Nature-Nurture

Ask anyone about their sexual orientation, and they will most likely say something like, "I was born this way."*

However, demonstrating how such a situation comes about is technically difficult. "Proving"** it is even more problematic. This blog is an ongoing collection of research abstracts related to the complex question of etiology of sexual orientation. Whatever the etiology, it is not solely environmental factors. If it were, everyone would be heterosexual.

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"[A]s much as people like to divide themselves into nature or nurture camps, what genes actually do in the brain reflects the interaction between hereditary and environmental information."

"The truth of the matter is that DNA is both inherited and environmentally responsive, and recent findings from animal studies go a long way toward resolving nature versus nurture by upsetting the assumption that the two work differently."

Gene E. Robinson, PhD, Director of the Neuroscience Program at the University of Illinois at Urbana-Champaign, in The New York Times op-ed article titled "The Behavior of Genes," published on Dec. 13, 2004

_________

"There are numerous theories about the origins of a person's sexual orientation; most scientists today agree that sexual orientation is most likely the result of a complex interaction of environmental, cognitive and biological factors. In most people, sexual orientation is shaped at an early age. There is also considerable recent evidence to suggest that biology, including genetic or inborn hormonal factors, play a significant role in a person's sexuality. In summary, it is important to recognize that there are probably many reasons for a person's sexual orientation and the reasons may be different for different people."

American Psychological Association, in "Answers to Your Questions for a Better Understanding of Sexual Orientation and Homosexuality"

"APA opposes all public and private discrimination on the basis of actual or perceived gender identity and expression and urges the repeal of discriminatory laws and policies."

Read the Full-text of APA’s APA Resolution on Transgender, Gender Identity, and Gender Expression Non-Discrimination.

More information on APA Policy Statements, as well as other activities and resources, is available at the APA Office of Lesbian, Gay, Bisexual, and Transgender Concerns.

_________

"No one knows what causes heterosexuality, homosexuality or bisexuality. Homosexuality was once thought to be the result of troubled family dynamics or faulty psychological development. Those assumptions are now understood to have been based on misinformation and prejudice. Currently there is a renewed interest in searching for biological etiologies for homosexuality. However, to date there are no replicated scientific studies supporting any specific biological etiology for homosexuality. Similarly, no specific psychosocial or family dynamic cause for homosexuality has been identified, including histories of childhood sexual abuse. Sexual abuse does not appear to be more prevalent in children who grow up to identify as gay, lesbian, or bisexual, than in children who identify as heterosexual." [emphasis mine]

American Psychiatric Association, in "Gay, Lesbian and Bisexual Fact Sheet," May 2000

_________

* Heterosexuals are more likely to say something along the lines of, "Of course I am this way. I am normal." This signifies nothing more than the obvious: the process of natural selection inherently favors reproductive success. Heterosexual inclinations favor reproductive success, so genes coding for heterosexual neurobehavioral-development necessarily flourish.

** Proof is confined to mathematics and specially constructed philosophical syllogisms. Within the sciences, the problem of induction renders proof a logical impracticality.

Homophobia

Homophobic aggression .
Homophobia - male homosexual arousal .
Homophobia statistics .
Psychology of Homophobia .

.. PubMed search ..

Homophobia statistics

From a university website:

Understanding Homophobia and Heterosexism.

Sexual Prejudice: Motivations.

Demographic Correlates

In contrast to heterosexuals with favorable attitudes toward gay people, those with negative attitudes are more likely to be:
• men
• older
• less well-educated
• residing in geographic areas where negative attitudes represent the norm (for example, rural areas or the Midwestern or Southern United States).

Political and Religious Values

In contrast to heterosexuals with favorable attitudes toward gay people, those with negative attitudes are:
• more likely to attend religious services frequently
• more likely to endorse orthodox religious beliefs, such as the literal truth of the Bible
• more likely to be a Republican than a Democrat or Independent
• more likely to describe themselves as politically conservative, rather than liberal or moderate.

