White matter microstructure in transmasculine and cisgender adolescents: A multiparametric and multivariate study.

Adolescence is a sensitive developmental period for neural sex/gender differentiation. The present study used multiparametric mapping to better characterize adolescent white matter (WM) microstructure. WM microstructure was investigated using diffusion tensor indices (fractional anisotropy; mean, radial, and axial diffusivity [AD]) and quantitative T1 relaxometry (T1) in hormone therapy naïve adolescent cisgender girls, cisgender boys, and transgender boys (i.e., assigned female at birth and diagnosed with gender dysphoria). Diffusion indices were first analyzed for group differences using tract-based spatial statistics, which revealed a group difference in AD. Thus, two multiparametric and multivariate analyses assessed AD in conjunction with T1 relaxation time, and with respect to developmental proxy variables (i.e., age, serum estradiol, pubertal development, sexual attraction) thought to be relevant to adolescent brain development. The multivariate analyses showed a shared pattern between AD and T1 such that higher AD was associated with longer T1, and AD and T1 strongly related to all five developmental variables in cisgender boys (10 significant correlations, r range: 0.21-0.73). There were fewer significant correlations between the brain and developmental variables in cisgender girls (three correlations, r range: -0.54-0.54) and transgender boys (two correlations, r range: -0.59-0.77). Specifically, AD related to direction of sexual attraction (i.e., gynephilia, androphilia) in all groups, and T1 related to estradiol inversely in cisgender boys compared with transgender boys. These brain patterns may be indicative of less myelination and tissue density in cisgender boys, which corroborates other reports of protracted WM development in cisgender boys. Further, these findings highlight the importance of considering developmental trajectory when assessing the subtleties of neural structure associated with variations in sex, gender, and sexual attraction.

Patient, Family, Caregiver, and Community Engagement in Research: A Sensibility Evaluation of a Novel Infographic and Planning Guide.

Background: Engaging patients, families, caregivers, and the community (PFCCs) throughout the research cycle ensures that research is meaningful for the target population. Although tools have been developed to promote PFCC engagement, many are lengthy, complex, and lack recommended behaviours. This study evaluated the sensibility of an infographic and accompanying planning guide for facilitating engagement of PFCCs in research. Methods: Thirteen rehabilitation researchers reviewed the PFCC engagement tool and planning guide, participated in a semi-structured interview, and completed a 10-item sensibility questionnaire. Interviews were transcribed, imported into NVivo, and analyzed using direct content analysis. Median scores and proportions of responses for each of the 10 items in the questionnaire were calculated. Results: Median scores for all questionnaire items were ≥ 4 on a 7-point Likert Scale. Participants reported the tool was easy to navigate, contained relevant items to promote PFCC engagement, and followed a logical sequence. Suggested modifications of the tool related to formatting, design, and changing the title. Conclusions: The tool was deemed sensible for overt format, purpose and framework, face and content validity, and ease of usage and provides guidance to engage PFCCs across the research cycle. Further studies are recommended to assess the effectiveness of the tool to engage PFCCs in research.

Historique: la mobilisation des patients, des familles, des proches et de la communauté (PFPC) tout au long du cycle de la recherche permet de garantir que celle-ci soit significative pour la population ciblée. Même si des outils ont été mis au point pour promouvoir la mobilité des PFPC, bon nombre sont longs, complexes et dénués d'indications sur les comportements recommandés. La présente étude visait à évaluer la sensibilité d'une infographie et du guide de planification qui l'accompagne pour favoriser la mobilisation des PFPC en recherche. Méthodologie: au total, 13 chercheurs en réadaptation ont examiné l'outil de mobilisation des PFPC et le guide de planification, ont participé à une entrevue semi-structurée et ont rempli un questionnaire de sensibilité en dix points. Les entrevues ont été transcrites, importées dans NVivo et évaluées au moyen d'une analyse directe de contenu. Les scores médians et les proportions des réponses ont été calculés pour chacun des dix points du questionnaire. Résultats: les scores médians à tous les points du questionnaire étaient d'au moins 4 sur une échelle de Likert en sept points. Les participants ont indiqué que l'outil était facile à parcourir, contenait des points appropriés pour favoriser la mobilisation des PFPC et respectait une séquence logique. Les modifications proposées portaient sur le formatage, le graphisme et le changement de titre. Conclusions: l'outil était considéré comme sensible pour son caractère explicite, son objectif et sa structure, sa validité apparente et de contenu et sa facilité d'utilisation. De plus, il contenait des conseils pour mobiliser les PFPC tout au long du cycle de recherche. II est recommandé de réaliser d'autres études pour évaluer l'efficacité de l'outil à mobiliser les PFPC en recherche.

