New Twin Study shows little Genetic / Utero Hormonal causation of homosexuality

From here:

Twin studies are favorites of mine because of the potential light they throw on the origin of SSA. The latest one (Santtila et al., 2008) is three times larger than any previous one – in fact bigger than all the rest put together. Does it change anything? Quick answer: No. It confirms the best recent studies: genetic factors are minor; non-genetic factors are major. That is, critical idiosyncratic reactions predominate.

The paper’s title was "Potential for homosexual response is prevalent and genetic." This implies to the average reader that homosexuality is perhaps hidden, but very common, and is predominantly genetic. We shall see this is not representative of their best conclusions.

This is the fifth systematically sampled twin study to look at SSA independently in men and women. Of the four previous studies, two were from Australia (Buhrich, Bailey & Martin, 1991, Bailey, Dunne & Martin, 2000 and two from the USA (Hershberger, 1997, Bearman & Bruckner, 2002).

This latest study is from Finland. Using the very centralized records typical of Scandinavian states, they assembled a large genuinely random sample of twins, (6,001 individual females and 3,152 males) primarily for a study on aggression. With that constraint they were permitted only two questions about SSA: "What same-sex sexual contact have you had in the last year?" and (in essence) "If there was no prospect of anyone finding out, and you were sexually propositioned by someone of the same sex you liked, what would be your chances of accepting?"

Before we go further let’s address one small difficulty. Unfortunately, different studies use different measures for SSA. Some ask for total number of partners – this one asked only the frequency of contacts in the last year. Other surveys ask how frequent same-sex fantasy was – this one asked respondents to fantasize (perhaps for the first time) about what sex might be like. The authors then call this "potential homosexuality," but you and I would probably conclude that such a measure is fairly clearly mostly indicating something other than SSA.

It obviously includes bisexual people and casts the net so wide that it could well be testing for something like novelty, curiosity, or sensation-seeking, rather than actual sexual orientation. 32.8% of men and 65.4% of women replied "yes" to that question in contrast to 3.1% of men and 1.2% of women who were actually homosexually active.

The results were:

Genetics Shared Environment Nonshared Environment
Men 27%(2.7-38) 0%(0-18) 73%(62-85)
Women 16%(8.3-24) 0%(0-3.6) 84%(76-91)

Men 37%(12-47) 0%(0-19) 63%(53-73)
Women 46%(32-52) 0%(0-11) 54%(48-60)

Table 1. Relative influence of various factors for the Santtila and Sandnabba (2008) data. Error ranges in parentheses are the 95% (2 sigma) error range.

The table shows the genetic effects are minor rather than predominant, and that non-shared factors (i.e. environmental factors particular to the individual) predominate. Given that the trend of twin study methodology will still be to overestimate the genetic content, we must say (considering the activity figures alone) that genetic effects are weak, and conclude the title of the paper is rather misleading. Even 27% in the above table is a maximum and has only indirect effects.

Twin study researchers usually involve the siblings of twin subjects as much as possible, because they are genetically related to the same degree as fraternal twins. This sibling/twin comparison is very interesting because it tests for any special twin environment. In this case, the siblings were tested along with the identical twins and fraternal twins and the results were meaningless, i.e. they did not yield results compatible with any plausible hypothesis of genetic influence in SSA. Although the authors do not specify exactly what the problem was it must have been severe ("…attempts at fitting univariate and bivariate extended-family scripts for categorical data were not successful….") This would usually be enough to knock such a study on the head, but rather incredibly the authors simply and blithely ignore the siblings for the rest of the paper and use the twins only.

As usual in these studies, family upbringing ("shared environment") was consistent with zero, but I contend again that many family factors are hidden in the "non-shared environment," and highly individualistic to the people concerned.

The results by my calculations do reinforce one conclusion drawn from previous studies. That is if one identical twin, male or female has SSA, the chances are only about 10 percent that the co-twin also has, i.e. identical twins usually differ for SSA.

In spite of my criticisms, some useful points emerge from the study. The sample is probably the least biased so far. The authors believe uterine hormonal influence theories do not hold because they should lead to greater similarly between identical twins, not less similarity as we see above. The final errors are less than in previous studies. Also we see a continuation of an already established trend; the later and better the study, the smaller the detected genetic influence on SSA.

In the meantime one should continue to assume genetic effects on SSA are minor, and other very individualist factors predominate.

Reference List

Bailey, J.M., Dunne, M.P., & Martin, N.G. (2000). Genetic and Environmental influences on sexual orientation and its correlates in an Australian twin sample. Journal of Personality and Social Psychology, 78, 524-536.

