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

Table 2. Wilcoxon Rank Sum Analysis for Total Finger Ridge Count (TFRC) in Twins Concordant For Sexual Orientation (Control Group)

[X.sub.i]     [Y.sub.i]      Sign of     Difference    Rank of
 TFRC          TFRC        difference   (Xi - Yi)    difference
twin with     twin with
high Klein    low Klein
Grid score    Grid score

100               85                       15            5
 84               79                        5            3
 96              102           -            6            4
 96               93                        3            2
 88               86                        2            1

Note. TFRC: [SR.sub. ] = 11; [SR.sub.-] = 4: T = 4: n = 5; ns.

DISCUSSION

The hypothesis tested in this study (that differences in prenatal environment are associated with differences in sexual orientation) is supported by the results of the TFRC comparison in females. Differences in TFRC are associated most strongly with differences in second trimester fetal size (Bracha et al., 1992; Schaumann & Alter, 1976). Fetal size in twins and singletons during the second trimester may be affected by several factors. In twins, factors which might have resulted in size differences include competition for intrauterine space, a monochorionic placenta which results in competition for nutrients and exogenous hormones, and differential assault by infectious agents (it is possible for one twin in a set to be infected by maternal viruses like influenza, while the other twin remains unaffected). These factors have been shown to result in greater differences in ridge count between cotwins (Arrieta et al., 1991; Bogle, Reed, & Norton, 1994; Bogle, Reed, & Rose, 1994; Bracha et al., 1992; Melnick & Myrianthopoulos, 1979; Sokol et al., 1995).

Since larger size is consistently associated with larger finger pads, which have higher ridge counts, this suggests that either the lesbian twins were subject to events which reduced their size, or the heterosexual cotwins suffered from a condition which would increase finger pad size, such as edema. If the heterosexual females were larger during the second trimester than their homosexual cotwins, this difference need not be reflected in third trimester size or birth weight.

There is some evidence from other studies to support the hypothesis that differences in hormones, including testosterone and other androgens, may be associated with differences in sexual orientation (Dorner & Hinz, 1968; Dorner, Rohde, Stahl, Krell, & Masius, 1975; Money, Schwartz, & Lewis, 1984). It has been suggested that female homosexuals have experienced some degree of masculinization in the auditory system as a result of exposure to prenatal androgens (McFadden & Pasanen, 1998). Excessive exposure to prenatal adrenal hormones, and prenatal exposure to exogenous androgens other than testosterone (e.g., the drug diethylstilbestrol, and the disorder congenital virilizing adrenal hyperplasia), have been shown to increase lesbian behavior (Erhardt et al., 1985; Money et al., 1984). High levels of androgens could be related to intrauterine stress, since stress results in the increased production of androgen-like cortico-steroids (Mauri & Volpe, 1994; Nyirenda & Secki, 1998; Ponirakis, Susman, & Stifter, 1998; Schneider, Roughton, Koehler, & Lubach, 1999; Wadhwa, Duncan-Schetter, Chicz-DeMet, Porto, & Sandman, 1996). Prenatal stress has been proposed as factor in the etiology of sexual orientation (Ellis, Ames, Peckman, & Burke, 1988).