Content area
Full text
This article provides a comprehensive review and critique of biological research on sexual orientation published over the last decade. We cover research investigating (a) the neurohormonal theory of sexual orientation (psychoneuroendocrinology, prenatal stress, cerebral asymmetry, neuroanatomy, otoacoustic emissions, anthropometrics), (b) genetic influences, (c) fraternal birth-order effects, and (d) a putative role for developmental instability. Despite inconsistent results across both studies and traits, some support for the neurohormonal theory is garnered, but mostly in men. Genetic research using family and twin methodologies has produced consistent evidence that genes influence sexual orientation, but molecular research has not yet produced compelling evidence for specific genes. Although it has been well established that older brothers increase the odds of homosexuality in men, the route by which this occurs has not been resolved. We conclude with an examination of the limitations of biological research on sexual orientation, including measurement issues (paper and pencil, cognitive, and psychophysiological), and lack of research on women.
Key Words: birth order, developmental instability, genetic, homosexuality, hormones, phallometry, sexual orientation, vaginometry
It has been over a decade since the first review of the evidence for a biological basis of sexual orientation was published in this journal (Gooren, Fliers, & Courtney, 1990). The focus of that review was on the role of hormones in explaining within-sex variations in sexual orientation. More recently Bailey and Pillard (1995) reviewed the extant data regarding genetic influences on sexual orientation. Since these reviews, there has been a considerable expansion in the evidence supporting the existence of biological influences. Such studies have explored genetic, neuroanatomic, endocrine, and morphological differences based on sexual orientation. Given the accumulation of new data since the last reviews, it seems an appropriate time to evaluate the evidence in this area. The focus of this review is on recent evidence (last 10 years) regarding the putative role of biological factors influencing sexual orientation. When necessary, less contemporary research will be included to contextualize newer evidence. Articles were identified by searching PubMed and PsychInfo for common terms related to sexual orientation (i.e., homosexuality, gay, etc.) along with relevant biological terms (genetic, brain, etc.), allowing only for the retrieval of articles published in the last 10 years. Authors who presented relevant papers at the International Behavioral Development Symposium convened...