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Tuesday, June 21, 2005

Biological Correlation of being gay
Chapter 23. Biological Correlates of being Gay - Biological Determinism?

Scientists are currently uncertain as to whether homosexuality is primarily caused by environmental or genetic factors. The uncertainty arises because much of the available data involves correlations. But the controls have gotten better, so that it is now possible to reject some of the competing models.

Two papers on homosexual behavior in (male) humans made headlines in 1991. The first, authored by Simon LeVay and published in the journal Science (253:1034-1037) described a small anatomical difference between the brains of homosexual and heterosexual men. The second study purported to demonstrate that homosexuality had an appreciable genetic or inherited basis (Bailey & Pillard, Arch. Gen. Psychiatry 48:1089-1096).

The issues underlying this topic are similar to those which underlie an earlier controversy over the inheritance of I.Q. scores. In both cases, interest is in the biological basis of differences in human behavior (intelligence, sexual orientation). And in both cases, science faces similar difficulties: demonstrating the causal basis of observed correlations. LeVay's study demonstrates a possible (anatomical) correlate of homosexuality, but as we will find, further evidence is desirable in concluding that the correlate is truly with respect to sexual orientation; there is no attempt at this stage to investigate even the simple matter of whether the brain difference is a cause of, or merely a correlate of sexual orientation. The second study demonstrates that the probability of homosexual orientation in a male who is the brother of an individual claiming to be homosexual (on a questionnaire) increases with the genetic relatedness of the two brothers. The authors interpret their results as evidence for the inheritance of sexual preference. This study also has problems.

Study 1 (LeVay) A correlation between brain structure and sexual orientation
The data in this paper consist of volume measurements of different regions or "nuclei" in a part of the brain called the hypothalamus. These nuclei are recognized merely as areas of high cell density in sections of the brain, and the boundaries for the nuclei are ambiguous. LeVay studied the volumes of 4 such nuclei: INAH 1, 2, 3, and 4. (INAH stands for "Interstitial Nucleus of the Anterior Hypothalamus".) Previous work by other authors had demonstrated (i) that the anterior hypothalamus is critical in the performance of various male-typical sex behaviors (i.e., that lesions impair heterosexual behavior without impairing sex drive), and (ii) that the volumes of INAH 2 and 3 are larger in males than in females. However, previous work had not looked for possible anatomical or physiological differences between men of hetero- and homosexual orientations.

From this previous work, LeVay formulated the following model/hypothesis:

either or both of INAH 2 & 3 should be larger in individuals whose sexual preference is for females (smaller in individuals whose preference is for males).

His material consisted of post-mortem tissue from 41 subjects:

19 declared homosexual males (1 bisexual; all AIDS deaths); mean age 38.2
16 presumed heterosexual males (6 AIDS deaths); mean age 42.8
6 presumed heterosexual females; mean age 41.2
Slices of the brain were coded, stored, and eventually sectioned. The scoring of INAH volumes was done blindly. The results were as follows (-*- indicates a statistically significant difference in nuclear volume, ~ indicates the absence of a significant difference):

Results for INAH 1, 2, and 4.
No statistically-significant differences were observed among the different groups.

Results for INAH 3.
homosexual male *- heterosexual male
homosexual male *- AIDS heterosexual male
AIDS heterosexual male ~ non-AIDS heterosexual male
female *- heterosexual male
female ~ homosexual male
All significant differences were in the predicted direction. So here is the first (and still the only) study to suggest an anatomical correlate of hetero- versus homosexual behavior.

Caveats and observations offered by LeVay

1) The model makes predictions about INAH volumes in women according to sexual preference, yet this female comparison was not undertaken in the present study. (Material was not available.)
2) The correlations of INAH 3 volume with sexual orientation could reflect confounding factors. One of the most obvious is AIDS, yet
i) there was no correlation between nuclear volume and time since AIDS diagnosis
ii) the correlation between INAH volume and sex was observed even within AIDS victims (a good control)
iii) AIDS apparently did not affect the volumes of the other nuclei
3) Samples may have been biased with respect to behaviors among heterosexual males.
4) INAH volume was not an absolute predictor of orientation, so behavior cannot be strictly correlated with nuclear volume.
5) INAH 2 was not dimorphic, though it was predicted to be.
So this study may give some insights to the biological basis of sexual orientation in humans, but it is only a first step in understanding the biological basis of these behaviors; the paper is in fact extremely modest in what it claims.

Study II (Bailey & Pillard): Inheritance of sexual preference
This study was designed to compare two models:

Model 1: Homosexuality is inherited.
Model 2: The environment a child is raised in determines whether that child will be homosexual or heterosexual.
The manner by which this study compared these models was motivated by previous work which found a high positive correlation between monozygotic (e.g. identical) twins in the incidence of homosexuality. In the present study, the authors looked at the concordance of homosexual behavior in 3 groups of men:

(a) monozygotic or identical twins raised together,
(b) dizygotic or fraternal twins raised together, and
(c) men with adopted brothers.
Each of these groups serves as a control for the other groups.

Model 1 predicts that the concordance of homosexuality should be greatest in group (a) and least in group (c). By contrast, in the absence of an inherited basis for this behavior, we might expect there to be no difference among these groups. Certainly, by restricting all comparisons to "brothers," much of the impact of common environment is removed, but we still might expect some systematic environmental differences among these groups that would partly confound the interpretation.

The design was to advertise in gay publications of the Midwest and Southwest for gay males with male cotwins or adopted brothers. The objective was to determine the sexual orientation of both those who responded to the ad and of their brothers. (It was also necessary to determine what kind of twins they were.) There are various messy aspects to these data (the sexual orientation of some men was assessed only by their twins, the assessment of monozygotic versus dizygotic twins was based on responses to a questionnaire, etc.). The final results are given in Table 11.1. These data are consistent with the model of moderate inheritance of sexual orientation: the heritability calculated from these numbers lies within the range 0.31- 0.74, which is consistent with a moderate to a high genetic determination of the trait (that whether an male becomes gay or heterosexual depends moderately to substantially on his genes).

Table 14.1 Resemblance between relatives in sexual orientation

Relationship
Rate of homosexuality

monozygotic
52% (29 out of 56)

dizygotic
22% (12 out of 54)

adoptive
11% (6 out of 57)

Any pair of these 3 sets of results provides a controlled evaluation. The fact that 3 groups are available provides a level of control that is superior to any of the pairs. The fact that all 3 groups are in relative agreement with each other (under the model of inheritance) adds substantial support to the model.

The authors noted one inconsistent result, with respect to non-twin brothers (not part of the data reported above). The rate of homosexuality was on 9.2% (13/142), significantly lower than the expected rate. (In the absence of environmental factors, the rate for non-twin brothers should be the same as for dizygotic twins.)

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