A new study by Andrew Francis of Emory University in the Journal of Sex Research casts doubt on both the fraternal birth-order effect and reparative drive theory.
Here is the abstract:
Using a nationally representative sample of young adults, I identify the family-demographic correlates of sexual orientation in men and women. Hence, I test the maternal immune hypothesis, which posits that the only biodemographic correlate of male homosexuality is the number of older brothers, and there are no biodemographic correlates of female homosexuality. For men, I find that having one older brother does not raise the likelihood of homosexuality. Although having multiple older brothers has a positive coefficient, it is not significant. Moreover, having any older sisters lowers the likelihood of homosexual or bisexual identity. For women, I find that having an older brother or having any sisters decreases the likelihood of homosexuality. Family structure, ethnicity, and education are also significantly correlated with male and female sexual orientation. Therefore, the maternal immune hypothesis cannot explain the entire pattern of family-demographic correlates. The findings are consistent with either biological or social theories of sexual orientation.
The sample is large and the measures of sexual orientation, while brief, cover behavior and attractions. Here is more on the sampling:
I use the National Longitudinal Study of Adolescent Health (Add Health), a nationally representative study of adolescent health in the United States (Udry, 2003). Adolescents in grades 7 through 12 were initially interviewed in 1995 and 1996 (Waves I and II) and were reinterviewed in 2001 (Wave III). The sample size of male respondents is about 5,000, and the sample size of female respondents is about 5,600. Table 1 displays summary statistics. At Wave III, all respondents in the sample were 18 years old or older. About 88% were between the ages of 20 and 24.
Sexual orientation was assessed with this item:
‘‘Choose the description that best fits how you think about yourself: 100% heterosexual; mostly heterosexual, but somewhat attracted to people of your own sex; bisexual, that is, attracted to men and women equally; mostly homosexual, but somewhat attracted to people of the opposite sex; or 100% homosexual.’’
As noted, the theory that the likelihood of any homosexuality is enhanced via more older brothers is not supported by this large, representative sample. In addition to a look at older brothers, Francis also examined other family demographics. Although unrelated to homosexual behavior or attraction, having older sisters was associated with a slightly reduced identification as less than 100% heterosexual. This finding contrasted with the 2002 Bearman and Bruckner study which found an elevation in homosexual romantic attraction for fraternal twin males with a female twin.
For females, Francis found that having siblings decreased slightly the likelihood of most same-sex outcomes. None of the correlates predict sexual orientation well. In every case, the size of the differences were trivial. With large samples, one does not need a large difference between groups to attain statistical significance.
Francis also examined family structure and found more trivial associations. For instance, he found a 3.8% increase in the likelihood of ever having a same-sex sexual partner among those who did not live with either parent. In contrast to reparative theory expectations, he reported that identifying as less than 100% heterosexual for males was associated with living with only dad. No romantic attraction or same-sex behavior was reported for males living with only mother.
There were other factors which Francis reported but the real take home point from this study is how little any of these variables predict sexual orientation. This study undermines reparative drive theory due to the unremarkable performance of the parental variables to predict orientation. One would expect to find great differences between male heterosexual participants and same-sex attracted participants if fathering/mothering were crucial to male sexual orientation as Joe Nicolosi teaches. In fact in this YouTube video, Nicolosi says that the main factor in the development of male homosexuality is a distant or hostile father.
The Francis article finds very little predictive power in family dynamics of any kind. There is no predictive power at all for those whose parents are separated. Living with dad should insulate against a homosexual outcome and living with mom alone should enhance the likelihood of same-sex attraction and/or behavior. In this sample, it does not.