Recalled Childhood Separation Anxiety in Relation to Sex, Sexual Orientation, and Gender Identity in Thailand.

Same-sex sexual attraction is an evolutionary paradox. It is influenced by genes but also associated with reduced reproduction. An altruistic disposition toward kin could increase relatives' reproduction, thus addressing this paradox. Developmentally, such a disposition may manifest as anxiety during separation from kin. This hypothesis has been supported among androphilic (i.e. sexually attracted to males) males, but few similar studies of gynephilic (i.e. sexually attracted to females) females exist. We examined recalled childhood separation anxiety in Thailand and employed the largest and most sexually and gender-diverse sample to date (N = 1403). Among heterosexuals, men recalled less concern for parents' and siblings' well-being during separation than women. Androphilic males evidenced elevated concern for parents' and siblings' well-being during separation, and transfeminine androphilic males known as sao praphet song reported heightened anxiety when separated from parents. Among females, transmasculine gynephilic females known as toms recalled less concern about parents' well-being during separation than feminine females who are attracted to toms and known as dees. Bisexual women recalled less anxiety when separated from parents than dees and lesbian and heterosexual women. These findings suggest a disposition toward heightened kin-directed altruism is evident in childhood among androphilic males, but not gynephilic females.

Not Who You Are, But Who You Are With: Re-examining Women's Less Satisfying Sexual Debuts.

Gender differences in appraisals of first intercourse are among the largest in sexuality research, with women indicating less satisfying "sexual debuts" than men. Dispositional or "actor-level" explanations for this gender gap are pervasive, yet research has largely examined heterosexual debuts in which actor gender and partner gender are confounded. We assessed whether women's less satisfying sexual debuts are better explained by actor gender or partner gender, comparing experiences of women who debuted with men (WDM) with those of men and women who debuted with women (MDW, WDW). Retrospective accounts of sexual debut were collected from 3033 adults. At first intercourse, we found that WDW had equal physical and emotional satisfaction to MDW, and more satisfaction than WDM, suggesting satisfaction gaps owing to partner gender, not actor gender. This pattern did not extend to a comparison event (first masturbation), where WDW and WDM had similar satisfaction, but less satisfaction than MDW, suggesting an actor gender gap. To identify sources of satisfaction gaps, we probed for corresponding differences in the circumstances of sexual debut. Sexual circumstances were more strongly implicated than nonsexual ones, with relative deprivation of glans stimulation explaining relative dissatisfaction at first intercourse, but not first masturbation, and orgasm explaining it at both. Findings challenge the view that the satisfaction gap at first intercourse reflects an inherent difference between genders. Indeed, they demonstrate similarities when partner gender does not differ and suggest strategies for ensuring equal sexual satisfaction-and equal sexual rights realization-at (hetero) sexual debut.

Cortical Structure Differences in Relation to Age, Sexual Attractions, and Gender Dysphoria in Adolescents: An Examination of Mean Diffusivity and T1 Relaxation Time.

