac.uk
H-Y antigen
H-Y antigen and homosexuality in men.
J Theor Biol. 1997 Apr 7;185(3):373-8.
Blanchard R, Klassen P.
In men, sexual orientation correlates with the number of older brothers, each additional older brother increasing the odds of homosexuality by approximately 33%. It is hypothesized that this fraternal birth order effect reflects the progressive immunization of some mothers to Y-linked minor histocompatibility antigens (H-Y antigen) by each succeeding male fetus, and the concomitantly increasing effects of H-Y antibodies on the sexual differentiation of the brain in each succeeding male fetus. This hypothesis is consistent with a variety of evidence, including the apparent irrelevance of older sisters to the sexual orientation of later-born males, the probable involvement of H-Y antigen in the development of sex-typical traits, and the detrimental effects of immunization of female mice to H-Y antigen on the reproductive performance of subsequent male offspring.
Department of Psychiatry, University of Toronto, Ontario, Canada. blanchardr@cs.clarke-inst.on.ca
Immune complexes containing H-Y antigen and maternal IgG in cord serum.
Clin Exp Immunol. 1982 Nov;50(2):450-3.
Farber CM, Wachtel SS, Cunningham-Rundles C.
Increased levels of immune complexes are more frequently detected in the serum of newborn males than they are in the serum of newborn females. In one survey of 545 newborns, 21 of 26 (81%) of the babies with high levels of immune complexes in cord serum were boys (Farber, Cambiaso & Masson, 1981). To evaluate the hypothesis that this was due in some cases to the synthesis by the mother of antibodies directed against the 'male-specific' H-Y antigen, we tested for presence of IgG antibodies bound to H-Y antigen in serum samples from 263 newborns including 124 girls and 139 boys. Sera from five of the 10 male newborns with high levels of immune complexes contained IgG bound to H-Y; none of the sera from newborn girls had detectable amounts of those immune complexes; and sera from women who had borne males manifested higher levels of IgG reactive with sources of soluble H-Y than sera from women who had borne females.
J Theor Biol. 1997 Apr 7;185(3):373-8.
Blanchard R, Klassen P.
In men, sexual orientation correlates with the number of older brothers, each additional older brother increasing the odds of homosexuality by approximately 33%. It is hypothesized that this fraternal birth order effect reflects the progressive immunization of some mothers to Y-linked minor histocompatibility antigens (H-Y antigen) by each succeeding male fetus, and the concomitantly increasing effects of H-Y antibodies on the sexual differentiation of the brain in each succeeding male fetus. This hypothesis is consistent with a variety of evidence, including the apparent irrelevance of older sisters to the sexual orientation of later-born males, the probable involvement of H-Y antigen in the development of sex-typical traits, and the detrimental effects of immunization of female mice to H-Y antigen on the reproductive performance of subsequent male offspring.
Department of Psychiatry, University of Toronto, Ontario, Canada. blanchardr@cs.clarke-inst.on.ca
Immune complexes containing H-Y antigen and maternal IgG in cord serum.
Clin Exp Immunol. 1982 Nov;50(2):450-3.
Farber CM, Wachtel SS, Cunningham-Rundles C.
Increased levels of immune complexes are more frequently detected in the serum of newborn males than they are in the serum of newborn females. In one survey of 545 newborns, 21 of 26 (81%) of the babies with high levels of immune complexes in cord serum were boys (Farber, Cambiaso & Masson, 1981). To evaluate the hypothesis that this was due in some cases to the synthesis by the mother of antibodies directed against the 'male-specific' H-Y antigen, we tested for presence of IgG antibodies bound to H-Y antigen in serum samples from 263 newborns including 124 girls and 139 boys. Sera from five of the 10 male newborns with high levels of immune complexes contained IgG bound to H-Y; none of the sera from newborn girls had detectable amounts of those immune complexes; and sera from women who had borne males manifested higher levels of IgG reactive with sources of soluble H-Y than sera from women who had borne females.
