June is the month to celebrate diversity as symbolized by a rainbow. Gay Pride parades were held the last weekend of June, 2017 in New York, San Francisco and where I live in Erie, PA. These events have reminded me of an area of left-handedness research that has not received much public attention. It is an active area of inquiry that spans several decades and has resulted in many published scholarly papers.
Are there prenatal factors that influence the development of a particular sexual orientation? If so, is there a connection to the development of left-handedness? One theory argues that handedness can be shifted from the right to the left side by prenatal exposure to differing levels of androgens, such as testosterone. High levels of testosterone weaken the development of the left hemisphere of the unborn fetus. The right hemisphere becomes preeminent leading to an increased probability of left-handedness. Prenatal fluctuations in androgens may also influence sexual orientation development. If so, there is a link between the emergence of left-handedness and a particular sexual orientation. Specifically, this theory predicts higher rates of left-handedness among gay men and lesbians when compared to heterosexual populations.¹
The decade of the 1990’s produced a number of published studies comparing rates of right- and left-handedness in homosexuals versus heterosexuals. However, few conclusions could be drawn from these studies. Finally, in 2000, a group of researchers conducted a meta-analysis of existing published studies. A meta-analysis is a special type of research project. Data from previously published papers are analyzed to assess support for or against, in this case, a link between variations in rates of left-handedness and sexual orientation. The authors examined published papers with cumulatively 7,000 homosexual and over 16,000 heterosexual participants. They divided these participants into two groups, right-handed and non-right-handed. The non-right-handed group included both left-handers and those participants who showed more mixed handedness patterns. Male homosexuals were 34% more likely to be non-right-handed and female homosexuals were 91% more likely to be non-right-handed than their heterosexual counterparts. These meta-analytic findings supported the androgen exposure theory for female but not for male homosexuals. Lesbians may be over-masculinized, with high levels of testosterone, which then results in higher rates of non-right-handedness. However, gay men are under-masculinized, with low testosterone levels. This state should cause lower not higher rates of non-right-handedness when compared to heterosexual males.
Studies published after the 2000 meta-analysis have reported no differences in rates of left-handedness when heterosexuals and homosexuals are measured and compared. A recent 2017 publication found that lesbians have slightly higher rates of mixed-handedness, sometimes called ambidexterity, when compared to heterosexual women. However, the results for men showed no relationship between sexual orientation and variations in rates of left- versus right-handedness. These researchers concluded that a connection between androgen exposure, sexual orientation and right- versus left-handedness, if one exists, is complex.
The theory that connects testosterone to differential left versus right hemisphere development in the fetus, and then to handedness, gained prominence in the 1980’s. Since that time, hundreds of research papers exploring various aspects of the theory have been published. Testosterone is the major masculinizing hormone. Therefore, this theory can successfully account for testosterone’s role in producing higher rates of left-handedness among males when compared to females. However, it is not successful in predicting variations in rates of left-handedness in persons with different sexual orientations.
¹Read the previous blog post of June 9, 2016 “Is there a season for the left?” for a more detailed background about the theory connecting testosterone to the development of left-handedness.