The Older Brother Effect

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For decades, scientists have been trying to figure out which biological factors determine sexuality. One of the more inexplicable discoveries in this arena is that homosexuality is more common in men who have older brothers, known as the “older brother effect”. Up until now, it’s been unclear why that is.

New research from Brock University in Canada suggests that women who give birth to boys multiple times progressively build up antibodies that will affect their future sons in utero. The researchers argue that these antibodies, formed in response to proteins present in male brains, may lead to changes in brain development that influence sexual orientation.

“It seems that some women during their first male pregnancy, or just after their first male birth, begin to detect this foreign substance (the NLGN4Y protein) and start to develop an immune response. And then later, with further male pregnancies, the high levels of antibodies directed toward this substance may change brain development in these later born males,” says lead researcher Tony Bogaert in a media release.

While the study has yet to be replicated by another research team, it suggests that the older brother effect is rooted in prenatal factors. In other words, this is yet more evidence that sexual orientation has a biological basis, a point that has long been contested by anti-gay types.

“The implications of this study, especially if and when it is replicated by an independent team, are profound,” explains Bogaert. “Along with more deeply understanding the exact origin of the older brother effect, it helps solidify the idea that, at least in men, there’s a strong biological basis to sexual orientation. “This is the culmination of more than 20 years of research where we started looking at the older brother, or fraternal birth order, effect. The current study adds to the growing scientific consensus that homosexuality is not a choice, but rather an innate predisposition.”

There’s no single factor determining whether a person is gay or not, but research like this continues to support the theory that sexual orientation is at the very least influenced by biological factors. Considering that anti-gay groups still push the notion that being gay is a choice that can be fixed with harmful conversion therapy and the like, this area of research is truly valuable.

Comments 24

  1. One of the theories I’ve heard in regards to this involvement was originated from the idea that “gays” themselves had a copper deficiency. When this was proven wrong/inconclusive -this then became the mother is copper deficient. When this became wrong/inconclusive – it became “factors we don’t know about. Let’s throw in muliple children”.

    Part of this issue however is that the majority of women have multiple children. Essentially this means that the majority of individuals have siblings as opposed to not.

    As such arguing this phenomenon as being a biological function is more naturally arguably as the monty hall problem – from a logical perspective.

  2. The huge flaw of this study is that it totally doesn’t work for gay females, or twins, so these antibodies could very well be a result or something else rather than a cause. The study was also apparently conducted on too few people to be statistically relevant.

    Still, there’s certainly something with this older brother effect and antibodies that require more studies. As far as I know, only the epigenetic marker theory of William Rice (also requiring more confirmation) has no such flaws and hasn’t been disputed since 2012.

    There might be a bit of reticence to explore this field further, it could be a rebirth of bad eugenics if parents could tell if their kid will be gay. On the other hand, these anti-gays are almost always anti-abortion too, so there’s not much they can do about it… ;)
    Free access to the PDF:

    Merry Christmas to all! I just celebrated the 50th birthday of my younger brother yesterday. The poor guy didn’t stand a chance: He had both an elder brother and a gay one at that… ;)

    1. I’m an only child. Let the guys in the lab coats, thick glasses and clipboards explain that!

  3. re: The Older Brother Effect

    What about the half/older brother effect? Not only am I gay, but my half brother who is 4 years older is also gay.

    And then I had a 1-year older sister who was put up for adoption — because my ‘good catholic’ mother wouldn’t have an abortion to correct a “mistake” — would she be gay also?

    1. Many things are wrong with your understanding of the article.

      “What about the half/older brother effect? Not only am I gay, but my half brother who is 4 years older is also gay.”

      In order for the immunological response that seems to partially influence sexual preference in males with older brothers to be relevant in your case:

      A) You should be related to your half-brother through your mother, not your father.

      B) More importantly, he should be the younger brother, not the older (e.g., should have more older brothers himself).

      “And then I had a 1-year older sister who was put up for adoption (…) would she be gay also?”

      No, the title of the study alone (Male Homosexuality and Maternal Immune Responsivity to the Y-Linked Protein NLGN4Y) indicates that the observed phenomenon is only seen in Y-linked inheritance (father to son).

      1. Dan:
        “Many things are wrong with your understanding of the article.”
        Actually, it’s you who completely misunderstood my reply (with a tiny bit of sarcasm thrown in for fun).

        “In order for the immunological response that seems to partially influence sexual preference in males with older brothers to be relevant in your case:”
        That’s not even a proper sentence with either/both a specific subject and/or verb.

        “A) You should be related to your half-brother through your mother, not your father.”
        I am. That’s why I asked the question in the first place.

        “B) More importantly, he should be the younger brother, not the older (e.g., should have more older brothers himself).”
        Except that he’s the OLDER brother by 4 years. And, of course, he doesn’t have any OLDER brothers because he’s the oldest, And he’s just as gay as I am.

        “‘And then I had a 1-year older sister who was put up for adoption (…) would she be gay also?'”
        One should see [have seen] that is clearly an exploratory question because of the 3 offspring, she is the middle one with no knowledge by me if she is either homosexual or bisexual … or in fact, heterosexual. If they have done such research to determine the “brother syndrome,” it would stand to reason that [most likely] they are also doing peripherally studies regarding older/younger sisters in relation to an older brother. Generally, most scientific studies will also look into the “other roads that intersect with the primary study.”