Personality and Attitudinal Characteristics

In contrast to heterosexuals with favorable attitudes toward gay people, those with negative attitudes:
• display higher levels of psychological authoritarianism
• are less sexually permissive
• are more supportive of traditional gender roles.

Perceptions and Experiences of Gay Men and Lesbians

In contrast to heterosexuals with favorable attitudes toward gay people, those with negative attitudes:
• are more likely to believe that a homosexual orientation is freely chosen
• are less likely to have had close personal friends or family members who are openly lesbian or gay.

Homophobia male homosexual arousal

Is homophobia associated with homosexual arousal?

J Abnorm Psychol. 1996 Aug;105(3):440-5.

Adams HE, Wright LW Jr, Lohr BA.

The authors investigated the role of homosexual arousal in exclusively heterosexual men who admitted negative affect toward homosexual individuals. Participants consisted of a group of homophobic men (n = 35) and a group of nonhomophobic men (n = 29); they were assigned to groups on the basis of their scores on the Index of Homophobia (W. W. Hudson & W. A. Ricketts, 1980). The men were exposed to sexually explicit erotic stimuli consisting of heterosexual, male homosexual, and lesbian videotapes, and changes in penile circumference were monitored. They also completed an Aggression Questionnaire (A. H. Buss & M. Perry, 1992). Both groups exhibited increases in penile circumference to the heterosexual and female homosexual videos. Only the homophobic men showed an increase in penile erection to male homosexual stimuli. The groups did not differ in aggression. Homophobia is apparently associated with homosexual arousal that the homophobic individual is either unaware of or denies.

Department of Psychology, University of Georgia, Athens 30602-3013, USA.

Homophobic aggression

Homophobia and physical aggression toward homosexual and heterosexual individuals.

J Abnorm Psychol. 2001 Feb;110(1):179-87.

Bernat JA, Calhoun KS, Adams HE, Zeichner A.

This study examined the relationship between homophobia (defined as self-reported negative affect, avoidance, and aggression toward homosexuals) and homosexual aggression. Self-identified heterosexual college men were assigned to homophobic (n = 26) and nonhomophobic (n = 26) groups on the basis of their scores on the Homophobia Scale (HS; L. W. Wright, H. E. Adams, & J. A. Bernat, 1999). Physical aggression was examined by having participants administer shocks to a fictitious opponent during a competitive reaction time (RT) task under the impression that the study was examining the relationship between sexually explicit material and RT. Participants were exposed to a male homosexual erotic videotape, their affective reactions were assessed, and they then competed in the RT task against either a heterosexual or a homosexual opponent. The homophobic group reported significantly more negative affect, anxiety, and anger-hostility after watching the homosexual erotic videotape than did the nonhomophobic group. Additionally, the homophobic group was significantly more aggressive toward the homosexual opponent, but the groups did not differ in aggression toward the heterosexual opponent.

Department of Psychology, University of Georgia, Athens, Georgia 30602-3013, USA. jbernat@egon.psy.uga.edu

Psychology of Homophobia

The psychological basis of homophobia: cultural construction of a barrier.

Integr Psychol Behav Sci. 2007 Dec;41(3-4):225-47; discussion 326-34.

Madureira AF

The main goals of this article are: (a) to analyze the intricate relations between sexuality, gender, culture and power, and more precisely, the connections between homophobia and sexism; and (b) to analyze the psychological and cultural basis of homophobia and its affective roots. The model of the semiotic regulatory system and the general notion of tension between the two processes (specified by Ernest Boesch)--Heimweh ("homeward road"--striving towards the known and the secure) and the Fernweh ("road to the far away"--adventure, encountering novelty) is used as the theoretical ways to analyze homophobia as a cultural barrier. It is suggested: homophobia is a boundary phenomenon of affective meaning making, a collective historical-cultural construction. Presented in the conclusion are some implications of the promotion of strategies against homophobia in our societies.

Institute of Psychology, University of Brasilia, Brasilia, DF, Brazil. afam2001@terra.com.br
.