Recent research found that the combination of masculine gender identity and gynephilia was associated with cortical T1 relaxation time, which is considered to reflect gray matter density. We hypothesized that mean diffusivity (MD), a diffusion tensor imaging metric that reflects the degree to which water movement is free versus constrained, in combination with T1 relaxation time would provide further insight regarding cortical tissue characteristics. MD and T1 relaxation time were measured in 76 cortical regions in 15 adolescents assigned female at birth who experience gender dysphoria (GD AFAB) and were not receiving hormone therapy, 17 cisgender girls, and 14 cisgender boys (ages 12-17 years). Sexual orientation was represented by the degree of androphilia-gynephilia and the strength of sexual attraction. In multivariate analyses, cortical T1 relaxation time showed a weak but statistically significant positive association with MD across the cortex, suggesting that macromolecule-rich cortical tissue also tends to show water movement that is somewhat more constrained. In further multivariate analyses, in several left frontal, parietal, and temporal regions, the combination of shorter T1 relaxation time and faster MD was associated with older age and greater gynephilia in GD AFAB individuals and cisgender boys and with stronger attractions in cisgender boys only. Thus, for these cortical regions in these groups, older age, gynephilia, and stronger attractions (cisgender boys only) were associated with macromolecule-rich tissue in which water movement was freer-a pattern that some prior research suggests is associated with greater cell density and size. Overall, this study indicates that investigating T1 relaxation time and MD together can further inform how cortical gray matter tissue characteristics relate to age and psychosexuality.

Patterns of Genital and Subjective Sexual Arousal in Cisgender Asexual Men.

Human asexuality has been defined as a lack of sexual attraction to others, although its nature is not well understood. Asexual men's genital and subjective sexual arousal patterns were compared to sexual men's to better understand asexual men's sexual response patterns. Using a penile plethysmograph to measure genital arousal, 20 asexual, 27 heterosexual, and 22 gay cisgender men (M age = 28.28, SD = 9.41) viewed erotic films depicting sexual activity or masturbation, and a subsample engaged in sexual fantasy of their choosing. Questionnaires assessing sexual function and behavior were also completed. Asexual men scored lower on sexual desire and orgasmic function, higher on sexual aversion, and did not differ on overall sexual satisfaction. Compared with gay and heterosexual men, asexual men demonstrated lower genital and subjective sexual arousal to the erotic films but displayed similar sexual arousal when engaging in sexual fantasy. Asexual men's lower levels of sexual excitation rather than their higher levels of sexual inhibition were associated with lower responses to the erotic films. These findings suggest asexual men have preferred sexual stimuli that differ from sexual men and have a similar capacity for sexual arousal as sexual men. Collectively these findings add to a growing literature aiming to understand the nature of asexuality.

Carving the Biodevelopment of Same-Sex Sexual Orientation at Its Joints.

Sexual orientation is a core aspect of human experience and understanding its development is fundamental to psychology as a scientific discipline. Biological perspectives have played an important role in uncovering the processes that contribute to sexual orientation development. Research in this field has relied on a variety of populations, including community, clinical, and cross-cultural samples, and has commonly focused on female gynephilia (i.e., female sexual attraction to adult females) and male androphilia (i.e., male sexual attraction to adult males). Genetic, hormonal, and immunological processes all appear to influence sexual orientation. Consistent with biological perspectives, there are sexual orientation differences in brain development and evidence indicates that similar biological influences apply across cultures. An outstanding question in the field is whether the hypothesized biological influences are all part of the same process or represent different developmental pathways leading to same-sex sexual orientation. Some studies indicate that same-sex sexually oriented people can be divided into subgroups who likely experienced different biological influences. Consideration of gender expression in addition to sexual orientation might help delineate such subgroups. Thus, future research on the possible existence of such subgroups could prove to be valuable for uncovering the biological development of sexual orientation. Recommendations for such future research are discussed.

Inter-Network Brain Functional Connectivity in Adolescents Assigned Female at Birth Who Experience Gender Dysphoria.

