Sons and lovers

Why are younger brothers of older sons more likely to be gay?

From a brief report in New Scientist, 1 July:

IF YOU’RE male, having more older brothers makes it more likely you’ll be gay. .. Each older brother increases your chance of being homosexual by about 30 per cent…This statistic has been dogged by the suggestion that it’s due to social rather than biological factors. Sceptics propose that rough-and-tumble play between brothers may lead the younger boys to become gay.

Now Anthony Bogaert, at Brock University in St Catharines in Canada, has largely ruled that out. He looked at a total of 944 homosexual and heterosexual men, including one group raised with non-biological male siblings. Bogaert reasoned that if simply being brought up with a lot of older brothers produced the effect, it shouldn’t matter if they had the same mother or not. In fact, it did matter: only the number of biological older brothers was linked to sexual orientation. This was true even when the biological older brothers lived separately. “It’s pretty strong in suggesting a prenatal origin,” he says.

Here’s the full text and the abstract of Bogaert’s paper in the Proceedings of the National Academy of Sciences.

The older brother observation is supported by several references. Bogaert’s study needs to be replicated, and qualified comments on his methods would be welcome. Meanwhile let’s assume he’s right. The great thing about evolutionary psychology is that anyone can play, so let’s speculate why.

One odd feature of the culture wars is that on homosexuality the usual bias is reversed: homophobes think homosexuality is learnt, liberals that it’s innate. But innate can mean several things.

There are lots of problems with a straight genetic explanation. At first sight a gene for homosexuality looks as suicidal as one for haemophilia, and would be quickly selected out. Hereditary genetic diseases have mercifully low incidences: 1 in 10,000 of the US population suffer from cystic fibrosis, the commonest. (Down’s syndrome has a higher incidence, 1 in 800 births, but it comes from a copying mistake not inheritance). Even cystic fibrosis probably only survives at this incidence because one copy of the gene (carried by 5% of the US population) conveys what used to be a major benefit: protection against typhoid. Better known is the protection against malaria offered by one copy of the gene for sickle-cell anaemia, carried by 1 in 12 African-Americans,

Male homosexuality is certainly much more prevalent than these; surveys vary but 5% is a fair guess. This looks much too high for a deleterious gene. So perhaps it’s an advantageous gene, good for society by providing psychological and religious insights, bonding, and hairdressing? The problem here is that such benefits accrue to the group, and while group selection is theoretically possible, most biologists think it’s unlikely, as it would in practice always be crowded out by individual selection. The genetic theory also has problems dealing with the birth-order effect.

Bogaert himself is careful to assert a prenatal effect not a genetic one. In fact he thinks it’s epigenetic:

A theory of male homosexuality consistent with the present findings is a maternal immune response to succeeding male pregnancies….No direct support exists for a maternal immune response that underlies the fraternal birth-order effect, but various lines of evidence exist in this theory’s favor.

Let’s run with this. In whose genetic interest would be this response? Not the mother’s or the father’s – their Darwinian interest is to have breeding sons. But it could well pay an elder son to have a younger brother who would not be a sexual competitor for females but remain a natural ally in his own reproductive strategy. An ally in what, for example? Hunting, for personal survival and buying sexual favours from women with meat; fighting, to dominate and intimidate rival males; and rape. So Bogaert’s maternal immune response could be the result of a sort of vaccine produced in the womb by male fetuses, and not accidentally.

I have no qualifications on this stuff – don’t blame Bogaert for my armchair theory. But if you don’t think such manipulation is possible, think of the tiny parasite toxoplasma gondii, that makes a rat lose its fear of cats so that it can migrate to its preferred host. And think about this, from Frank Sulloway, the expert on the social effects of birth order:

In the Taiwanese aphid (Pseudoregma alexanderi), offspring exist in two forms, one of which is a soldier caste that defends the other caste from attack.

So much for the stereotype of homosexual as wimp.