"Homophobia": a dialogical-semiotic approach.

Integr Psychol Behav Sci. 2007 Dec;41(3-4):262-71; discussion 326-34.

Gonçalves MM, Machado C

The target article from Madureira--The psychological basis of homophobia: Construction of a cultural barrier--is a very rich theoretical proposal, drawing on from different fields, from cultural psychology to the dialogical theories of the self. Despite its richness, in this commentary we will question some of its implicit and explicit assumptions. It seems to us that the author treats "homophobia" as a hegemonic cultural discourse, ignoring that a mixture of opposing discourses are nowadays available at a cultural level towards lesbians and gays, with important consequences at interpersonal and intrapersonal level. In addition, the very concept of "homophobia" (with its implicit assumptions from the field of psychopathology: "phobia") suggests a very static and intrapersonal way of understanding the diversity of the social mechanisms that underlie sexual prejudice. The way the author describes and tries to clarify the semiotic processes implied in the construction of the interpersonal and psychological barrier involved in "homophobia", in our opinion, also needs to be further developed. The use of the opposition between HEIMWEH and FERNWEH in the explanation of the barrier it is not, from our view, a description of a (semiotic) process but more a description of an outcome. In order to overcome these problems, we suggest some possible ways to approach this phenomenon from a semiotic and dialogical perspective.

Department of Psychology, University do Minho, 4710 Braga, Portugal. mgoncalves@iep.uminho.pt
.

Biology

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 ..

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

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.

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

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

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.

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.

Blanchard R.

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.

Blanchard R.

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

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.

Blanchard R, Bogaert AF.

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.

Statistics & Models

Family size - homosexuality, gender dysphoria .
Fraternal birth order - homosexual feelings .
Fraternal birth order - homosexual feelings - men, women .
Male homosexuality & maternal fecundity .

.. PubMed search ..

Fraternal birth order - homosexual feelings - men, women

Fraternal birth order and ratio of heterosexual/homosexual feelings in women and men.

J Homosex. 2006;51(4):161-74.

McConaghy N, Hadzi-Pavlovic D, Stevens C, Manicavasagar V, Buhrich N, Vollmer-Conna U.

Studies of the 2-3% of persons who identify as homosexual found men but not women had more older brothers than persons who identify as heterosexual. The present study investigated the birth order in the approximately 20% of men and women who anonymously report some homosexual feelings, few of whom identify as homosexual. The number of older brothers and sisters was investigated in seven cohorts: 319 male twins; and 49, 54, and 61 female and 66, 116, and 50 male medical students. Both women and men who anonymously reported homosexual feelings had a greater mean number of older brothers and sisters than did those who reported no homosexual feelings. The difference was stronger in relation to brothers than sisters. The birth order effect was not related to the strength of the subjects' degree of homosexual compared with heterosexual feelings. Its presence in women could not be accounted for by the widely accepted hypothesis that the birth order effect is due to a maternal immune reaction provoked only by male fetuses. The lack of relationship between the strength of the effect and degree of homosexual feelings in the men and women suggests the influence of birth order on homosexual feelings was not due to a biological, but a social process in the subjects studied. Investigating the neglected significant percentage of predominantly heterosexual men and women who anonymously report some homosexual feelings may aid in understanding the factors influencing sexual orientation, and identity.

University of New South Wales, New South Wales, Australia.

Paraphilia - Males

Neurodevelopmental correlates of paraphilic sexual interests in men.

Arch Sex Behav. 2008 Feb;37(1):166-72.

Rahman Q, Symeonides DJ.

