That’s the question that the approximately 2% to 3% of us (according to recent estimates) on the planet want to know. Science wants to know the reasons of heterosexual and homosexual development, too, and some intriguing answers are found in Gay, Straight, and the Reason Why: The Science of Sexual Orientation by neuroscientist Simon LeVay.
In the nature vs. nurture debate over sexuality and other gender traits, nature seems to be winning the battle, and science is employing a variety of experiments and research to show how sex hormone levels during fetal development, genetics, and brain systems determine whether we develop as straight, gay or bi.
Dr. LeVay doesn’t necessarily break new ground here. What he does is provide an excellent round-up of the most up-to-date scientific theories and experiments, and then adds his own expert analysis.
Among the findings presented in the book:
• There is no actual evidence that family dynamics, learning, early sexual experiences or free choice play a role in the development of an individual’s sexuality.
• Testosterone plays the leading role in the sexual differentiation of the brain, and research points to testosterone levels during fetal development as having an impact on the development of an individual’s sexuality.
• Environmental factors that impact biological factors (such as prenatal stress, which alters testosterone levels) could have an impact on gender traits and sexuality in human beings. (They do in laboratory animals.)
• Other hormones, and the chromosomal sex of brain cells of the fetus, also play a role in determining sexuality.
• Estimates of heritability of homosexuality range from 30% to 50%, similar to heritability for many other psychological traits.
• Sexual orientation is linked to other gendered traits, and gay people express both gender-shifted traits and gender-typical traits.
The findings LeVay shares concerning the effect of hormones on fetal development were popularly introduced back in the 90′s when Brain Sex: The Real Difference Between Men and Women was published. I read that book, and even gave a copy to my niece and her new husband as a wedding gift. (I know. I’m weird!)
Perhaps most importantly, LeVay presents the additional questions that need further study. This line of scientific inquiry has been going on for some decades now, and yet is still in its infancy. There is much more to discover.