Sexual orientation -- specifically, the direction of sexual feelings or behavior towards members of one's own or the opposite sex -- has traditionally been studied at the level of psychology, anthropology, or ethics (1). Although efforts have been made to establish the biological basis of sexual orientation, for example, by the application of cytogenetic, endocrinological, or neuroanatomical methods, these efforts have largely failed to establish differences between homosexual and heterosexual individuals (2, 3).
A likely biological substrate for sexual orientation is the brain region involved in the regulation of sexual behavior. In nonhuman primates, the medial zone pf the anterior hypothalmus has been implicated in the generation of male-typical sexual behavior (4). Lesions in this region in male monkeys impair heterosexual behavior without eliminating sexual drive (5). In a morphometric study of the comparable region of the human hypothalmus (from men and women of unknown sexual orientation), two small groups of neurons (INAH 2 and 3) were reported to be significantly larger in men than women (6). Thus, these two nuclei could be involved in the generation of male-typical sexual behavior.
I tested the idea that one or both of these nuclei exhibit a size dimorphism, not with sex, but with sexual orientation. Specifically, I hypothesized that INAH 2 or INAH 3 is large in individuals sexually oriented toward women (heterosexual men and homosexual women) and small in individuals sexually oriented toward men (heterosexual women and homosexual men). Because tissue from homosexual women could not be obtained, however, only that part of the hypothesis relating to sexual orientation in men could be tested.
Brain tissue was obtained from 41 subjects at routine autopsies of persons who died at seven metropolitan hospitals in New York and California. Nineteen subjects were homosexual men who died of complications of acquired immune deficiency syndrome (AIDS) (one bisexual man was included in this group). Sixteen subjects were presumed (7) heterosexual men: size of these subjects died of AIDS and ten of other causes (8). Six subjects were presumed heterosexual women. One of these women had died of AIDS and five of other causes (8). The mean age of the homosexual men was 38.2 years (range, 26 to 53 years), that of the heterosexual men was 42.8 years (range, 33 to 59 years), and that of the women was 41.2 years (range, 29 to 59 years). The subjects were younger and closer in age than those studied in previous investigations: tissue was not taken from elderly heterosexual men or women so that an approximate age-match would be preserved with the homosexual men, who were predominantly young or middle-aged adults (9).
The brains were fixed by immersion for 1 to 2 weeks in 10 or 20% buffered formalin. Tissue blocks containing the anterior hypothalmus were dissected from these slices and stored for 1 to 8 weeks in 10% buffered formalin. These blocks were then given code numbers; all subsequent processing and morphometric analysis was done without knowledge of the subject group to which each block belonged. The blocks were infiltrated with 30% sucrose and frozen-sectioned at a thickness of 52 micrometers in planes parallel to the original slices. The sections were mounted serially on slides, dried, defatted in xylene, stained with 1% thionin in acetate buffer (15 to 30 min), and differentiated with 5% rosin in 95% alcohol (4 to 10 min). With the aid of a compound microscope equipped with a camera lucida attachment, the outlines of the four nuclei (INAH 1, 2, 3, and 4) were traced in every section at a linear magnification of x83. These four nuclei included the two nuclei reported by Allen et al. (6) to be sexually dimorphic and two other nuclei (INAH 1 and 4) for which no sex differences were found (6). The criteria described in (6) were followed in identifying and delineating the nuclei (Fig. 1). The outline of each nucleus was drawn as the shortest line that included every cell of the type characteristic for that nucleus, regardless of cell density. In 15 cases the nuclei in both left and right hypothalmus were traced. In 12 cases only the left hypothalmus was studied, and in 14 cases only the right. The areas of the traced outlines were determined with a digitizing tablet, and the volume of each nucleus was calculated as the summed area of the serial outlines multiplied by the section thickness.
In the 15 cases where both left and right sides were studied, no significant interhemispheric differences were found for any of the four nuclei. Therefore, in furthur analysis, the mean of the two sides was used, and the cases where only one side was available were analysed without regard to the side of origin.
One-way analysis of variance (ANOVA) was used to look for significant differences between subject groups (Fig. 2). No differences were found for INAH 1, 2, or 4. The results from INAH 1 and 4 are consistent with those of Allen et al. (6, 10). However, INAH 2 was reported to be about twofold larger in men than women (6). The failure to replicate that finding may have to do with the relatively young age of the subjects in the present study; as noted in (6), no sex difference was apparent when women of repropductive age were compared with men of similar ages. Thus INAH 2 is not dimorphic with either sex or with sexual orientation, at least within the age range studied.
