A twin registry study of male and female sexual orientation

Journal of Sex Research, Spring, 1997 by Scott L. Hershberger

Biological approaches to the etiology of individual differences in human sexual orientation have received significant empirical support during the last few years. One impetus for this work has been the desire to explain the continual occurrence throughout time and place of homosexuality despite its apparent selective disadvantages (Ruse, 1988). The approaches have been diverse: Support has been obtained from neuroendocrine (LeVay, 1991), molecular genetic (Hamer, Hu, Magnuson, Hu, & Pattatucci, 1993), and behavioral genetic (Bailey & Pillard, 1991) perspectives. Arguably, behavioral genetic studies have provided the strongest evidence to date in affirming the role biology plays in influencing human sexual behavior (Bailey, 1995).

Behavioral genetic studies rely on the relative similarity of family members of differing genetic resemblance to estimate the heritability and environmentality underlying a phenotype. Most commonly, behavioral genetic studies compare the similarity of monozygotic (MZ) and dizygotic (DZ) twins. My primary purpose for this article is to report the results of a twin study of sexual orientation to evaluate the influence of heredity in both men and women.

Table 1 presents the pairwise concordances found for sexual orientation in MZ and same-sex DZ twins from seven of the largest twin studies. Although the magnitude of heritability suggested by the studies varies greatly, most suggest the presence of some genetic influence on sexual orientation. The only exception appears to be King and McDonald (1992). To date, the Bailey, Pillard, Neale, and Agyei (1993) study has been the only published, noncase study of female twin concordance for sexual orientation.

Table 1 Probandwise Concordances From Twin Studies of Sexual Orientation

Study                               MZ(a)     %      DZ(a)     %

Males:
Kallmann, 1952(a),(b)               37/37    100     4/26     15
Heston & Shields, 1968(b)             2/4     50      1/7     14
Buhrich, Bailey, & Martin, 1991      8/17     47      0/3      0
Bailey & Pillard, 1991              29/56     52    12/54     22
King & McDonald, 1993                2/16     13     2/16     13
Whitam, Diamond, & Martin, 1993     22/34     65     4/14     29

Females:
Bailey et al., 1993                 34/71     48     6/37     16

Study                                  Ascertainment

Males:
Kallmann, 1952(a),(b)                Word of mouth
Heston & Shields, 1968(b)            Serial admission
Buhrich, Bailey, & Martin, 1991      Twin registry
Bailey & Pillard, 1991               Advertisement
King & McDonald, 1993                Advertisement
Whitam, Diamond, & Martin, 1993      Advertisement

Females:
Bailey et al., 1993                  Advertisement

Another difference among these studies is the method used to obtain participants, a possible source for the different heritabilities implied by the studies. As the phrase suggests, no attempt is made to obtain a representative sample of gay/lesbian probands when a "word-of-mouth" approach is used. Four sexual orientation twin studies were advertised in homophile publications to request volunteers, a strategy that could lead to nonrandom participation. It is not known how well the characteristics of volunteer participants approximate the characteristics of individuals who do not volunteer for sexual orientation studies and who, further, never have the opportunity of volunteering because they do not read these publications. These readers may be more affluent, more open about their sexual orientation, or more educated. If more open, the co-twins may be more knowledgeable about their twins' sexual orientation, facilitating greater rates of participation by them than for twins who are less knowledgeable concerning their co-twin's sexual orientation.

Two strategies may be used to mitigate but not completely remove bias in obtaining participants. The first strategy, used commonly in studies of psychiatric disorders, is to obtain systematically a diagnosis for each individual passing through a site such as a hospital and to recruit for the study those who are eligible. The individuals who come to the site must be representative, and those who have the disorder and those who do not cannot differ in their rates of participation. Heston and Shields (1968) obtained participants using this strategy, examining serial admissions to a psychiatric hospital for schizophrenia. They did not, of course, systematically sample for sexual orientation. A second strategy is to gather a sample of participants representative of the general population and then query them as to their sexual orientation. If no differences exist in the response rates of individuals who are positive for the phenotype compared to individuals who are not positive, then the results may be generalized to the population represented by the sample. Provided that they truly represent the population they are supposed to, twin registries can be useful here. However, both serial admissions and twin registries as methods of ascertainment suffer if twins who are more alike tend to volunteer for participation more frequently than twins who are less alike. Further, if this effect is stronger for MZs than DZs, spuriously high heritability estimates may be obtained (Kendler & Eaves, 1989; Neale, Eaves, Kendler, & Hewitt, 1989).