Handedness and fraternal birth order impact on sexual orientation
Fraternal birth order and the maternal immune hypothesis of male homosexuality.
Horm Behav. 2001 Sep;40(2):105-14.
In men, sexual orientation correlates with an individual's number of older brothers, each additional older brother increasing the odds of homosexuality by approximately 33%. It has been hypothesized that this fraternal birth order effect reflects the progressive immunization of some mothers to Y-linked minor histocompatibility antigens (H-Y antigens) by each succeeding male fetus and the concomitantly increasing effects of such maternal immunization on the future sexual orientation of each succeeding male fetus. According to this hypothesis, anti-H-Y antibodies produced by the mother pass through the placental barrier to the fetus and affect aspects of sexual differentiation in the fetal brain. This explanation is consistent with a variety of evidence, including the apparent irrelevance of older sisters to the sexual orientation of later born males, the probable involvement of H-Y antigen in the development of sex-typical traits, and the detrimental effects of immunization of female mice to H-Y antigen on the reproductive performance of subsequent male offspring. The maternal immune hypothesis might also explain the recent finding that heterosexual males with older brothers weigh less at birth than heterosexual males with older sisters and homosexual males with older brothers weigh even less than heterosexual males with older brothers.
Centre for Addiction and Mental Health-Clarke Site, Department of Psychiatry, Faculty of Medicine, University of Toronto, 250 College Street, Toronto, Ontario M5T 1R8, Canada. Ray_Blanchard@camh.net
Review and theory of handedness, birth order, and homosexuality in men.
Laterality. 2008 Jan;13(1):51-70.
Research has repeatedly shown that older brothers increase the odds of homosexuality in later-born males. This phenomenon has been called the fraternal birth order effect. The most highly developed explanation of this phenomenon is the maternal immune hypothesis, which proposes that the fraternal birth order effect reflects the progressive immunisation of some mothers to male-specific antigens by each succeeding male foetus and the concomitantly increasing effects of anti-male antibodies on the sexual differentiation of the brain in each succeeding male foetus. Recent studies indicate that older brothers increase the odds of homosexuality in right-handed males but not in non-right-handed males. The present article explores how the maternal immune hypothesis might be extended or modified to account for the apparent interaction of older brothers and handedness. Two possibilities are considered: (1) non-right-handed foetuses are insensitive to the presence of maternal anti-male antibodies, and (2) mothers of non-right-handed foetuses do not produce anti-male antibodies.
Centre for Addiction and Mental Health, Toronto, ON, Canada. Ray_Blanchard@camh.net
The sex ratio of older siblings in non-right-handed homosexual men.
Arch Sex Behav. 2008 Dec;37(6):970-6. Epub 2006 Nov 28.
Blanchard R, Lippa RA.
This study tested the prediction, based on prior research, that non-right-handed homosexual men will report fewer than expected older brothers. Participants were 2486 heterosexual and homosexual, right-handed and non-right-handed, male and female adults, representing five samples collected for various projects by the second author. Data on sibship composition, sexual orientation, and hand-preference were gathered in the original research using on-line (Internet) or self-administered paper-and-pencil questionnaires. The non-right-handed homosexual men reported 83 older brothers per 100 older sisters, which was significantly lower than the human sex ratio of 106 live-born males per 100 live-born females. In contrast, the right-handed homosexual men reported 125 older brothers per 100 older sisters, which was significantly higher than the expected ratio. One possible explanation of these results is that older brothers increase the odds of homosexuality in right-handed males but decrease the odds of homosexuality in non-right-handed males. A second possibility is that older brothers decrease the probability that non-right-handed homosexual males will be represented in survey research. The latter scenario could arise if the combination of some biological factor associated with older brothers and some biological factor associated with non-right-handedness is so toxic that it kills the fetus or predisposes the individual to a condition (e.g., mental retardation, major mental illness) that makes him less likely to be available for research recruitment at Gay Pride parades (etc.) than other members of the gay community.