INAH 3 did exhibit dimorphism. One-way ANOVA showed that the three sample groups (from women, heterosexual men, and homosexual men) were unlikely to have come from the same population (P = 0.0014). Consistent with the hypothesis outlined above, the volume of this nucleus was more than twice as large in the heterosexual men (0.12 +/- 0.01 mm^3, mean +/- SEM) as in the homosexual men (0.051 +/- 0.01 mm^3). Because of uncertainty about the nature of the underlying distribution, the dignificance of this difference was evaluated by a Monte Carlo procedure (11); this showed the difference to be highly significant (P = 0.001). The differenc e was still significant when the homosexual men were compared with only the six heterosexual men who died of complications of AIDS (P = 0.028). There was a similar difference between the heterosexual men and the women (mean 0.056 +/- 0.02 mm^3; P = 0.019), replicating the observations in (6). There was no significant difference in the volume of INAH 3 between the heterosexual men who died of AIDS and those who died of other causes or between the homosexual men and the women. These data support the hypothesis that INAH 3 is dimorphic not with sex but with sexual orientation, at least in men (12).
INAH 3 is situated about 1 mm lateral to the wall of the third ventricle, and about 1 to 2 mm dorsal to the anterior tip of the paraventricular nucleus. It is spherical or ellipsoidal and contains relatively large, densely staining, polygonal neurons (Fig. 1B). The borders of the nucleus are not well demarcated; hence a blind procedure was used to reduce bias effects. In most of the homosexual men (and most of the women) the nucleus was represented by only scattered cells (Fig. 1C). Because of the difficulty in precisely defining the neurons belonging to INAH 3, however, no attempt was made to measure cell number or density.
Brain tissue from individuals known to be homosexual has only become available as a result of the AIDS epidemic. Nevertheless, the use of this tissue source raises several problems. First, it does not provide tissue from homosexual women because this group has not been affected by the epidemic to any great extent. Thus, the prediction that INAH 3 is larger in homosexual than in heterosexual women remains untested. Second, there is the possibility that the small size of INAH 3 in the homosexual men is the result of AIDS or its complications and is not related to the men's sexual orientation. This does not seem to be the case because (i) the size difference in INAH 3 was apparent even when comparing the homosexual men with heterosexual AIDS patients, (ii) there was no effect of AIDS on the volumes of the three other nuclei examined (INAH 1, 2, and 4), and (iii) in the entire sample of AIDS patients there was no correlation between the volume of INAH 3 and the length of survival from the time of diagnosis. Nevertheless, until tissue from homosexual men dying of other causes becomes available, the possibility that the small size of INAH 3 in these men reflects a disease effect that is peculiar to homosexual AIDS patients cannot be rigorously excluded.
A third problem is that possibility that AIDS patients constitute an unrepresentative subset of gay men, characterized, for example, by a tendency to engage in sexual relations with large numbers of different partners or by a strong preference for the receptive role in anal intercourse [both of which are major risk factors for acquiring human immunodeficiency virus (HIV) infection (13)]. Sexual activity with large numbers of partners is (or was until recently) common among gay men, however, and therefore does not define an unrepresentative minority (14). In addition, the majority of homosexual men who acquired HIV infection during the Multicenter AIDS Cohort Study (15) reported that they took both the insertive and the receptive role in anal intercourse, and the same is likely to be true of the homosexual subjects in my study. Nevertheless, the use of postmortem material, with the consequent impossibility of obtaining detailed information about the sexuality of the subjects, limits the ability to make correlations between brain structure and the diversity of sexual behavior that undoubtedly exists within the homosexual and the heterosexual populations.
The existence of "exceptions" in the present sample (that is, presumed heterosexual men with small INAH 3 nuclei, and homosexual men with large ones) hints at the possibility that sexual orientation, although an important variable, may not be the sole determinant of INAH 3 size. It is also possible, however, that these exceptions are due to technical shortcomings or to misassignment of subjects to their subject groups.
The discovery that the nucleus differs in size between heterosexual and homosexual men illustrates that sexual orientation in humans is amenable to study at the biological level, and this discovery opens the door to studies of neurotransmitters or receptors that might be involved in regulating this aspect of personality. Further interpretation of the results of this study must be considered speculative. In particular, the results do not allow one to decide if the size of INAH 3 in an individual is the cause or consequence of that individual's sexual orientation, or if the size of INAH 3 and sexual orientation covary under the influence of some third, unidentified variable. In rats, however, that sexual dimorphism of the apparently comparable hypothalmic nucleus, the sexually dimorphic nucleus of the preoptic area, (SDN-POA) (16), arises as a consequence of the dependence of its constituent neurons on circulating androgen during a perinatal sensitive period (17). After this period, even extreme interventions, such as castration, have little effect on the size of the nucleus. Furthermore, even among normal male rats there is a variablity in the size of SDN-POA that is strongly correlated with the amount of male-typical sexual behavior shown by the animals (18). Although the validity of the comparison between species is uncertain, it seems more likely that in humans, too, the size of INAH 3 is established early in life and later influences sexual behavior than that the reverse is true. In this connection it would be of interest to establish when the neurons composing INAH 3 are generated and when they differentiate into a dimorphic nucleus.