Gender dysphoria (GD) is characterized by distress due to an incongruence between experienced gender and sex assigned at birth. Brain functional connectivity in adolescents who experience GD may be associated with experienced gender (vs. assigned sex) and/or brain networks implicated in own-body perception. Furthermore, sexual orientation may be related to brain functional organization given commonalities in developmental mechanisms proposed to underpin GD and same-sex attractions. Here, we applied group independent component analysis to resting-state functional magnetic resonance imaging (rs-fMRI) BOLD timeseries data to estimate inter-network (i.e., between independent components) timeseries correlations, representing functional connectivity, in 17 GD adolescents assigned female at birth (AFAB) not receiving gender-affirming hormone therapy, 17 cisgender girls, and 15 cisgender boys (ages 12-17 years). Sexual orientation was represented by degree of androphilia-gynephilia and sexual attractions strength. Multivariate partial least squares analyses found that functional connectivity differed among cisgender boys, cisgender girls, and GD AFAB, with the largest difference between cisgender boys and GD AFAB. Regarding sexual orientation and age, the brain's intrinsic functional organization of GD AFAB was both similar to and different from cisgender girls, and both differed from cisgender boys. The pattern of group differences and the networks involved aligned with the hypothesis that brain functional organization is different among GD AFAB (vs. cisgender) adolescents, and certain aspects of this organization relate to brain areas implicated in own-body perception and self-referential thinking. Overall, brain functional organization of GD AFAB was generally more similar to that of cisgender girls than cisgender boys.

The Risks and Benefits of Being "Early to Bed": Toward a Broader Understanding of Age at Sexual Debut and Sexual Health in Adulthood.

BACKGROUND: A younger age at sexual intercourse has frequently been linked to adverse sexual health outcomes. Yet, little is known about its associations with healthy sexual function, and less still about the timing of pre-coital sexual debuts and adult sexual health. AIM: We examined associations between an earlier sexual debut and subsequent sexual health, using broad operationalizations of each that capture pre-coital experiences and positive outcomes. Connections to sexual health risk and healthy sexual function were assessed through the lens of the dual-control model of sexual response. METHODS: Data on age at first sexual intercourse, first sexual contact, first sexual stimulation, and first orgasm, as well as sexual health risk and healthy sexual function were gathered from 3,139 adults. OUTCOMES: Adverse sexual events (reproductive illness, infection, or injury affecting sexual activity; pregnancy termination and/or loss; non-volitional sex) and current sexual difficulties (Female [FSFI] and/or Male Sexual Function Index [MSFI] scores; Sexual Excitation and/or Sexual Inhibition Inventory for Women and Men [SESII-W/M] scores). RESULTS: When defined narrowly as first sexual intercourse, earlier sexual debut was associated with adverse sexual events, including non-volitional sex, pregnancy termination and/or loss, and reproductive illness, infection, or injury affecting sexual activity. However, it was also related to healthier sexual function, including less pain during vaginal penetration, better orgasmic functioning, and lower sexual inhibition. When sexual debut was broadened to include pre-coital experiences, earlier sexual contact, like earlier sexual intercourse, was associated with non-volitional sex. However, earlier sexual stimulation and orgasm were unrelated to adverse outcomes. Rather, these related to fewer sexual desire difficulties, and greater sexual excitation. Exploratory mediation analyses revealed later sexual intercourse and orgasm were connected to sexual difficulties through higher sexual inhibition and lower sexual excitation, respectively. CLINICAL IMPLICATIONS: When sexual functioning is impaired, delay of both coital and noncoital debuts may warrant assessment, and sexual excitation and inhibition may be targets for intervention. To facilitate healthy sexual development of young people, non-coital debuts with and without a partner may warrant inclusion in risk management and health promotion strategies, respectively. STRENGTHS & LIMITATIONS: Although this research operationalized sexual debut and sexual health broadly, and examined associations between them, it is limited by its cross-sectional retrospective design and non-clinical convenience sample. CONCLUSION: From a risk-based perspective, earlier sexual intercourse is adversely related to sexual health. Yet, it is also associated with healthy sexual function. Indeed, earlier sexual initiation may confer more benefits than risks when sexual debuts beyond intercourse are considered. Peragine DE, Skorska MN, Maxwell JA, et al. The Risks and Benefits of Being "Early to Bed": Toward a Broader Understanding of Age at Sexual Debut and Sexual Health in Adulthood. J Sex Med 2022;19:1343-1358.

New Data on Birth Order in Homosexual Men and Women and a Reply to Vilsmeier et al. (2021a, 2021b).