        1. Generally, people should bother to read and understand a scientific article before they make jokes about it.

          I like how you quote yourself though. Looks real pro.

  4. This study also bears fruit to what I’ve been saying for a very long time in here and other forums:

    A [any] society’s make up will have no less than 15%, and easily up to 20% (if not more) homosexuals within said society. But obviously, because of the necessity of hiding one’s true sexual nature [i.e., the “closet”] no census or poll will necessarily bear this out — although, recently, even Iran! recently recognized that within their “closed” society, 17% disclosed their homosexual tendencies.

  5. I have to disagree.

    You seem to be drawing your own conclusions with no evidence to back them up. The research does not suggest anywhere that their findings “should” indicate a higher percent of homosexuals than those reported in the polls, much less a “minimum” like you indicate.

    As a matter of fact, the article that summarizes the findings states the exact opposite:

    ” However, the psychologist cautions that the effects are modest and the likelihood of a male child being born gay is still small — even if they have multiple male siblings. “The vast majority of men with older brothers are still heterosexual, but it says something very broad about sex and gender development.” ”

    Sure, polls do seem affected by cultural and political contexts (nationally, the US has 3% aprox. homosexual population[1], while cities like San Francisco have up to 15%[2]). But this is far from objective, empirical and replicable evidence that there must be a minimum of homosexuals in any given society (closeted or not).



    1. Dan:

      You post as if you actually know and understand science and politics. But it’s clear that you really don’t know jack shit about either — especially when both are intertwined and have been throughout human history.

      And the proof of what I just said is in your comments — you neither know how to read other comments properly nor do you know how to understand and accept obvious political information (propaganda):

      “nationally, the US has 3% aprox. homosexual population”

      That statement is just fucking laughable. Anyone with even a 7th grade education in the truth would know and agree with me. That number comes from the religious-controlled polls that are, in reality, PURE BULLSHIT. The religious put out these numbers because they NEED THESE BULLSHIT NUMBERS to continue marginalizing the entire homosexual and bisexual population in this [America’s] society that been hell-bent to be controlled by our over 325,000 churches.

    2. Clearly, by your ignorant replies, you suffer from a lifetime of inadequate learning abilities with obvious arrogance … but with nothing to back it up. You should consider suing your parents — for total inompetence in raising offspring. You obviously don’t possess any natural curiosity whatsoever — the necessary “ingredient” to learning anything.

      But, hey, thanks for proving me correct in my replies.

  6. I worry about “them” finding a biological source for homosexuality. If they can identify a gay gene, or a gay protein, or a gay anything, they can find a way to “cure” it in vitro with some kind of intervention.

    Don’t kid yourself – there’s nothing haters would like more than to find a genuine physical reason for gayness. They can fix that. But if it’s just a lifestyle choice, there’s no cure for that and they know it. And we’re all entitled to live as we CHOOSE – the pursuit of happiness is in the eye of the beholder.

    1. Exactly my thought as I was reading this. I despise conversion therapy and those who advocate it – but I truly believe if science finds a gene that enables homosexuality they, some anyway, would also work hard to destroy that very thing.

  7. Another théorie du jour. Evidently women also produce more males during times of war, so perhaps their bodies do have some unknown control over their fetuses. Personally, I think there may be many reasons why some of us are gay and some of us are not quite, occasionally, dyed in the wool, or would rather be burned at the stake. But why should it matter? Why do we owe anyone an explanation. Why not direct all this energy at finding out why others get so bent out of shape over it. (Those two boys are delicious!)

  8. I am my mother’s third son and “queer as a three dollar bill.” My straight adopted son is third of five brothers. The youngest two of this group have had sexual identity issues. I know of two other multiple male sibling groups that have gay members. I feel that this is only part of the story. The brain, probably at birth, decides our overall sexual preference, however our bodies react to stimulus and don’t care with whom it’s have sex. An example of this might be two convicts who live for years as a couple and then upon their respective releases return to their heterosexual lifestyles. According to Dr. Kinsey a large percentage of boys experiment with homosexual behavior at the onset of puberty. Sex is a complicated subject.

  9. gayness being biological wont mean anything to antigay types. they will simply see it as an “inborn defect”. nature or choice, for them it will always be a problem.

  10. @Dan
    🎅🏻My bad… Thanks for the correction & Happy Holidays!

  11. I’m a gay male with one older sibling, an older brother. So this theory fits me, and I think it’s quite interesting, but it reveals one factor among many (though what is important here is that it is a biological factor) which influence sexual orientation.

    I don’t think we will ever be able to point to any one thing and say that is what causes homosexuality, but studies like this one certainly help establish the fact that biology plays a role and bigots can no longer deny that. It is not behaviour to be corrected or punished. It is natural and healthy.

    1. This is what I was trying to get through “Dan” but he doesn’t possess enough basic intelligence to understand the diversity of biology, regardless of what he posts.

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