Author: James Wimberley

James Wimberley (b. 1946, an Englishman raised in the Channel Islands. three adult children) is a former career international bureaucrat with the Council of Europe in Strasbourg. His main achievements there were the Lisbon Convention on recognition of qualifications and the Kosovo law on school education. He retired in 2006 to a little white house in Andalucia, His first wife Patricia Morris died in 2009 after a long illness. He remarried in 2011. to the former Brazilian TV actress Lu Mendonça. The cat overlords are now three. I suppose I've been invited to join real scholars on the list because my skills, acquired in a decade of technical assistance work in eastern Europe, include being able to ask faux-naïf questions like the exotic Persians and Chinese of eighteenth-century philosophical fiction. So I'm quite comfortable in the role of country-cousin blogger with a European perspective. The other specialised skill I learnt was making toasts with a moral in the course of drunken Caucasian banquets. I'm open to expenses-paid offers to retell Noah the great Armenian and Columbus, the orange, and university reform in Georgia. James Wimberley's occasional publications on the web

13 thoughts on “Sons and lovers”

  1. If the maternal immune response theory is valid, the evolutionary advantage accrues to the mother, and is due to having a robust immune system, which balances the cost of an increased probablity of one of her offspring not reproducing. No direct advantage to anybody due to the homosexuality itself is required, just as RH disease doesn't require that giving birth to a sick baby be beneficial, just a cost of something else that is advantageous.
    Note that a younger brother isn't much of a "competitor" from an evolutionary standpoint, because he shares half your genes, and his reproductive success is a partial substitute for your own.

  2. The trouble with specifically seeking genetic/developmental explanations of homosexuality, per se, is that it leaves aside, as somehow settled or uninteresting, genetic/developmental explanations of sexuality.
    If you think homosexuality is not, in part, genetic, then, do you think sexuality is not, in part, genetic? Ditto, for developmental explanations.
    The notion that sexual attraction, as experienced by self-identified homosexuals, is globally different in its etiology from sexual attraction as experienced by self-identified heterosexuals, seems an odd premise to adopt at the outset of even a speculative investigation.
    It would be more sensible to start by trying to analyze what constitutes sexual attraction/attractiveness and willingness/eagerness to engage in sexual intercourse with a particular individual. Even the most casual survey would suggest that there might be a number of discrete and variable elements.
    Then, we might ask which particular element was involved in the birth order observations.
    We might also begin to ask what combination(s) of discrete elements tend to lead an individual to identify as heterosexual or homosexual, in a culture in which such categories are available, keeping in mind that other cultures and eras seem to have done quite nicely with completely different schemes for categorizing sexual behaviors and identities.
    I would not be so certain that there would be any particular evolutionary pressure against homosexuality. Being handsome is reproductively advantageous, but being handsome is a fortuitous combination of factors, genetic and developmental; the homely are not being weeded out of the human population.
    You offer one notion of how a dominant elder brother might be aided by a less-heterosexually-inclined younger brother. But, in a complex sexuality in a social species, there are probably multiple strategies in play. Highly attractive (handsome, fit) males have the option of seeking out many partners, offering superior genetic fitness to females, but little committment; lesser mortals might offer a single female more committment and support. Highly dominant males might seek to accumulate many wives, while subservient males might choose to serve, and occasionally cuckold, their lord and master.
    Since the reproductive "game" in a social species is very complex, and there are many possible winning strategies, sexuality might have evolved to be quite flexible, and be both developmentally and behaviorally malleable. When monogamy, poligamy and even polyandry have been recorded, when one generation of human family units typically wants 12 children and a few generations later, family units want, at most, two, one has to admire the flexibility implied.

  3. When this was summarized on 60 Minutes a couple of months ago, it included a further twist: the Older Brother Effect is nullified by left-handedness. Somehow.

  4. No. It's too early to play. You need to have a mechanism. You need some sense of causality before you can surmise where the causality came from.
    At this point all there is is a correlation. You don't know which way the correlation works. The hypotheses so far have been along the lines of "my mother made me a homosexual." But it may not be that. It may be that women have a way of recognizing and selectively aborting gay fetuses, but the mechanism for doing that weakens with each boy carried to term. (It may, of course, not be that, either.) The evolutionary mechanism for developing that would be very different from a mechanism that developed encouraging homosexuality in later boys.