Law and Mental Health Program, Centre for Addiction and Mental Health, College Street Site, 250 College Street, Toronto, Ontario, M5T 1R8, Canada. Ray_Blanchard@camh.net
Sex ratio of older siblings in heterosexual and homosexual, right-handed and non-right-handed men.
Arch Sex Behav. 2008 Dec;37(6):977-81.
Blanchard R.
This study investigated why older brothers, which increase the odds of homosexuality in later-born males who are right-handed, have no effect or the opposite effect on later-born males who are non-right-handed. The specific question was whether the different results for the non-right-handed men have to do with the heterosexual non-right-handers or the homosexual non-right-handers. The human sex ratio at birth (106 males per 100 females) was used as a gold standard for determining which groups differ from the general population and in which direction. All usable data from previous studies were combined to obtain the largest possible sample (N = 8,201). The observed ratio of older brothers to older sisters was 105 for the heterosexual right-handers, 128 for the homosexual right-handers, 127 for the heterosexual non-right-handers, and 96 for the homosexual non-right-handers. The ratios for the homosexual right-handers and the heterosexual non-right-handers differed significantly from the expected value. These results suggest that both heterosexual and homosexual non-right-handers contribute to the older brothers x handedness x sexual orientation interaction.
Law and Mental Health Program, Centre for Addiction and Mental Health, 250 College Street, Toronto, Ontario, M5T 1R8, Canada. Ray_Blanchard@camh.net
Arch Sex Behav. 2008 Dec;37(6):970-6. Epub 2006 Nov 28.
Blanchard R, Lippa RA.
This study tested the prediction, based on prior research, that non-right-handed homosexual men will report fewer than expected older brothers. Participants were 2486 heterosexual and homosexual, right-handed and non-right-handed, male and female adults, representing five samples collected for various projects by the second author. Data on sibship composition, sexual orientation, and hand-preference were gathered in the original research using on-line (Internet) or self-administered paper-and-pencil questionnaires. The non-right-handed homosexual men reported 83 older brothers per 100 older sisters, which was significantly lower than the human sex ratio of 106 live-born males per 100 live-born females. In contrast, the right-handed homosexual men reported 125 older brothers per 100 older sisters, which was significantly higher than the expected ratio. One possible explanation of these results is that older brothers increase the odds of homosexuality in right-handed males but decrease the odds of homosexuality in non-right-handed males. A second possibility is that older brothers decrease the probability that non-right-handed homosexual males will be represented in survey research. The latter scenario could arise if the combination of some biological factor associated with older brothers and some biological factor associated with non-right-handedness is so toxic that it kills the fetus or predisposes the individual to a condition (e.g., mental retardation, major mental illness) that makes him less likely to be available for research recruitment at Gay Pride parades (etc.) than other members of the gay community.
Law and Mental Health Program, Centre for Addiction and Mental Health, College Street Site, 250 College Street, Toronto, Ontario, M5T 1R8, Canada. Ray_Blanchard@camh.net
Sex ratio of older siblings in heterosexual and homosexual, right-handed and non-right-handed men.
Arch Sex Behav. 2008 Dec;37(6):977-81.
Blanchard R.
This study investigated why older brothers, which increase the odds of homosexuality in later-born males who are right-handed, have no effect or the opposite effect on later-born males who are non-right-handed. The specific question was whether the different results for the non-right-handed men have to do with the heterosexual non-right-handers or the homosexual non-right-handers. The human sex ratio at birth (106 males per 100 females) was used as a gold standard for determining which groups differ from the general population and in which direction. All usable data from previous studies were combined to obtain the largest possible sample (N = 8,201). The observed ratio of older brothers to older sisters was 105 for the heterosexual right-handers, 128 for the homosexual right-handers, 127 for the heterosexual non-right-handers, and 96 for the homosexual non-right-handers. The ratios for the homosexual right-handers and the heterosexual non-right-handers differed significantly from the expected value. These results suggest that both heterosexual and homosexual non-right-handers contribute to the older brothers x handedness x sexual orientation interaction.
