I’m going to start cracking open my folders of pdfs on gay science (poor Nietzsche…). I think I’ll start with one of the largest non-genetic factors contributing to human gayness, what’s been called “The most consistent biodemographic correlate of sexual orientation” (10): big brothers.
I’d like to not simply give dry findings, but give a bit of the history behind them, to both emphasize the case and simply because the progress of science is interesting stuff (to me ;-)). Also I’ll not go tracking down the full text of the older studies (pre-1980), only those I can find online.
As far as I can tell, the first significant published hint of homosexuality having anything to do siblings came in a study by the German Jensch, in the midst of WW2 (1). Earlier findings had been published but this seems to be the first major work, using a rather large group, 2,072 gays (one has to wonder about the likely awful way in which he found so many gays in 1941 Germany). Jensch found a ratio in their family of 114 male sibling live births to 100 female. In the general population, the ratio is about 106 per 100.
A number of smaller studies followed, some showing less and some more of an apparent correlation. Then in a paper entitled Homosexualitat als genetisches Problem (Homosexuality as a genetic problem), which I’m sure is a read every bit as delightful as the title, especially in his native tongue :-). Lang presented another large set of data, and found, in a group of 1,777 gays, a ratio of 126 brothers to 100 sisters (2).
So by the 1960s it’s pretty clear and repeatedly shown that gays have more brothers than the general public. But these studies were missing a great deal of the true picture, as we’ll see.
About this time researchers began looking at a possible effect in birth order. The Slater index is the number of older sibling one has divided by their total number of siblings. Thus, if birth order had no effect, the average gay's Slater index should be 0.50.
In 1962, Slater found the average of his humbly named index was 0.58 for a group of 337 gay men (3). Over the years this study was repeated, with Tsoi et al. in 1977 again finding an index of 0.58 (4), and Hare and Moran in 1979 finding an index of 0.55 (5).
Incidentally, I’ve a couple older brothers and my Slater index is 1, and from a typically large LDS family :-).
So we now have two pieces of the puzzle: gay men 1. have more brothers and 2. are born later in the line of their siblings. But again, something is missing. It will take putting these two pieces together to make the next big step in this process. Still, at this point one should pause, as there are some important implications already.
A couple of popular theories as to why a boy is gay are losing ground here (though you’ll still see them hanging on today). For one, it was assumed that if boys grew up around women, without maleness around, they’d grow up queer, taking on the effeminate attributes they predominately observed. The evidence shows though, a house full of sisters or being an only child is more likely to produce heterosexual men, and it’s brothers that contribute to a boy being gay. Another theory, and one with some data to back it up, was that being gay correlates with an absent father. It does correlate, but likely because being born last makes you more likely than your siblings to grow up with an absent father, say, due to his death or abandonment.
Moving on, it seems Blanchard et al. was one of the first to put the two pieces together and find the connection between older brothers and homosexuality. He published two studies in quick succession (5,6), 1995 and 1996. Each study had two matched groups of homosexuals with heterosexual, 458 subjects. Among his results (my emphasis):
“Logistic regression analysis showed that homosexuality was positively correlated with the proband's number of older brothers but not with older sisters, younger brothers, younger sisters, or parental age at the time of the proband's birth. Each additional older brother increased the odds of homosexuality by 33%.
So the reality was painted into an even smaller corner. It seems the past studies all included sisters in their Slater indexes, and weren’t accounting for birth order in their research on brothers, thus diluting the actual effect in both cases. It is also important to note that this effect is only found for gay men, and a similar effect for women has proven either slight or nonexistent (another important hint that will lead to new hypotheses; as we all know, only women carry homosexual children ;-)).
So what is it about older brothers? Is this a congenital, more nature thing, or a learned, more nurture thing? Many on the anti-gay side promote nurture first, you know, not something God intended, something “reparable”, and something kind of sordid would help too. They already went after our fathers (and still do); now it’s our brother’s turn. Here’s one: maybe those older brothers are molesting their younger siblings and that mental trauma makes the kid addicted to sex with men.
So this, of course, was examined (as, sure, I suppose it should be ruled out, and I was a bit too quick-tempered above :-)).
Skipping a number of studies that repeated the above research reported on by Blanchard, Bogaert compiled a similar study, in 2003 (8). But this time the participants were questioned on their early “same-sex play” and other sexual experience. His paper reports on two separate and large national probability samples (one of 18,876 People and the other of 3,432). He reports:
“fraternal birth order predicted same-sex attraction in men, with each additional older brother increasing the odds of homosexual attraction by an average of 38%. Results also indicated that the fraternal birth order/same-sex attraction relationship in men was independent of sexual behavior, including early same-sex behavior. No sibling characteristics predicted sexual orientation in women. Results suggest experience-based theories (e.g., early same-sex play) of the fraternal birth order effect in men are unlikely to be correct.”