Bearman, P.S., & Bruckner, H. (2002). Opposite-sex twins and adolescent same-sex attraction. American Journal of Sociology, 107, 1179-1205.

Buhrich, N., Bailey, J.M., & Martin, N.G. (1991). Sexual orientation, sexual identity, and sex-dimorphic behaviors in male twins. Behavior Genetics, 21, 75-96.

Hershberger, S.L. (1997). A twin registry study of male and female sexual orientation. Journal of Sex Research, 34, 212-222.

Santtila, P., Sandnabba, N.K., Harlaar, N., Varjonen, M., Alanko, K., & von der Pahlen, B. (2008). Potential for homosexual response is prevalent and genetic. Biological Psychology, 77(1), 102-105.

9 Comments on “New Twin Study shows little Genetic / Utero Hormonal causation of homosexuality

  1. Evening Peter,

    me again, with a ragbag of thoughts and comments.

    It’s interesting that both ‘sides’ in the gay debate (excuse the reductionism for brevity’s sake) are so keen on pinning down the genetic component of the origins of homosexuality. It seems to me that it’s not terribly important – or at least that by itself it’s not a secure basis for argument. On the one hand: if homosexuality were found to be solely or largely genetic, it wouldn’t follow that it’s a benign or ‘good’ trait, or that same-sex sex is right. Cystic fibrosis for instance is a genetic condition (hope that’s right…) – yet nobody argues that this makes it good. On the other hand: if homosexuality were found to be largely or solely non-genetic, it wouldn’t follow that it’s a pathology, or that it is always mutable, or that anti-discrimination laws protecting gay people shouldn’t be passed.

    As an aside at this point – it seems strange for some (especially some Christians) to argue that because (on the basis of what’s known at the moment) homosexuality’s origins aren’t solely genetic, there’s no basis for a gay identity or for anti-discrimination laws. But if one argues along such lines, surely one would also have to argue that there should be no anti-discrimination laws relating to religion, marital status, country of origin, or some conditions that come under the ‘disability’ heading (autism would be an example). I haven’t heard anyone advocating the former position, go on to advocate the latter one – but it would be logically consistent to do so.

    I’m not qualified to comment on the specifics of the study cited in the article you quote. I think the words of Francis Collins are notable though. See this link to Ex-gay watch, from May 07:
    His words don’t contradict the study cited above, but he does acknowledge the limits of where the science is at at the moment, and doesn’t seek to use it as a knockdown justification for his position (which both NARTH and some gay activists do).

    Not very coherent I fear, but see how it strikes you…

    in friendship, Blair

  2. I think the argument from those of us who are conservative on this issue is this – The liberal justice issue is based upon the assumption that “gay” is something natural and biological, a normal human variant of existence and (crucially) NOT part of the fallen world. If the underlying basis of this argument can be shown to be false then that in some way helps undermine the theology that is based upon it.

  3. But Peter, the article you quote above does not show that ‘gay’ isn’t “something natural and biological” – unless the thinking was that only traits which are 100% attributable to genes, are “natural and biological”. We’d probably consider handedness or height “natural” traits but (I read) neither is 100% heritable.

    In any case, and at the risk of being repetitive: as I was trying to say in comment #3, the sole fact that a given trait is 100% genetically caused (as cystic fibrosis for example is) does not make it a good trait. And the sole fact that a characteristic isn’t 100% genetically caused (e.g. handedness, height) does not make it a bad or pathological one.

    If that is true then it seems to me that the “liberal justice” argument, and the “conservative” refutation of it that you refer to above, is something of a red herring – or at least inadequate, and that it has no ‘knockdown’ status. We need more than knowledge of to what extent a characteristic is genetically caused, in order to discern whether it is “part of the fallen world”. I guess this would take us back to Scripture and ‘discernment of spirits’ among other things, as well as scientific knowledge (which I’m not trying to discount).

    in friendship, Blair

  4. Clearly being gay is not something you are born to. So to some extent is a choice, not for example as being born as of African decent. As a choice, especially sexual preference, people will tend to judge you based on their moral values.

    If your neighbor likes to hang himself by meat hooks in his garage for pleasure, then there are bound to be people think that it's strange. Should this neighbor no be allowed to go around to your school preaching his lifestyle to grade school kids?

    But if this same neighbor was gay many would say, that yes the children need to learn about alternative lifestyles and how noble a cause it is.

    They would compare his struggles to that of the civil rights movement. Where people are discriminated for just being born the wrong color.

    I personally feel that gays like most other people with sexual preferences, should exercise what they want to do in private. And should not be bothered, nor should they bother other people with it.

    Everyone has sexual preferences, most of us however do not think we are special and feel the need to covert others to our preferences. This is what I find objectionable.

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