Law and Mental Health Program, Centre for Addiction and Mental Health, 250 College Street, Toronto, Ontario, M5T 1R8, Canada. Ray_Blanchard@camh.net
Proportion of homosexual men who owe their sexual orientation to fraternal birth order: An estimate based on two national probability samples.
Am J Hum Biol. 2004 Mar-Apr;16(2):151-7.
Homosexuality in men correlates with an individual's number of older brothers, greater numbers of older brothers being associated with a greater probability of homosexuality. There are reasons to believe that this relationship is causal rather than merely statistical, that is, that older brothers produce the increase in the probability of homosexuality for later-born males. It is possible, under this assumption, to estimate the proportion of homosexual men who can attribute their sexual orientation to their birth order among their brothers (fraternal birth order). This statistic, the population attributable fraction (PAF), was computed on the combined archival data of 2,256 heterosexual and 71 homosexual men examined in survey studies of sexual behavior in the UK and the USA. The PAF was 28.6%, with 95% confidence limits of 14.8% and 48.0%. These limits encompass the PAF of 15.1% previously estimated with a Canadian sample. The results indicate that the proportion of homosexual men whose sexual orientation is attributable to fraternal birth order constitutes a minority, but not a negligible minority, of all homosexual men. The fraternal birth order effect may reflect the progressive immunization of some mothers to Y-linked antigens by each succeeding male fetus, and the concomitantly increasing effects of antimale antibodies on the sexual differentiation of the brain in each succeeding male fetus.
Department of Psychiatry, University of Toronto, Toronto, Ontario, Canada. Ray_Blanchard@camh.net
Finger length ratios 2D:4D
The use of digit ratios as markers for perinatal androgen action.
Reprod Biol Endocrinol. 2006 Feb 26;4:10. [FFTA]
McIntyre MH.
Since the ratio of the second-to-fourth finger length was first proposed as a marker for prenatal androgen action in 1998, over 100 studies have been published that have either further tested the association between the digit ratio and prenatal androgens, or employed digit ratios as a marker to investigate the association between prenatal androgens and a variety of outcomes, including behavior, fertility, and disease risks. Despite the clear demand for an adult marker of prenatal androgen action and increased use of digit ratios as such a marker, its validity remains controversial. This review (1) evaluates current evidence for the relationship between digit ratios and prenatal androgens (using experimentation with animal models, amniotic testosterone, and congenital adrenal hyperplasia case-control studies), (2) describes opportunities for future validation tests, and (3) compares the potential advantages and disadvantages of digit ratio measures with more established methods for studying the effects of prenatal androgens. [Free Full Text Article on PubMed]
Department of Epidemiology, Harvard School of Public Health, 677 Huntington Avenue, Boston, MA 02115, USA. mmcintyr@hsph.harvard.edu
Conclusion (from FFTA)
The validity of digit ratios as markers for perinatal androgen action is supported by a number of lines of recently reported evidence, but further support is needed.
(1) Sex differences in digit ratios develop early in childhood and are unaffected by pubertal growth. This seems to be firmly established.
(2) High amniotic testosterone-to-estradiol ratio predicts low 2D:4D by two years old. While current evidence is supportive, further studies using amniotic testosterone or other direct methods should be conducted.
(3) CAH affected girls have lower 2D:4D than unaffected controls. This observation is not firmly established and further studies could help to resolve inconsistencies in the literature.
(4) The CAG repeat polymorphism of the androgen receptor gene influences 2D:4D as predicted. Further studies could help to confirm this finding.
Questions remain that might also be answered through animal or in vitro experimentation, provided that the utility of particular models can be established. For example, which periods of androgen exposure during the perinatal period produce observed sex differences in digit ratios? How do the development of the right and left sides differ to yield such varying results (even among laboratory mice)?