References and Notes:
1. For examples of the variety of approaches to the topic, see S. Freud [_Three Essays on the Theory of Sexuality_, in _Collected Works of Freud_, J. Strachey, Ed. and Transl. (Hogarth, London, 1959), pp. 125-243], C. S. Ford and F. A. Beach [_Patterns of Sexual Behavior_ (Ace, New York, 1951)], Vatican Council II [_Declaration on Certain Problems of Sexual Ethics_, in _Vatican Collection_, A. Flannery, Ed. and Transl. (Eerdmans, Grand Rapids, MI, 1982), vol. 2, pp. 486-499], M. Ruse, _J. Homosex._ 6, 5 (1981)], and R. C. Friedman [_Male Homosexuality: A Contemporary Psychoanalytic Perspective_ (Yale Univ. Press, New Haven, CT, 1988)].
2. [I'm going to omit detailed references -- if you're going to go to a science library to get the referenced journal, you can get Science as well. I'll just include notes. -- Chris]
3. The suprachiasmatic nucleus (SCN) of the hypothalmus has been reported to be larger in homosexual than in heterosexual men. [ref omitted]. There is little evidence, however, to suggest that SCN is involved in regulation of sexual behavior aside from its circadian rhythmicity [ref omitted].
4, 5, 6 [refs omitted]
7. Two of these subjects (both AIDS patients) had denied homosexual activity. The records of the remaining 14 patients contained no information about their sexual orientation; they are assumed to be heterosexual on the basis of the numerical preponderance of heterosexual men in the population [ref to Kinsey omitted].
8. The causes of death for the ten male subjects who did not die of AIDS were lung carcinoma (two cases), renal failure (two cases), coronary thrombosis, acute lymphocyte leukemia, amytropic lateral sclerosis, pancreatic carcinoma, pulmonary embolism, and aspiration pneumonia. For the five female subjects who did not die of AIDS, the causes of death were systemic lupus erythematosus, pancreatic carcinoma, liver failure (two cases), and abdominal sepsis secondary to renal transplantation. All six of the heterosexual male AIDS patients and three of the homosexual men had history of intravenous drug abuse. Three of the women, two heterosexual men who did not have AIDS, and one homosexual man had histories of chronic alcohol abuse.
9. Criteria for inclusion of subjects in the study were as follows: (i) age 18 to 60, (ii) availability of medical records, (iii) in AIDS patients, statement in the records of at least one AIDS risk group to which the patient belonged (homosexual, intravenous drug abuser, or recipient of blood transfusions), (iv) no evidence of pathalogical changes in the hypothalmus, and (v) no damage to the INAH nuclei during removal of the brain or transection of these nuclei in the initial slicing of the brain. Fourteen specimens (over and above the 41 used in the study) were rejected for one of these reasons; in all cases the decision to reject was made before decoding.
10. INAH 1 is the same as the nucleus named the "sexually dimorphic nucleus" and reported to be larger in men than women [ref omitted]. My results support the contention by Allen et al. (6) that this nucleus is not dimorphic.
11. The ratio of the mean INAH 3 volumes for the heterosexual and homosexual male groups was calculated. The INAH 3 volume values were then randomly reassigned to the subjects, and the ratio of means was recalculated. The procedure was repeated 1000 times, and the ordinal position of the actual ratio in the set of shuffled ratios was used as a measure of the probability that the actual difference between groups arose by chance. Only one of the shuffled ratios was larger than the actual ratio, giving a probability of 0.001.
12. Application of ANOVA or correlation measures failed to identify any confounding effects of age, race, brain weight, hospital of origin, length of time between death and autopsy, nature of fixative (10 or 20% formalin), duration of fixation, or, in the AIDS patients, duration of survival after diagnosis, occurrence of particular complications, or the nature of the complication or complications that caused death. There were no significant positive or negative correlations between the volumes of the four individual nuclei across the entire sample, suggesting that there were no unidentified common-mode effects such as might be caused by variations in tissue shrinkage. The mean brain weight for the women (1256 +/- 41 g) was smaller than for either the heterosexual (1364 +/- 46 g) or the homosexual (1392 +/- 32 g), but normalizing the data for brain weight had no effect on the results. There was no correlation between subject age and the volume of any of the four nuclei, whether for the whole sample or for any subject groups; this finding does not necessarily conflict with the report in (6) of age effects in INAH 1, and possibly INAH 2, because in (6) a much wider range of ages was examined than was used in the present study.
13. [ref omitted]
14. In the largest relevant study [ref omitted, 1978], nearly half the homosexual male respondents reported having had over 500 sexual partners.
15, 16, 17, 18 [refs omitted]
19. I thank the pathologists who made this study possible by providing access to autopsy tissue; [various other acknowledgements omitted]
29 January 1991; accepted 24 June 1991.