The fraternal birth order effect (FBOE) is the repeated finding that older brothers increase the odds of homosexuality in later-born males. It has been our working assumption, based on the majority of previous studies, that a similar FBOE does not occur in females. In an elaborate quantitative review posted last year to a preprint server, Vilsmeier et al. (2021a) concluded that there is no valid evidence for an FBOE in men or women. Ablaza et al. (2022) subsequently published a study of population-level data from the Netherlands with conclusions completely opposite to those of Vilsmeier et al., namely, that there is robust evidence of an FBOE in both men and women. The present research was initially undertaken to refute the assertion of Vilsmeier et al. that there is no proof of an FBOE in men and to investigate how they obtained such a discrepant conclusion. We found evidence that the discrepancy may relate to Vilsmeier et al.'s use of the large and demonstrably unreliable sample published by Frisch and Hviid (2006). After the publication by Ablaza et al., we expanded our article to address their finding of an FBOE in women. We argue that our preferred explanation of the FBOE in men-that it reflects the progressive immunization of some mothers to Y-linked antigen by each succeeding male fetus and the concomitantly increasing effects of anti-male antibody on sexual differentiation in the brain in each succeeding male fetus-could plausibly be extended to female homosexuality.

A Learning Experience? Enjoyment at Sexual Debut and the Gender Gap in Sexual Desire among Emerging Adults.

Gender differences in experience of first intercourse are among the largest in sexuality research, with women recalling less pleasure and satisfaction than men. This "enjoyment gap" has not been considered in explanations of gender differences in sexual desire. Yet, reinforcement and incentive learning feature prominently in recent models of women's sexual desire, and nonhuman animal models demonstrate their impact at sexual debut. We examined whether women's lower sexual desire is explained by their gender or by gendered experience of enjoyment at sexual debut. Emerging adults (N = 838) provided retrospective accounts of physical (orgasm) and affective (satisfaction) enjoyment at (hetero)sexual debut. We replicated gender differences across behavioral, general, and multidimensional measures of trait sexual desire; however, they were contingent on experience and measurement method. When its cognitive multidimensional properties were appreciated, women's sexual desire varied with experience of orgasm at sexual debut and diverged from men's only when orgasm did not occur. Such effects were not observed for satisfaction, nor for men. Nor did effects of a control event - masturbatory debut - extend beyond solitary sexual desire. Findings underscore the importance of orgasm equality, and suggest its absence at sexual debut may play an unacknowledged role in differentiating sexual desire.

Mental rotation task performance in relation to sexual and gender diversity in Thailand.

Neurohormonal theory argues that organizational effects of hormone exposure influence sexual orientation and gender identity, as well as sex differences in visuospatial cognition. This study examined mental rotation task (MRT) performance in a diverse Thai sample (N = 980). Thai culture has several third genders: individuals assigned male at birth (AMAB) who are feminine and attracted to cis men (i.e., sao praphet song); individuals assigned female at birth (AFAB) who are masculine and attracted to feminine individuals (i.e., toms); AFAB individuals who are feminine and attracted to toms (i.e., dees); and sexual orientation categories similar to Western culture (e.g., gay, lesbian, bi). On the MRT, straight cis men outperformed straight cis women. Results were consistent with organizational effects among AMAB individuals, with straight cis men outperforming gay cis men and sao praphet song. Among AFAB individuals, however, only bi and lesbian cis women outperformed dees. Overall, support for neurohormonal theory was limited among AFAB individuals, but MRT performance among AMAB individuals was consistent with organizational effects. This study informs our understanding of visuospatial sex/gender differences and the applicability of neurohormonal theory across cultures.

An anthropometric study of sexual orientation and gender identity in Thailand.

