Information about the birth order,
handedness, and sexual orientation study published
in the April 2007 issue of the Archives of Sexual Behavior
The complete reference for the article is:
Blanchard, R., & Lippa, R. A. (2007). Birth Order, Sibling Sex Ratio, Handedness, and Sexual Orientation of Male and Female Participants in a BBC Internet Research Project. Archives of Sexual Behavior, 36, 163-176.
Abstract:
This study investigated the relations among sexual orientation, fraternal birth order (number of older brothers), and hand-preference. The participants were 87,798 men and 71,981 women who took part in a Web-based research project sponsored by the British Broadcasting Corporation (BBC). The results yielded some evidence confirming prior findings that non-right-handedness is associated with homosexuality in men and women, that older brothers increase the odds of homosexuality in men, and that the effect of older brothers on sexual orientation is limited to right-handed men. The evidence was weaker than in previous studies, however, probably because the usual relations among the variables of interest were partially obscured by the effects of other factors. Thus, the homosexual men and women had higher rates of non-right-handedness than their heterosexual counterparts, but the strongest handedness finding for both sexes was a marked tendency for participants who described themselves as ambidextrous also to describe themselves as bisexual. The birth order data were strongly affected by a tendency for the male participants to report an excess of older sisters, and the female participants to report an excess of older brothers. Statistical analyses confirmed that this was an artifact of the parental stopping rule, “Continue having children until you have offspring of both sexes.” In subsequent analyses, participants were divided into those who did and did not have younger siblings, on the grounds that the data of the former would be less contaminated by the stopping rule. In the former subsample, the right-handed homo/bisexual males showed the typical high ratio of older brothers to older sisters, whereas the non-right-handed homo/bisexual males did not.
A brief description of the study:
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 a new study published in the Archives of Sexual Behavior, Canadian psychologist Ray Blanchard and California psychologist Richard Lippa extended recent research on handedness, birth order, and sexual orientation by analyzing data from over 200,000 men and women who completed a survey posted on the BBC Science and Nature website. Survey participants reported 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:
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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%). |
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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. |
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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 “parental stopping rule.” After controlling for this effect, Blanchard and Lippa 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. |
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Left-handed men did not show the fraternal birth order effect. However, they showed some evidence of a “family size effect”: Both additional brothers and sisters—i.e., larger family sizes—increased their odds of being gay or bisexual. |
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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. |
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 fetuses. Because male tissue is “foreign” in a woman’s body, a mother’s immune system may “remember” male fetuses, and this chemical memory may then influence the development of subsequent male fetuses.
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.
Some results from the BBC Internet survey were presented in the 2005 BBC documentary, Secrets of the Sexes. Social scientists who participated in the BBC research project continue to analyze the data.
A more detailed report of Blanchard and Lippa’s findings can be found in: Blanchard, R. and Lippa, R. A. (2007). Birth Order, Sibling Sex Ratio, Handedness, and Sexual Orientation of Male and Female Participants in a BBC Internet Research Project. Archives of Sexual Behavior, xx, xx-xx.
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. Richard Lippa is a Professor of Psychology at California State University, Fullerton.
This article appeared in a special section of the April 2007 issue of the Archives of Sexual Behavior, guest edited by Richard Lippa, devoted to studies that analyzed data from the 2005 BBC Internet survey. Results from this survey were presented in the BBC One documentary, Secrets of the Sexes. About a quarter of a million people across the world responded to the BBC survey, which investigated psychological sex differences--how men and women differ in their cognitive abilities, personality traits, sexual attitudes and behavior, mate preferences, and attitudes. For further information about the BBC studies and their results, see the BBC Science and Nature website.
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