Interesting work has already been done using 2D:4D, and it might one day prove, after further validation, to be a simple, reliable, and broadly accepted method for studying early human sex differentiation in large, representative samples. Furthermore, while interest in early testosterone has been greatest among psychologists, epidemiologists have also begun to investigate the role of early development in disease. Given that more direct measurement of early testosterone levels is unworkable in most epidemiological study designs, further research would be encouraged by the availability of markers such as digit ratios.
Hand asymmetry in heterosexual and homosexual men and women: relationship to 2D:4D digit ratios and other sexually dimorphic anatomical traits.
Arch Sex Behav. 2008 Feb;37(1):119-32.
Martin JT, Puts DA, Breedlove SM.
Sexual differentiation leads to the development of distinctive anatomical structures (e.g., gonads and genitalia); it also produces less obvious anatomical shifts in brain, bones, muscles, etc. This study is a retrospective analysis of growth patterns in the hands in relation to sex and sexual orientation. Using data from three published studies, we analyzed four hand traits in adults: hand width, hand length, second digit length, and fourth digit length. Using these measurements, we derived estimates of trait laterality (directional asymmetry or DA) and developmental instability (fluctuating asymmetry or FA). High FA is a putative indicator of interference with the cellular and molecular mechanisms regulating development. We focused on how these derived variables were related to sex, sexual orientation, and putative markers of early sex steroid exposure (e.g., the second to fourth digit ratio or 2D:4D). Our data point to three principal conclusions. First, individual differences in DA appeared to be a major source of variation in the 2D:4D ratio. The 2D:4D ratios of heterosexual men differed depending on whether they had leftward or rightward DA in their digits. Homosexual women showed the same pattern. Individuals with leftward DA in both digits had lower 2D:4D ratios than those with rightward DA. This effect was absent in heterosexual women and homosexual men. This led to sex differences in 2D:4D and sexual orientation differences in 2D:4D in the leftward DA group, but not in the rightward DA group. The second conclusion was that DA in digit length and hand width varied with sex; women were more likely to have rightward asymmetry than men. Homosexual men and women were generally sex typical in DA. The third conclusion was that homosexuality is unlikely to be a result of increased developmental instability. Although limited in scope, the present evidence actually suggests that homosexuals have lower FA than heterosexuals, raising the question of whether the positive fitness components associated with low FA may contribute to selection that maintains homosexuality in a population.
Department of Basic Medical Sciences, College of Osteopathic Medicine of the Pacific, Western University of Health Sciences, 309 E. 2nd Street, Pomona, CA 91766, USA. jmartin@westernu.edu
Sexual orientation and fluctuating asymmetry in men and women.
Arch Sex Behav. 2008 Feb;37(1):158-65.
Hall PA, Schaeff CM.
Previous research suggests that individuals' sexual orientation may be affected by developmental instability (DI) induced by exposure to prenatal stresses. We tested this relationship using fluctuating asymmetry (FA), the small random deviations from symmetry that arise in otherwise bilaterally symmetrical traits as a consequence of developmental noise and developmental instability. Differences among individuals reflect variation in their exposure to and ability to accommodate for stresses experienced during development as well as to developmental noise that arises due to cellular stocasticity. FA measurements for 156 heterosexual and 132 homosexual men and women participants provided strong support for the developmental instability hypothesis: FA was significantly higher in both male and female homosexuals (men: four of seven bilateral traits and composite FA values (cFA); women: five of seven bilateral traits and composite FA values). Although finger-length ratios (FLRs), an indirect marker for prenatal hormones, were sex-atypical (e.g., feminized) for homosexual men, we failed to detect any relationship between FA levels and 2D:4D finger-length ratios (FLRs). Hence, although elevated levels of developmental stress appear to be linked to shifts in sexual orientation, the underlying mechanism does not seem to be connected to sex-atypical prenatal hormones. Additional analyses with sex atypical individuals are needed to confirm this.
