Eric Vilain: Professor of Human Genetics, Pediatrics and Urology Director, Center for Gender-Based Biology, Chief, Medical Genetics – Department of Pediatrics, David Geffen School of Medicine at UCLA

Human sexual orientation, one’s preference for male or female sexual partners, is a largely stable behavioral trait with a significant genetic component. It is a highly sexually dimorphic trait, and as such an interesting model for brain sexual differentiation.

There is much uncertainty about the factors that shape sexual orientation. Although many studies have helped to explain the experience of lesbian, gay, and bisexual (LGB) individuals, empirically supported explanations for the development of an LGB identity are lacking.

Often the media hypes the results of biological research on sexual orientation. Yet, the reports on biological findings are filtered through the journalists’ understanding of the science and at times are misrepresented. Further, parties who are both for and against the rights of sexual minorities reinterpret these media reports. This can make it difficult to sort fact from fiction.

The purpose of this presentation is to provide an overview on what the biological research has shown in regards to sexual orientation, to assess the current state of the research, and to highlight strengths/limitations of this research. To accomplish this, four main areas will be considered.

Hormones: Research has investigated whether prenatal hormones affect sexual orientation. Although hormones have an impact on behavior, only gross hormonal variations seem to affect sexual orientation.
Anatomical associations: Despite media attention, evidence that non-heterosexual individuals have distinctive anatomical features is far from conclusive. In regards to the anatomical association studies, only non-right handedness and specific anatomical features of the hypothalamus have a consistent relationship with a non-heterosexual orientation.
Birth order: A consistent finding is that having an older brother increases the odd of male homosexuality. Differing theoretical viewpoints on this issue will be discussed.
Genetics: Finally, the most promising evidence for biological influences comes from recent advances in the field of genetics. The validity of studies using “family trees” and the potential role of the expression of genes on sexual orientation will be discussed. Finally, twin studies will be reviewed, as well our own epigenetic data on twin pairs discordant for sexual orientation.