The etiology of anomalous, or paraphilic, sexual preferences in men is unclear although a growing literature points to their prenatal neurodevelopmental ontogenesis. The present study explored whether this was also apparent in a community sample of 200 heterosexual men by examining their sexual fantasies using the Wilson Sex Fantasy Questionnaire (WSFQ) and several demographic and somatic neurodevelopmental markers, including sibling sex composition, handedness, maternal and paternal age at birth, second to fourth finger length ratios, and fluctuating asymmetry of finger lengths and wrist widths. Responses to the WSFQ were used to quantify the extent of paraphilic interest by computing a variance-quotient (or VQ) previously shown to differentiate paraphilic from conventional heterosexual males. High paraphilic scorers had a significantly greater number of older brothers, higher right-hand 2D:4D, and a trend for lower Edinburgh Handedness Inventory (EHI) scores compared to low-paraphilic scorers. Correlational analysis revealed a significant positive association of VQ scores with number of older brothers and significant negative associations with number of younger brothers and EHI scores (elevated paraphilic interests were correlated with elevated non-right handedness). Correlations between VQ scores and other variables were not significant. It is suggested that processes such as developmental instability and maternal immunity may play a role in variant sexual preferences among otherwise healthy heterosexual men.

Department of Biological Sciences, Queen Mary College, University of London, Mile End Road, London, UK. q.rahman@qmul.ac.uk

Male homosexuality & maternal fecundity

New evidence of genetic factors influencing sexual orientation in men: female fecundity increase in the maternal line.

Arch Sex Behav. 2009 Jun;38(3):393-9. Epub 2008 Jun 17.

Iemmola F, Camperio Ciani A.

There is a long-standing debate on the role of genetic factors influencing homosexuality because the presence of these factors contradicts the Darwinian prediction according to which natural selection should progressively eliminate the factors that reduce individual fecundity and fitness. Recently, however, Camperio Ciani, Corna, and Capiluppi (Proceedings of the Royal Society of London, Series B: Biological Sciences, 271, 2217-2221, 2004), comparing the family trees of homosexuals with heterosexuals, reported a significant increase in fecundity in the females related to the homosexual probands from the maternal line but not in those related from the paternal one. This suggested that genetic factors that are partly linked to the X-chromosome and that influence homosexual orientation in males are not selected against because they increase fecundity in female carriers, thus offering a solution to the Darwinian paradox and an explanation of why natural selection does not progressively eliminate homosexuals. Since then, new data have emerged suggesting not only an increase in maternal fecundity but also larger paternal family sizes for homosexuals. These results are partly conflicting and indicate the need for a replication on a wider sample with a larger geographic distribution. This study examined the family trees of 250 male probands, of which 152 were homosexuals. The results confirmed the study of Camperio Ciani et al. (2004). We observed a significant fecundity increase even in primiparous mothers, which was not evident in the previous study. No evidence of increased paternal fecundity was found; thus, our data confirmed a sexually antagonistic inheritance partly linked to the X-chromosome that promotes fecundity in females and a homosexual sexual orientation in males.

Department of General Psychology, University of Padova, via Belzoni 80, Padua, Italy.


Evidence for maternally inherited factors favouring male homosexuality and promoting female fecundity.

Proc Biol Sci. 2004 Nov 7;271(1554):2217-21.

Camperio-Ciani A, Corna F, Capiluppi C.

The Darwinian paradox of male homosexuality in humans is examined, i.e. if male homosexuality has a genetic component and homosexuals reproduce less than heterosexuals, then why is this trait maintained in the population? In a sample of 98 homosexual and 100 heterosexual men and their relatives (a total of over 4600 individuals), we found that female maternal relatives of homosexuals have higher fecundity than female maternal relatives of heterosexuals and that this difference is not found in female paternal relatives. The study confirms previous reports, in particular that homosexuals have more maternal than paternal male homosexual relatives, that homosexual males are more often later-born than first-born and that they have more older brothers than older sisters. We discuss the findings and their implications for current research on male homosexuality.

Department of General Psychology, Università di Padova, via Venezia 8, 35100 Padua, Italy. andrea.camperio@unipd.it

Fraternal birth order - homosexual feelings

Fraternal birth order and ratio of heterosexual/homosexual feelings in women and men.

J Homosex. 2006;51(4):161-74.