Fluctuating asymmetry, second to fourth finger length ratios and human sexual orientation.
Psychoneuroendocrinology. 2005 May;30(4):382-91. Epub 2005 Jan 13.
Rahman Q.
Sexual orientation in humans may be influenced by levels of prenatal sex steroids which canalise neurodevelopment along sex-typical (heterosexual) or sex-atypical (homosexual) lines. Some evidence for sexual-orientation-related differences in putative somatic markers of prenatal sex hormones supports this view. A competing theory asserts that human homosexuality is due to developmental instability (DI) because it represents a shift from the species-typical pattern of heterosexual orientation. Evidence for elevated rates of non-right handedness among homosexuals provides limited support for this account. The current study tested both theories by examining nine bilateral somatic traits in 120 healthy heterosexual and homosexual men and women in order to compute second to fourth finger length ratios (2D:4D), a measure ascribed to levels of prenatal sex steroids, and fluctuating asymmetry (FA), a measure of DI. Homosexual men and women had significantly lower right hand 2D:4D ratios (even after controlling for handedness, height and weight differences) in comparison to heterosexuals, but sexual orientation did not relate to composite FA scores. The findings constrain the number of possible neurodevelopmental pathways responsible for sexual orientation in humans.
School of Psychology, University of East London, The Green, London, UK. q.rahman@uel.ac.uk
Digit length ratios predict reactive aggression in women, but not in men.
Horm Behav. 2004 Dec;46(5):558-64.
Considerable evidence suggests that digit length and dermatoglyphic asymmetry patterns in the hand are affected by early exposure to androgens. Because androgens play an important role in sexual differentiation of morphological and behavioral traits, digit length patterns often display sex differences. When present, sex differences in digit lengths are more pronounced on the right side as compared to the left side. Moreover, the ratio of the second to fourth digit length (2D:4D) in the right hand is inversely correlated with testosterone (T) in men. Because T is implicated in agonistic behavior, 2D:4D may be used as a marker of androgen exposure and subsequent behavioral variation in adulthood. Consequently, we investigated the relationships among 2D:4D, directional asymmetry of 2D:4D (left hand 2D:4D minus right hand 2D:4D) as a variant of T, and human reactive aggression. One hundred young men (n = 51) and women (n = 49) participated in our experiments (mean age = 20.1 years). Participants called two noncompliant confederates to solicit donations for a fictitious charity organization and selected follow-up letters after the calls. The force exerted when hanging up the phone and the "tone" of the follow-up letters were used to assess reactive aggression. High aggression scores were associated with high directional asymmetry of 2D:4D and masculinized (low) right hand 2D:4D, only in females and under high provocation. Directional asymmetry of 2D:4D was positively correlated with T in males (pooled data, n = 97). Taken together, these data confirm the predominantly right-sided influence of androgens on digit length and suggest that digit length ratios may be associated with female reactive aggression when sufficient provocation is present.
Reprod Biol Endocrinol. 2006 Feb 26;4:10. [FFTA]
McIntyre MH.
Since the ratio of the second-to-fourth finger length was first proposed as a marker for prenatal androgen action in 1998, over 100 studies have been published that have either further tested the association between the digit ratio and prenatal androgens, or employed digit ratios as a marker to investigate the association between prenatal androgens and a variety of outcomes, including behavior, fertility, and disease risks. Despite the clear demand for an adult marker of prenatal androgen action and increased use of digit ratios as such a marker, its validity remains controversial. This review (1) evaluates current evidence for the relationship between digit ratios and prenatal androgens (using experimentation with animal models, amniotic testosterone, and congenital adrenal hyperplasia case-control studies), (2) describes opportunities for future validation tests, and (3) compares the potential advantages and disadvantages of digit ratio measures with more established methods for studying the effects of prenatal androgens. [Free Full Text Article on PubMed]
Department of Epidemiology, Harvard School of Public Health, 677 Huntington Avenue, Boston, MA 02115, USA. mmcintyr@hsph.harvard.edu
Conclusion (from FFTA)
The validity of digit ratios as markers for perinatal androgen action is supported by a number of lines of recently reported evidence, but further support is needed.