The biodevelopment of psychological sex differentiation is putatively reflected in several anthropometrics. We examined eight anthropometrics in 1404 Thai participants varying in sex, sexual orientation, and gender identity/expression: heterosexual men and women, gay men, lesbian women, bisexual women, sao praphet song (transgender birth-assigned males), toms (transgender birth-assigned females), and dees (birth-assigned females attracted to toms). Exploratory factor analyses indicated the biomarkers should be analyzed independently. Using regressions, in birth-assigned males, less male-typical second-to-fourth digit ratios in the left hand were associated with sexual orientation towards men regardless of gender identity/expression, whereas shorter height and long-bone growth in the arms and legs were more evident among sao praphet song-who are both sexually oriented towards men and markedly feminine. In birth-assigned females, there were no clear sexual orientation effects, but there were possible gender-related effects. Groups of individuals who tend to be more masculine (i.e., toms, lesbians) showed more male-typical patterns on weight and leg length than some groups of individuals who tend to be less masculine (i.e., heterosexual women, dees). Thus, it appears the various anthropometrics inform separate biodevelopmental processes that differentially relate to sexual orientation and gender identity/expression depending on the measure in question as well as birth-assigned sex.

A Multi-Modal MRI Analysis of Cortical Structure in Relation to Gender Dysphoria, Sexual Orientation, and Age in Adolescents.

Gender dysphoria (GD) is characterized by distress due to an incongruence between experienced gender and sex assigned at birth. Sex-differentiated brain regions are hypothesized to reflect the experienced gender in GD and may play a role in sexual orientation development. Magnetic resonance brain images were acquired from 16 GD adolescents assigned female at birth (AFAB) not receiving hormone therapy, 17 cisgender girls, and 14 cisgender boys (ages 12-17 years) to examine three morphological and microstructural gray matter features in 76 brain regions: surface area (SA), cortical thickness (CT), and T1 relaxation time. Sexual orientation was represented by degree of androphilia-gynephilia and sexual attraction strength. Multivariate analyses found that cisgender boys had larger SA than cisgender girls and GD AFAB. Shorter T1, reflecting denser, macromolecule-rich tissue, correlated with older age and stronger gynephilia in cisgender boys and GD AFAB, and with stronger attractions in cisgender boys. Thus, cortical morphometry (mainly SA) was related to sex assigned at birth, but not experienced gender. Effects of experienced gender were found as similarities in correlation patterns in GD AFAB and cisgender boys in age and sexual orientation (mainly T1), indicating the need to consider developmental trajectories and sexual orientation in brain studies of GD.

Facial Attractiveness of Sisters of Thai Gay Men and Sao Praphet Song.

Androphilia refers to sexual attraction toward adult males. Androphilic males' female genetic relatives might offset the fitness cost of androphilia by having elevated numbers of offspring. Increased attractiveness relative to other women may enable these females to marry up the social hierarchy, providing greater access to resources to support more offspring. In Thailand, there is a marked status hierarchy and male androphiles are highly visible - identifying as gay men or as a third gender known as sao praphet song. Thus, a field experiment was conducted in which 208 heterosexual men rated the facial attractiveness of 25 sisters of Thai gay men or sao praphet song and 25 comparison women with no known same-sex attracted relatives. Contrary to the prediction, the comparison group was rated as more attractive than the sisters, casting doubt on hypotheses that posit greater attractiveness of female relatives of male androphiles as a proximate mechanism facilitating the evolutionary maintenance of genes influencing male androphilia. Also, men from rural areas rated the women, regardless of group, as more attractive than men from urban areas, consistent with some prior studies showing rural-urban differences in perceptions of attractiveness.

Direct Reproduction and Sexual Orientation and Gender Diversity in Thailand.

