Dermatoglyphic Analysis of Total Finger Ridge Count in Female Monozygotic Twins Discordant for Sexual Orientation

Journal of Sex Research, Nov, 2000 by Lynn S. Hall

Studies to date suggest that genes play a role in the expression of the sexual orientation trait, but since monozygotic (identical) twins show concordance rates for homosexuality of less than 100%, environmental variables are also important (Bailey & Pillard, 1991; Bailey, Pillard, Neale, & Agyei, 1993; Hershberger, 1997). Relevant environmental variables have been difficult to assess, but could include both prenatal and postnatal events. In order to test the hypothesis that second trimester environmental events contribute to the etiology of sexual orientation, this study utilized a dermatoglyphic model originally designed to test for second trimester fetal size in monozygotic twins discordant for schizophrenia (Bracha, Torrey, Gottesman, Bigelow, & Cunnif, 1992). In the study presented here monozygotic twins concordant and discordant for sexual orientation were assessed for the dermatoglyphic trait total finger ridge count (TFRC).

Dermatoglyphic traits such as finger ridge count develop between the 10th and 17th weeks postconception (Babler, 1991). Thus, they principally reflect events occurring during the second trimester. Dermatoglyphic features are inherited through a polygenic system with individual genes contributing an additive genetic component (Chakraborty, 1991; Schaumann & Alter, 1976). While generally under genetic control, ridge formation is influenced by individual differences in developmental stability (Markow, 1992; Schaumann & Alter, 1976). For example, if a fetus experiences edema, ridge count may increase. Conversely, intrauterine growth retardation may result in a decrease in ridge count; therefore, ridge count can be used as a marker for differences in fetal size (Bracha et al., 1992).

Normally, dermatoglyphic patterns are highly similar among monozygotic twins (Nylander, 1971; Plato, Schwartz, & Wertelecki, 1976; Schaumann & Alter, 1976), among whom total finger ridge counts are usually correlated at 0.96 (Bouchard, Lykken, McGue, Segan, & Tellegan, 1990). However, it has been demonstrated that monozygotic twins will show discordance in total finger ridge count if the prenatal environment of the twins differs significantly (Bracha et al., 1992). Since dermatoglyphic characteristics do not change after the second trimester, dermatoglyphic discordance among presumed genetically identical twins can be used as a marker for prenatal environmental differences.

Since this type of dermatoglyphic model is based on an analysis of genetically identical individuals (monozygotic twins), it provides a marker for prenatal environmental causality independent of genetic causality. When prenatal environment is similar for monozygotic twins they will possess nearly identical finger ridge configuration. This is usually the case. But in a nonuniform prenatal environment, monozygotic twins express different finger ridge configuration. If the concordance or discordance of dermatoglyphic features corresponds to the concordance or discordance of other characteristics, it is possible to conclude that the prenatal environmental differences which caused the differences in dermatoglyphics also contributed to the differences in the correlated phenotype.

This study applies the dermatoglyphic model described above to investigate the hypothesis that the expression of sexual orientation in females is influenced by second trimester prenatal environmental factors. This hypothesis is tested using two groups of female monozygotic twins, those who are discordant for sexual orientation (one homosexual and one heterosexual), and those who are concordant for sexual orientation (both homosexual). According to the null hypothesis, there should be no significant differences in finger ridge count in either group of twins. The expectations based on the model employed are that twins discordant for sexual orientation will be consistently discordant for TFRC, with the homosexual twin exhibiting lower counts than the heterosexual twin.

METHODS

Recruitment and Sampling

This study was conducted using both adult male and female twins concordant and discordant for homosexuality (results for males are reported elsewhere) (Hall, 1999, in press). Heterosexual twins account for the majority of subjects for which dermatoglyphic data has been collected, and the assumptions regarding high intratwin correlations have been made based on a largely heterosexual sample. Homosexuals who were monozygotic (identical) twins, regardless of the sexual orientation of their cotwin, were recruited through newspaper and magazine advertisements, and through the internet. Advertisements were placed in homophile publications in the following areas: New York, Boston/Provincetown, San Francisco, Chicago, Philadelphia, Baltimore, Washington DC, and Los Angeles. Internet advertisements were directed to gay news groups, web sites, and bulletin boards.

When a potential subject inquired about the study, an initial interview was conducted over the telephone to inform the subject of the details of the study, which included information on how data would be collected and what type of questions they would be asked. The interview also included questions about the subject and his/her twin with regard to age, how they chose to identify themselves (e.g., as gay, straight, or bisexual), where both twins lived, and if there were any possible problems associated with their participation in the study. If respondents were interested in continuing, they were asked for permission to contact the cotwin (regardless of the twin's sexual orientation). If both twins agreed to participate, a personal, interview was set up to collect data.