  5. Bruce Wilder wrote, "The trouble with specifically seeking genetic/developmental explanations of homosexuality, per se, is that it leaves aside, as somehow settled or uninteresting, genetic/developmental explanations of sexuality."
    What makes you think that anyone doing serious research on these issues views the genetics/developmental issues behind sexuality as "settled or uninteresting"?
    "…the homely are not being weeded out of the human population."
    Cf the theory of sexual selection.

  6. Brett: too strong an immune system is risky, and the threats it has to handle are very numerous, so I don'r buy that part of your objection. Your Haldane/Hamilton altruistic fitness point is stronger. A heterosexual brother is admittedly less of a competitor than an unrelated male, and other things being equal a male should welcome his kid brother's reproductive success. But they are not equal; the pool of available females may be small, and the ally payoff large. We can't go further from our armchairs.
    Bruce: of course it's complex. The facts that some firstborn sons are gay, and some daughters, alone rule out the vaccine theory as a one-shot explanation. But trying to grasp everything at once is an impracticable scientific strategy. Much better to get some hard data – as Bogaert invaluably has done – and worry at them. We now know from his work, not guess, that homosexuality has nothing to do with growing up with older brothers.
    Coommonsense: thanks for the additional data point.
    My wife has come up with the ingenious point that a homosexual younger son could be in his mother's interest, as Paleolithic social security. The problem is that the benefit is personal, but not apparently reproductive.
    I thought for a moment that the findings shed a new light on the account in Genesis 39 of the failure by Potiphar's wife to seduce Joseph, "the son of [Jacob's] old age." Perhaps Joseph just wasn't interested? But Joseph was the first-born son of Rachel, Jacob's second wife (Gen. 35:24).

  7. Let's run with this. In whose genetic interest would be this response? Not the mother's or the father's – their Darwinian interest is to have breeding sons. But it could well pay an elder son to have a younger brother who would not be a sexual competitor for females but remain a natural ally in his own reproductive strategy.
    I favor what I call the Funny Uncle Theory.
    Your own children carry 50% of your genetic material. Your nieces and nephews carry 25% of your genetic material. (These numbers increase slightly if there are additional ancestral relationships.)
    If you're a late-birth-order male in a subsistence environment, your own children will be competing for resources with your older siblings' children, potentially reducing all their chances for success. If instead you invest effort in feeding and protecting your nieces and nephews, you can increase their chances of survival and success, which does mean that a birth-order-linked-homosexuality-gene (which obviously is expressed in the mother, not in the homosexual male) could get spread around.

  8. I know only children who are gay, so I tend to be skeptical that the older-brother effect is a necessary or sufficient cause.
    However, I don't have much doubt that there can be a genetic basis for homosexuality, despite the fact that in a zero-sum evolutionary game it would seem to be bred out.
    Social animals have somewhat different evolutionary strategies than solitary ones. Termites and ants aren't extinct, even though they have undreds of non-reproducing males and females. Scouting behavior in prairie dogs is suicidal from the individual standpoint but incredibly advantageous from the standpoint of colony survival. Humans, too, are social animals whose evolution has depended on cooperation within groups, and the potential for non-reproducing members of the tribe, to serve as protectors or caretakers for other members of the tribe, strikes me as plausibly advantageous.

  9. It's a bit of a stretch to bring in the example of ants and other social insects. Sterile worker ants are so-called "supersisters" (sounds like a good name for a band) that share 75% of their genes, which is more than they would share with any sexually-produced offspring. Hence it is more advantageous for them, from a selfish gene perspective, to look after their sisters than to reproduce.

  10. Point taken – people aren't ants, and obviously operate in very different ways. I just wanted to bring up the idea that evolution isn't always the zero-sum game it's depicted as.

  11. Homosexuality and the number of older brothers and sisters, or, the difference between "significant" and "not significant" is not itself statistically significant

    This paper, "Biological versus nonbiological older brothers and men's sexual orientation," by Anthony Bogaert, appeared recently in the Proceedings of the National Academy of Sciences and was picked up by several news organizations, including Scientifi…

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