Human same-sex sexual attraction is considered to be an evolutionary paradox. This paradox rests on same-sex attracted individuals having lowered direct reproduction, indicating reduced direct fitness of genes that influence same-sex attraction. Yet, relatively few empirical studies have examined the relation between same-sex sexual attraction and direct reproduction. The present study examined direct reproduction (i.e., number of children produced) in a diverse Thai sample (N = 1623) of heterosexual men and women, gay and bisexual men, and lesbian and bisexual women, as well as unique Thai gender categories: sao praphet song (i.e., feminine androphilic third-gender males), toms (i.e., masculine gynephilic third-gender females), and dees (i.e., feminine females who are sexually attracted to toms). In males, heterosexual men were more likely to have produced children than other groups, especially as they aged, and none of the gay men or sao praphet song reported having children. In females, there was some degree of reproduction in each of the groups considered. On average, heterosexual and bisexual women had similar numbers of children, whereas heterosexual women had significantly more children than lesbian women, toms, and dees. Number of children was significantly positively associated with age among heterosexual women, bisexual women, toms, and dees. With age, the discrepancies in average number of children produced between heterosexual women and each of lesbian women and toms became significantly more pronounced. Discussion focuses on how these findings, in conjunction with those of prior studies, inform our understanding of evolutionary selection pressures related to human same-sex sexuality.

Handedness and Birth Order Among Heterosexual Men, Gay Men, and Sao Praphet Song in Northern Thailand.

Previous research has examined handedness and birth order to inform sexual orientation and gender identity/role expression development; however, sexual orientation and gender identity/role expression have rarely been disentangled to provide a more nuanced perspective. In Thailand, we investigated sexual orientation and gender identity simultaneously via comparison of 282 heterosexual men, 201 gay men, and 178 sao praphet song-i.e., androphilic, markedly feminine males recognized as a "third" gender. Handedness was examined as: extremely left-handed, moderately left-handed, ambidextrous, moderately right-handed, or extremely right-handed. Birth order was examined as numbers of older and younger brothers and sisters, by using Berglin's, fraternal, and sororal indices, and by examining the older brother odds ratio and sibling sex ratio. Compared with heterosexual men, gay men and sao praphet song were more likely to be extremely right-handed. Sao praphet song were also more likely to be extremely left-handed than heterosexual and gay men. Heterosexual men and sao praphet song had later sororal birth order compared with the expected Thai population value, suggesting stopping rules influenced when probands' mothers ceased having children. These findings provide new insights and replicate previous findings in a non-Western sample. Regarding handedness, in males, mechanisms related to extreme right-handedness likely influence the development of androphilia, whereas mechanisms related to both extreme right- and extreme left-handedness likely explain the combination of androphilia and feminine gender identity/role expression. Regarding birth order, similar to the conclusions of some prior research, stopping rules pose a challenge for testing the fraternal birth order effect.

A short review of biological research on the development of sexual orientation.

We review research supporting biological mechanisms in the development of sexual orientation. This research includes studies on neural correlates, prenatal hormones and related physical/behavioral correlates, genetics, and the fraternal birth order effect (FBOE). These studies, taken together, have provided substantial support for biological influences underlying the development of sexual orientation, but questions remain unanswered, including how biological mechanisms may differ in contributing to men's and women's sexual orientation development.

Fraternal Birth Order, Only-Child Status, and Sibling Sex Ratio Related to Sexual Orientation in the Add Health Data: A Re-analysis and Extended Findings.

The fraternal birth order (FBO) effect related to men's sexual orientation refers to the finding that the number of older brothers that a man has increases his chance of being androphilic. The FBO effect has generally been well replicated in diverse samples; one instance of non-replication was by Francis (2008) using Waves I and III of the Add Health data. We attempted to replicate the FBO effect in the Add Health data taking into account family size and other limitations of Francis' (2008) analyses. Also, we examined other sibling characteristics related to the FBO effect: sibling sex ratio and only-child status. We used two subsamples from Waves I (n = 20,745) and IV (n = 15,701) of the Add Health data, consisting of adolescents who were followed longitudinally from 1994 to 1995 until 2008. Wave I data were used to compute numbers of younger and older brothers and sisters from household roster information. Wave IV information about sexual orientation identity was used. Analyses were conducted within men and within women. We found modest support for the FBO effect in men, but not in women, using the older brother odds ratio, logistic regression analyses, and sibling sex ratio, which provided the strongest support for FBO. We found that gynephilic/biphilic women, but not androphilic/biphilic men, were more likely to be only-children compared to androphilic women and gynephilic men, respectively. We discuss limitations of the Add Health data and purported mechanisms for the FBO effect in men and the only-child effect in women.