McConaghy N, Hadzi-Pavlovic D, Stevens C, Manicavasagar V, Buhrich N, Vollmer-Conna U.

Studies of the 2-3% of persons who identify as homosexual found men but not women had more older brothers than persons who identify as heterosexual.

The present study investigated the birth order in the approximately 20% of men and women who anonymously report some homosexual feelings, few of whom identify as homosexual. The number of older brothers and sisters was investigated in seven cohorts: 319 male twins; and 49, 54, and 61 female and 66, 116, and 50 male medical students. Both women and men who anonymously reported homosexual feelings had a greater mean number of older brothers and sisters than did those who reported no homosexual feelings. The difference was stronger in relation to brothers than sisters. The birth order effect was not related to the strength of the subjects' degree of homosexual compared with heterosexual feelings. Its presence in women could not be accounted for by the widely accepted hypothesis that the birth order effect is due to a maternal immune reaction provoked only by male fetuses. The lack of relationship between the strength of the effect and degree of homosexual feelings in the men and women suggests the influence of birth order on homosexual feelings was not due to a biological, but a social process in the subjects studied. Investigating the neglected significant percentage of predominantly heterosexual men and women who anonymously report some homosexual feelings may aid in understanding the factors influencing sexual orientation, and identity.

University of New South Wales, New South Wales, Australia.

Family size - homosexuality, gender dysphoria

Sibship size, sibling sex ratio, birth order, and parental age in homosexual and nonhomosexual gender dysphorics.

J Nerv Ment Dis. 1992 Jan;180(1):40-7.

Blanchard R, Sheridan PM.

This study investigated whether demographic variables previously reported to differentiate ordinary homosexuals from heterosexuals also differentiate gender-dysphoric homosexuals from gender-dysphoric persons with other sexual orientations. Subjects were outpatients aged 16 and older who were referred by physicians to a specialty clinic for the assessment of gender identity disorders. The subjects were divided into three groups: 204 homosexual women, 193 homosexual men, and 273 nonhomosexual men; the last category included men sexually attracted to females, to both sexes, and to neither sex. Demographic data on patients' families of origin were extracted from their clinical charts and from structured interview protocols. A multiple-range test at the .05 level showed that the homosexual men had significantly more siblings than the homosexual women, who, in turn, had significantly more siblings than the nonhomosexual men. The sibling sex ratio of the homosexual men, 131 brothers per 100 sisters, was significantly higher than the sex ratio of live births for the population as a whole (106 men per 100 women; p = .01); the sibling sex ratios of the other groups did not differ from the expected value. The homosexual men had a significantly later birth order than the nonhomosexual men (p = .004); the homosexual women, who fell in between, did not differ significantly from either male group. There were no between-groups differences in paternal or maternal age at the time of the subject's birth. The results concerning sibling sex ratio and birth order are consistent with previous findings for homosexual men.

Gender Identity Clinic, Clarke Institute of Psychiatry, Toronto, Ontario, Canada.

Sexuality

Female sexual arousal .
Gender dysphoria .
Sexual arousal erotic aggressive stimuli .
Sexual responses - SF, SM, GF, GM .

.. PubMed search ..

Sexual responses - SF, SM, GF, GM

Excerpts from NYT magazine article, What Do Women Want?:

[Chivers] showed the short movie to men and women, straight and gay. To the same subjects, she also showed clips of heterosexual sex, male and female homosexual sex, a man masturbating, a woman masturbating, a chiseled man walking naked on a beach and a well-toned woman doing calisthenics in the nude.

The genitals of the volunteers were connected to plethysmographs — for the men, an apparatus that fits over the penis and gauges its swelling; for the women, a little plastic probe that sits in the vagina and, by bouncing light off the vaginal walls, measures genital blood flow. An engorgement of blood spurs a lubricating process called vaginal transudation: the seeping of moisture through the walls. The participants were also given a keypad so that they could rate how aroused they felt.

