Handedness, birth order, and sexual orientation
Psychologists Richard A Lippa and Ray Blanchard analysed data from the BBC web experiment, Sex ID. Here, Richard Lippa summarises their findings, which were published in the April 2007 issue of the Archives of Sexual Behavior.
Recent studies have documented that gay men and lesbians are more likely to be left-handed (or ambidextrous) than heterosexuals are. Also, gay men are more likely to have an excess of older brothers, compared to heterosexual men—a phenomenon termed 'the fraternal birth order effect'. The most recent studies that look at both factors at the same time suggest that the fraternal birth order effect is true for right-handed gay men but not for left-handed gay men – that is, older brothers increase a man's odds of being gay only for right-handers.
In the Sex ID study, we asked participants to report their sexual orientation, their degree of right- versus left-handedness, and how many older brothers and sisters they had.
Some of the major findings of the new study:
- More gay men than heterosexual men reported being left-handed (13% versus 11%). Similarly, more lesbian women than heterosexual women reported being left-handed (11% versus 10%).
- Bisexual men and women showed a strong tendency to describe themselves as ambidextrous. For example, 12% of bisexual men reported that they had mixed hand preferences, whereas only 8% of gay men and heterosexual men did. Even more strikingly, 16% of bisexual women reported that they had mixed hand preferences, compared to 12% of lesbians and 8% of heterosexual women.
- The BBC data showed that parents tend to keep having children until they have both a boy and a girl – a phenomenon known as a the 'parental stopping rule'. After controlling for this effect, we found new evidence for the fraternal birth order effect, particularly in right-handed men. Among right-handed men, each additional older brother increased a man's odds of being homosexual or bisexual by 15%. In contrast, older sisters had no effect.
- Left-handed men did not show the fraternal birth order effect. However, they showed some evidence of a 'family size effect' – larger family sizes (more brothers and sisters) increased their odds of being gay or bisexual.
- Gay and bisexual men tended to have more siblings than heterosexual men did. In contrast, bisexual women tended to have fewer siblings than heterosexual and lesbian women. This finding fits in with the results of other recent studies showing that the relatives of gay men may be more fecund than the relatives of heterosexual men.
My colleague Ray Blanchard speculates that the fraternal birth order effect – the finding that older brothers increase the odds that a man will be gay – may result from mothers' immune systems responding to male foetuses. Because male tissue is 'foreign' in a woman's body, a mother's immune system may 'remember' male foetuses, and this chemical memory may then influence the development of subsequent male foetuses.
Explanations for links between handedness and sexual orientation have focused on the effects of sex hormones and 'developmental instability' before birth. According to the sex hormone theory, variations in male hormone levels early in development influence both handedness and sexual orientation. According to developmental instability theory, prenatal factors such as exposure to infectious diseases and environmental chemicals may perturb early brain development, and this can sometimes lead to unusual outcomes, such as left handedness and homosexuality.
Ray Blanchard is a Professor of Psychiatry at the University of Toronto and Head of Clinical Sexology Services in the Law and Mental Health Program at the Centre for Addiction and Mental Health in Toronto.
Lippa is Professor of Psychology at California State University, Fullerton. Further information about his research and his analysis of the BBC data can be found at: http://psych.fullerton.edu/rlippa/index.html.
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