(1) Sex differences in digit ratios develop early in childhood and are unaffected by pubertal growth. This seems to be firmly established.
(2) High amniotic testosterone-to-estradiol ratio predicts low 2D:4D by two years old. While current evidence is supportive, further studies using amniotic testosterone or other direct methods should be conducted.
(3) CAH affected girls have lower 2D:4D than unaffected controls. This observation is not firmly established and further studies could help to resolve inconsistencies in the literature.
(4) The CAG repeat polymorphism of the androgen receptor gene influences 2D:4D as predicted. Further studies could help to confirm this finding.
Questions remain that might also be answered through animal or in vitro experimentation, provided that the utility of particular models can be established. For example, which periods of androgen exposure during the perinatal period produce observed sex differences in digit ratios? How do the development of the right and left sides differ to yield such varying results (even among laboratory mice)?
Interesting work has already been done using 2D:4D, and it might one day prove, after further validation, to be a simple, reliable, and broadly accepted method for studying early human sex differentiation in large, representative samples. Furthermore, while interest in early testosterone has been greatest among psychologists, epidemiologists have also begun to investigate the role of early development in disease. Given that more direct measurement of early testosterone levels is unworkable in most epidemiological study designs, further research would be encouraged by the availability of markers such as digit ratios.
Hand asymmetry in heterosexual and homosexual men and women: relationship to 2D:4D digit ratios and other sexually dimorphic anatomical traits.
Arch Sex Behav. 2008 Feb;37(1):119-32.
Martin JT, Puts DA, Breedlove SM.
Sexual differentiation leads to the development of distinctive anatomical structures (e.g., gonads and genitalia); it also produces less obvious anatomical shifts in brain, bones, muscles, etc. This study is a retrospective analysis of growth patterns in the hands in relation to sex and sexual orientation. Using data from three published studies, we analyzed four hand traits in adults: hand width, hand length, second digit length, and fourth digit length. Using these measurements, we derived estimates of trait laterality (directional asymmetry or DA) and developmental instability (fluctuating asymmetry or FA). High FA is a putative indicator of interference with the cellular and molecular mechanisms regulating development. We focused on how these derived variables were related to sex, sexual orientation, and putative markers of early sex steroid exposure (e.g., the second to fourth digit ratio or 2D:4D). Our data point to three principal conclusions. First, individual differences in DA appeared to be a major source of variation in the 2D:4D ratio. The 2D:4D ratios of heterosexual men differed depending on whether they had leftward or rightward DA in their digits. Homosexual women showed the same pattern. Individuals with leftward DA in both digits had lower 2D:4D ratios than those with rightward DA. This effect was absent in heterosexual women and homosexual men. This led to sex differences in 2D:4D and sexual orientation differences in 2D:4D in the leftward DA group, but not in the rightward DA group. The second conclusion was that DA in digit length and hand width varied with sex; women were more likely to have rightward asymmetry than men. Homosexual men and women were generally sex typical in DA. The third conclusion was that homosexuality is unlikely to be a result of increased developmental instability. Although limited in scope, the present evidence actually suggests that homosexuals have lower FA than heterosexuals, raising the question of whether the positive fitness components associated with low FA may contribute to selection that maintains homosexuality in a population.
Department of Basic Medical Sciences, College of Osteopathic Medicine of the Pacific, Western University of Health Sciences, 309 E. 2nd Street, Pomona, CA 91766, USA. jmartin@westernu.edu
Sexual orientation and fluctuating asymmetry in men and women.
Arch Sex Behav. 2008 Feb;37(1):158-65.
Hall PA, Schaeff CM.