The men, on average, responded genitally in what Chivers terms “category specific” ways. Males who identified themselves as straight swelled while gazing at heterosexual or lesbian sex and while watching the masturbating and exercising women. They were mostly unmoved when the screen displayed only men. Gay males were aroused in the opposite categorical pattern.

All was different with the women. No matter what their self-proclaimed sexual orientation, they showed, on the whole, strong and swift genital arousal when the screen offered men with men, women with women and women with men. They responded objectively much more to the exercising woman than to the strolling man, and their blood flow rose quickly — and markedly, though to a lesser degree than during all the human scenes except the footage of the ambling, strapping man — as they watched the apes. And with the women, especially the straight women, mind and genitals seemed scarcely to belong to the same person. The readings from the plethysmograph and the keypad weren’t in much accord. During shots of lesbian coupling, heterosexual women reported less excitement than their vaginas indicated; watching gay men, they reported a great deal less; and viewing heterosexual intercourse, they reported much more. Among the lesbian volunteers, the two readings converged when women appeared on the screen. But when the films featured only men, the lesbians reported less engagement than the plethysmograph recorded. Whether straight or gay, the women claimed almost no arousal whatsoever while staring at the bonobos [a film of bonobos mating].

Richard Lippa, a psychologist at California State University, Fullerton, has employed surveys of thousands of subjects to demonstrate over the past few years that while men with high sex drives report an even more polarized pattern of attraction than most males (to women for heterosexuals and to men for homosexuals), in women the opposite is generally true: the higher the drive, the greater the attraction to both sexes, though this may not be so for lesbians.

. . . a recent study by one of [Chivers'] mentors, Michael Bailey, a sexologist at Northwestern University: while fM.R.I. scans were taken of their brains, gay and straight men were shown pornographic pictures featuring men alone, women alone, men having sex with men and women with women. In straights, brain regions associated with inhibition were not triggered by images of men; in gays, such regions weren’t activated by pictures of women. Inhibition, in Bailey’s experiment, didn’t appear to be an explanation for men’s narrowly focused desires. Early results from a similar Bailey study with female subjects suggest the same absence of suppression. For Chivers, this bolsters the possibility that the distinctions in her data between men and women — including the divergence in women between objective and subjective responses, between body and mind — arise from innate factors rather than forces of culture.

Testosterone, so vital to male libido, appears crucial to females as well, and in drug trials involving postmenopausal women, testosterone patches have increased sexual activity. But worries about a possibly heightened risk of cancer, along with uncertainty about the extent of the treatment’s advantages, have been among the reasons that the approach hasn’t yet been sanctioned by the F.D.A.

(emphasis mine)

Gender and sexual orientation differences in sexual response to sexual activities versus gender of actors in sexual films.

J Pers Soc Psychol. 2007 Dec;93(6):1108-21.

Chivers ML, Seto MC, Blanchard R.

In this study, the authors investigated the hypothesis that women's sexual orientation and sexual responses in the laboratory correlate less highly than do men's because women respond primarily to the sexual activities performed by actors, whereas men respond primarily to the gender of the actors. The participants were 20 homosexual women, 27 heterosexual women, 17 homosexual men, and 27 heterosexual men. The videotaped stimuli included men and women engaging in same-sex intercourse, solitary masturbation, or nude exercise (no sexual activity); human male-female copulation; and animal (bonobo chimpanzee or Pan paniscus) copulation. Genital and subjective sexual arousal were continuously recorded. The genital responses of both sexes were weakest to nude exercise and strongest to intercourse. As predicted, however, actor gender was more important for men than for women, and the level of sexual activity was more important for women than for men. Consistent with this result, women responded genitally to bonobo copulation, whereas men did not. An unexpected result was that homosexual women responded more to nude female targets exercising and masturbating than to nude male targets, whereas heterosexual women responded about the same to both sexes at each activity level.

Law and Mental Health Program, Centre for Addiction and Mental Health, Toronto, Ontario M6J 1H4, Canada. meredith_chivers@camh.net

Bailey JM Chivers ML

Lippa R Hershberger S