So, they found no correlation between adult sexual orientation and early sex play, related to an older brother or not, but again they found that older brother effect. Still, the question was only winnowed down; it seems it has nothing to do with sexual activity. Instead, could this effect somehow involve something less scandalous, something like early identification or de-identification with older brothers?
At this point a review was published. In 2004, Blanchard conducted a rather large meta-analysis representing 10,143 men in 14 different studies. He also takes into account a fetal birth characteristic, weight (9). In part, he reports:
“ Research on birth order, birth weight, and sexual orientation suggests that the developmental pathway to homosexuality initiated by older brothers operates during prenatal life. Calculations assuming a causal relation between older brothers and sexual orientation have estimated the proportion of homosexual men who owe their sexual orientation to fraternal birth order at 15% in one study and 29% in another.”
FYI: I’m holding back on some of the other material in this paper (and some other papers) as I’ll get to it in posting on the biological hows and whys of this big brother effect within a week. I mainly want to establish the phenomena here.
Anyway, so there’s another effect that seemingly comes with having an older brother, lower birth weight. The male baby weighs less than it would if it had older sisters. This is another hint. There seems to be something that goes on in utero with regards to having an older brother that affects us in an easily measurable way. Is it the same thing that will account for some of our homosexuality? Even if low birth weight comes with older brothers, the likelihood of, say, a post-birth identification or de-identification psychological mechanism behind homosexuality would only be weakened.
Gratefully, such questions were settled to a significant degree by Bogaert, and just this year (2006) (10).
“Four samples of homosexual and heterosexual men (total n 944), including one sample of men raised in nonbiological and blended families (e.g., raised with halfor step-siblings or as adoptees) were studied. Only biological older brothers, and not any other sibling characteristic, including nonbiological older brothers, predicted men’s sexual orientation, regardless of the amount of time reared with these siblings. These results strongly suggest a prenatal origin to the fraternal birthorder effect.”
Note first, this was published in PNAS. Fancy ;-).
So, men raised with older biological brothers were compared to gay men raised as adoptees, or with half- or step brothers. The only sort of brother that seemed to significantly correlate with being gay was the older biological brother, even if the gay man wasn’t raise in his same family. That’s a pretty good bit of work to show that, no, it’s not being raised with older brothers in the home that helps makes one gay. It’s only having the same mother as another male human. It’s sharing a womb, not a room (har har :-)).
Next: the research on why and how this effect exists.
Lastly, occupational habit makes me disclaim: questions are never settled in any science, even with regards to laws established for decades and observed daily. We simply have a really good idea and a heck of a lot of data on this one, more than enough to reasonably count on it. I also want to be clear: this is only one effect among many; it’s just one of the more significant effects. So, if you don’t have older brothers, don’t get all offended; I’m not calling you a hetero ;-).
1. Jensch, K. (1941). "Weiterer Beitrag zur Genealogie der Homosexualitat
[A further contribution to the genealogical study of homosexuality]." Archivfur Psychiatrie und Nervenkrankheiten 112: 679-696.
2. Lang, T. (1960). "Homosexualitat als genetisches Problem [Homosexuality as a genetic problem]." Acta Geneticae Medicae et Gemellologiae 9: 370-381.
3. Slater, E. (1962). "Birth order and maternal age of homosexuals." Lancet: 69-71.
4. Tsoi, W., L. Kok, et al. (1977). "Male transsexualism in Singapore: A description of 56 cases." British Journal of Psychiatry 131: 405-409.
5. Hare, E. and P. Moran (1979). "Parental age and birth order in homosexual patients: A replication of Slater's study." British Journal of Psychiatry 134: 178-182.
6. Blanchard, R., K. Zucher, et al. (1995). "Birth order and sibling sex ratio in homosexual male adolescents and probably prehomosexual feminine boys." Developmental Psychology 31(1): 22-33.
7. Blanchard, R. and A. F. Bogaert (1996). "Homosexuality in men and number of older brothers." American Journal of Psychiatry 153: 27-31.
8. Bogaert, A. F (2003). "Number of older brothers and sexual orientation: New tests and the attraction/behavior distinction in two national probability samples." Journal of Personality and Social Psychology 84(3): 644-652.
9. Blanchard, R. (2004). "Quantitative and theoretical analyses of the relation between older brothers and homosexuality in men." Journal of Theoretical Biology 230: 173-187.
10. Bogaert, A. F. (2006). "Biological versus nonbiological older brothers and men’s sexual orientation." Proceedings of the National Acadamy of Science 103(28): 10771-10774.