A new scientific paper published in Adaptive Human Behavior and Physiology provides evidence that the hormones present during early development in the womb can affect a person’s sexual orientation. These hormones appear to mainly work by influencing gene expression patterns in the developing brain, ultimately influencing whether someone will be attracted to the same or opposite sex.
The question of what influences human sexual orientation has been a topic of scientific inquiry for decades. Researchers have explored various physiological, genetic, and environmental factors to better understand the development of sexual preferences. One particular area of interest is the role of hormones, specifically prenatal (in the womb) and early postnatal (shortly after birth) hormone exposure.
Previous research had yielded inconclusive results on the relationship between hormones and sexual orientation. This study sought to provide a comprehensive examination of the topic, drawing from a wide range of existing literature and conducting new analyses to clarify the role of hormones in determining sexual orientation.
“My co-authors and I are interested in understanding how gonadal hormones influence the development of the human brain, psychology, and behavior. Sexual orientation is an especially useful trait to examine, not only because it influences a fundamental human drive, but also because it is one of the most sexually differentiated psychological traits,” explained David A. Puts, a professor of anthropology and director of the Behavioral Endocrinology and Evolution Lab at Penn State.
The researchers adopted a multifaceted approach to investigate the relationship between hormones and sexual orientation. They began by conducting a thorough review of existing research on the topic, summarizing and synthesizing findings from numerous studies.
To investigate prenatal hormone exposure, the researchers examined a range of biomarkers, including digit ratios (2D:4D), handedness, otoacoustic emissions (OAEs), and auditory evoked potentials. These biomarkers served as possible indicators of early hormone action and were assessed in relation to participants’ sexual orientation.
The researchers also explored cases where individuals were exposed to atypical hormone levels due to medical conditions. For instance, they examined individuals with congenital adrenal hyperplasia (CAH), idiopathic hypogonadotropic hypogonadism (IHH), and polycystic ovary syndrome (PCOS). These conditions provided unique insights into the potential effects of varying hormone levels on sexual orientation.
The researchers also conducted a meta-analysis, a statistical method used in scientific research to combine and analyze the results of multiple individual studies on a specific topic, of 13 different studies that had investigated the relationship between non-heterosexuality and PCOS.
PCOS is a medical condition that affects females. It is characterized by hormonal imbalances, including elevated levels of androgens (hormones such as testosterone produced in higher concentrations in males). PCOS can lead to various health issues, including irregular menstrual cycles, ovarian cysts, and fertility problems.
PCOS is a relatively common condition, and it affects a diverse group of females. By examining PCOS in relation to sexual orientation, researchers could potentially gain insights into whether there is a relationship between hormonal profiles (specifically, elevated androgens) and sexual orientation within a broad and varied population.
Overall, the review and meta-analysis provided evidence that hormone exposure during early development has an influence on who people are attracted to. But the researchers found no consistent evidence to suggest that hormonal concentrations in adulthood could influence sexual orientation. This implies that the development of sexual orientation is primarily influenced by prenatal and early postnatal hormone exposure.
Puts outlined two main takeaways from the research: “First, prenatal or early postnatal testosterone is probably responsible for the vast majority of the variation in human sexual orientation because it accounts for the sex difference. That is, testosterone probably tends to make people attracted to females, so people with high testosterone during early development, usually males, tend to be more attracted to females, whereas people with low testosterone during early development, usually females, tend to be less attracted to females and more attracted to males.”
“Second, we show that gonadal hormones probably influence sexual orientation directly by regulating patterns of gene expression in the developing brain. This is in contrast to the views of many social scientists, who hypothesize that gender-based patterns of socialization are the primary reason that males and females diverge psychologically and behaviorally. Implicit in these accounts is the idea that gonadal hormones influence psychology by affecting the development of external genitalia and as a result, patterns of gender socialization.”
“We show that this is unlikely for sexual orientation — and so for at least some psychological traits — by comparing people raised as the same gender but who vary in their exposure to early sex hormones,” Puts continued. “The clearest evidence was from people who were raised as girls but differed in their early testosterone exposure. Across a variety of endocrine and other conditions, people raised as girls were more likely to be attracted to females if they were exposed to higher levels of testosterone early in development, and the likelihood seemed to increase with the degree of testosterone exposure.”
But the review also sheds light on the role of sex hormones such as estrogen, which play a crucial role in growth and reproductive development of women. In particular, the findings hinted at a potential link between higher estrogen levels in early development and increased attraction to males.
“Most sexual differentiation of the brains and behaviors of laboratory mammals is driven by testosterone from the testes,” Puts said. “However, in monkeys and apes, including humans, it is possible that ovarian hormones such as estrogens also play important roles, but very little is known about this. We reviewed evidence, including some from our lab, that prenatal or early postnatal ovarian estrogens may increase attraction to males. The potential effects of estrogens on the sexual differentiation of brains and behavior in anthropoid primates is a fairly new and understudied topic, and one well worth pursuing by others, in our view.”
While the study provides valuable insights into the potential role of hormones in shaping sexual orientation, the complexity of the topic leaves many questions unanswered.
“Although hormone signaling in the developing brain may explain most of the sex difference in sexual attractions, and hence most of the human variation in sexual orientation, we understand the within-sex variation less well,” Puts said. “Among typically-developing females, what makes some attracted to males and others attracted to females? We showed that a relatively small amount of this within-sex variation may be caused by testosterone or similar hormones.
“For example, women with polycystic ovary syndrome–an endocrine condition associated with elevated prenatal androgens–are more likely to be attracted to females, but this was a fairly weak relationship. But we don’t know what explains most of the within-sex variation. Likewise for males. Differences in early testosterone exposure may explain why some males are attracted to other males, but it appears likely that other, unknown factors are important as well.”
“I’m hoping that the paper can get some broad attention, as it speaks strongly to major questions in the social sciences and helps us understand our sexuality–one of the largest and most salient sources of human diversity,” Puts added.
The study, “Organizational Effects of Gonadal Hormones on Human Sexual Orientation“, was authored by Ashlyn Swift-Gallant, Toe Aung, Kevin Rosenfield, Khytam Dawood, and David Puts.