Previous research suggests that individuals' sexual orientation may be affected by developmental instability (DI) induced by exposure to prenatal stresses. We tested this relationship using fluctuating asymmetry (FA), the small random deviations from symmetry that arise in otherwise bilaterally symmetrical traits as a consequence of developmental noise and developmental instability. Differences among individuals reflect variation in their exposure to and ability to accommodate for stresses experienced during development as well as to developmental noise that arises due to cellular stocasticity. FA measurements for 156 heterosexual and 132 homosexual men and women participants provided strong support for the developmental instability hypothesis: FA was significantly higher in both male and female homosexuals (men: four of seven bilateral traits and composite FA values (cFA); women: five of seven bilateral traits and composite FA values). Although finger-length ratios (FLRs), an indirect marker for prenatal hormones, were sex-atypical (e.g., feminized) for homosexual men, we failed to detect any relationship between FA levels and 2D:4D finger-length ratios (FLRs). Hence, although elevated levels of developmental stress appear to be linked to shifts in sexual orientation, the underlying mechanism does not seem to be connected to sex-atypical prenatal hormones. Additional analyses with sex atypical individuals are needed to confirm this.
Fluctuating asymmetry, second to fourth finger length ratios and human sexual orientation.
Psychoneuroendocrinology. 2005 May;30(4):382-91. Epub 2005 Jan 13.
Rahman Q.
Sexual orientation in humans may be influenced by levels of prenatal sex steroids which canalise neurodevelopment along sex-typical (heterosexual) or sex-atypical (homosexual) lines. Some evidence for sexual-orientation-related differences in putative somatic markers of prenatal sex hormones supports this view. A competing theory asserts that human homosexuality is due to developmental instability (DI) because it represents a shift from the species-typical pattern of heterosexual orientation. Evidence for elevated rates of non-right handedness among homosexuals provides limited support for this account. The current study tested both theories by examining nine bilateral somatic traits in 120 healthy heterosexual and homosexual men and women in order to compute second to fourth finger length ratios (2D:4D), a measure ascribed to levels of prenatal sex steroids, and fluctuating asymmetry (FA), a measure of DI. Homosexual men and women had significantly lower right hand 2D:4D ratios (even after controlling for handedness, height and weight differences) in comparison to heterosexuals, but sexual orientation did not relate to composite FA scores. The findings constrain the number of possible neurodevelopmental pathways responsible for sexual orientation in humans.
School of Psychology, University of East London, The Green, London, UK. q.rahman@uel.ac.uk
Digit length ratios predict reactive aggression in women, but not in men.
Horm Behav. 2004 Dec;46(5):558-64.
Considerable evidence suggests that digit length and dermatoglyphic asymmetry patterns in the hand are affected by early exposure to androgens. Because androgens play an important role in sexual differentiation of morphological and behavioral traits, digit length patterns often display sex differences. When present, sex differences in digit lengths are more pronounced on the right side as compared to the left side. Moreover, the ratio of the second to fourth digit length (2D:4D) in the right hand is inversely correlated with testosterone (T) in men. Because T is implicated in agonistic behavior, 2D:4D may be used as a marker of androgen exposure and subsequent behavioral variation in adulthood. Consequently, we investigated the relationships among 2D:4D, directional asymmetry of 2D:4D (left hand 2D:4D minus right hand 2D:4D) as a variant of T, and human reactive aggression. One hundred young men (n = 51) and women (n = 49) participated in our experiments (mean age = 20.1 years). Participants called two noncompliant confederates to solicit donations for a fictitious charity organization and selected follow-up letters after the calls. The force exerted when hanging up the phone and the "tone" of the follow-up letters were used to assess reactive aggression. High aggression scores were associated with high directional asymmetry of 2D:4D and masculinized (low) right hand 2D:4D, only in females and under high provocation. Directional asymmetry of 2D:4D was positively correlated with T in males (pooled data, n = 97). Taken together, these data confirm the predominantly right-sided influence of androgens on digit length and suggest that digit length ratios may be associated with female reactive aggression when sufficient provocation is present.
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