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

Although this study does not directly support the hypothesis that prenatal testosterone levels are higher in lesbians, it could be interpreted as being consistent with a prenatal stress hypothesis, since prenatal stress is associated with smaller fetal size (Bogin, 1988). Additionally, two of the effects of testosterone in males are lower birth weight and reduced skeletal maturity in comparison to females (Bogin, 1988), so the evidence of smaller size in lesbian females is consistent with the hypothesis that they may have been exposed to higher levels of androgens than their heterosexual twins.

Some of the factors (including differences in testosterone level) which are associated with intrauterine stress are also associated with dermatoglyphic asymmetry in humans (Hall, in press; Sorenson-Jamison, Meier, & Campbell, 1993) and captive primates (Newell-Morris, Fahenbruch, & Sackett, 1989). While the females in this study did not exhibit any significant asymmetry in ridge count, it should be noted that the males in this study did (Hall, 1999, in press), and asymmetry in finger ridge count has been reported for homosexual males in another unrelated study (Hall & Kimura, 1994). Another interesting difference between male and female twins in this study is that males showed no differences in total finger ridge count, suggesting that there were no size differences during the second trimester. Presumably, these differing results are related to a differing etiology of sexual orientation in males and females. This is why female results are being reported separately from male results.

CONCLUSION

This pilot study was designed to test the hypothesis that prenatal factors influence the etiology of sexual orientation in females. There were no statistically significant differences in total finger ridge count for the female twins concordant for sexual orientation. Thus, for the twins concordant for sexual orientation, the null hypothesis that there are no differences in dermatoglyphics between twins cannot be rejected. However, for twins discordant for sexual orientation there were significant differences in TFRC, resulting in a rejection of the null hypothesis.

Because monozygotic twins presumably have identical genes, any dermatoglyphic differences between them must be due to differences in prenatal environmental experiences. The only time during an individual's life that dermatoglyphic traits can be modified is the prenatal period, and the cause of this modification must be the intrauterine environment in which the individual develops. Thus, the existence of significant dermatoglyphic differences among female twins discordant for sexual orientation indicates that the prenatal environment of one twin was not identical to the prenatal environment of the other twin, despite the fact that both developed simultaneously within the same mother.

Although this was a small study, the differences between lesbian and heterosexual twins were remarkably consistent, suggesting that there is a common prenatal environmental factor influencing sexual orientation in this group of twins. The nature of that factor remains unknown, although differences in ridge count are most likely due to differences in fetal size. While it may not be appropriate to extrapolate the results of this study to lesbians in general (and due to small sample size results should be interpreted carefully), the results suggest interesting directions for future research.