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

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.

In order to be included in the study, participants had to meet certain requirements.

1. Both twins had to be alive and agree to participate.

2. Both twins had to be free of any confounding factors which may independently result in discordance in finger ridge count (e.g., discordance for schizophrenia which effects total finger ridge count, or discordance for other factors which are known to influence directional asymmetry such as dyslexia).

3. Twins had to have hands which could be printed and which had ridge counts (in rare cases, when all fingers contain patterns known as arches, or tented arches there is no ridge count).

4. Twins had to meet certain criteria for establishing sexual orientation. They had to be predominantly heterosexual or predominantly homosexual based on Kinsey Scale and Klein Grid categorization.

Data Collection

Sets of twins who agreed to participate in the study were interviewed and asked to fill out several questionnaires about their medical and psychological history, zygosity, sexual orientation, childhood behavior, and handedness. Twins were interviewed in their own cities, usually in their homes, although some asked to be interviewed at work or in a public place, and some came to the interviewer's hotel. All twin data were assigned a code number for the purpose of maintaining confidentiality.

General Questionnaires

A general medical and psychological questionnaire was filled out by each subject. This assessed characteristics like age and general health status, and allowed the researcher to eliminate from analysis any individuals who may have confounding factors which could independently lead to finger ridge discordance (e.g., schizophrenia). This questionnaire also included demographic information so samples could be described and compared. A personal questionnaire similar to those used by Bailey and Pillard (1991) and Hamer, Hu, Magnuson, Hu, and Pattatucci (1993) was also used, and a questionnaire to determine lateral dominance was used to assess handedness (Plato, Fox, & Garruto, 1984).

Zygosity

A questionnaire, developed by Nichols and Bilbro (1966) and Cederlof, Friberg, Johnson, and Kaj (1961), was used to determine zygosity following the procedures used by Bailey and Pillard in their 1991 study (Bailey & Pillard, 1991; Cederlof et al., 1961; Nichols & Bilbro, 1966). The questionnaire to determine zygosity has been estimated to be about 90% to 95% accurate (Cederlof et al., 1961; Lykken, 1978; Martin & Martin, 1975; Ooki, Asaka, Yamuda, Asaka, & Hayakawa, 1990).

Sexual Orientation

While twins were accepted in this study regardless of whether they were discordant or concordant for homosexuality, each twin had to be either predominantly homosexual or predominantly heterosexual. This procedure of sampling from the two extreme ends of the behavioral spectrum is commonly used for analysis of traits displaying some continuous characteristics, and tends to increase the probability of success with analysis (Lander & Shork, 1994). Several measures used to assess sexual orientation were utilized in this study to make it possible to more accurately categorize subjects.

Sexual orientation was assessed in three ways. Subjects were initially asked during the telephone interview how they identified themselves. Only subjects who identified as gay/lesbian/homosexual or straight/heterosexual were enrolled in the study. Once enrolled in the study two other measures, the Kinsey Scale (Kinsey, Pomeroy, & Martin, 1948) and the Klein Grid (Klein, Sepekoff, & Wolf, 1985) were used. Subjects filled out all of the questionnaires with the help of the interviewer who explained each of the items and the scales. The subjects were kept in the study and classified as homosexual if they scored 5 and 6 on the Kinsey Scale or between 4 and 6 on the Klein Grid, and as heterosexual if they scored 0 or 1 on the Kinsey Scale and between 0 and 2 on the Klein Grid. No twins were eliminated as a result of scores.

Dermatoglyphics

Fingerprints were taken of the right and left hands of all of the twins according to the techniques described by Hauser (1990) and the American Dermatoglyphics Association (ADA: 1990). A ridge count was ascertained for all fingers following the procedures described by Cummins and Midlo (1943) and the ADA (1990). Finger ridge count is determined by drawing a line from the triradius (a point at which ridges come together forming a triangular configuration) through the center of the finger print pattern and counting each ridge which crosses the line. Total finger ridge count is determined for each hand separately by adding the ridge counts for each finger. Ridge counting is an objective procedure, but it is tedious and counter error can occur. To verify counts, all subjects were counted twice by the investigator, at different times. About one quarter of the subjects were randomly recounted a third time, and a random sample of ten subjects were counted by a graduate student instructed by the investigator. Counts which were off by more than 2 ridges were recounted again until agreement was made. This only happened in a few cases in which it was difficult to determine whether some marks constituted a ridge or cracked skin.

Each participant's prints were assigned a code number, so that during the quantification procedure (ridge count and pattern identification) the identity of the individuals and their relationship with each other was not known. This maintained the anonymity of the subjects, and assured a blind assessment of the data.

Statistical Analysis

After determining the ridge counts for all subjects, the coded data were identified and aggregated according to twin pairs. Statistical analysis was based on the Wilcoxon Rank Sum one-tailed statistic for matched pairs. This test is a non-parametric statistic that does not assume that the samples are normally distributed. It is very robust when used with small sample sizes (Welkowitz, Ewen, & Cohen, 1982). The test takes into account the magnitude and direction of the differences between individuals in the matched pair.

To use this statistic, the data from each twin set have to be separated and placed into two groups, an X group, and a Y group. The Klein Grid and Kinsey Scale scores were the basis for the assignment of twins into either the X or Y category. To provide consistency in the analyses, the twin with the higher Klein Grid Score was assigned the X value, and the twin with the lower score was assigned the corresponding Y value.

In the analysis of twins discordant for sexual orientation (study groups), the homosexual twin was always placed in the X category because they always had the highest Klein grid scores, and the heterosexual cotwin was always placed into the corresponding Y category because they always had the lowest Klein Grid scores. Among twins concordant for sexual orientation (control groups), it was important to place the twins into a category based on an empirical and consistent criteria, since statistically significant results could be artificially obtained as a result of biased categorization. To avoid this problem, twins concordant for sexual orientation were placed in X or Y categories based on the same criteria used for twins discordant for sexual orientation. Therefore, the twin with the higher Klein Grid Score was assigned the X value, and the twin with the lower score was assigned the corresponding Y value. The Klein Grid was used as the primary measure largely because of the difficulty of assigning twins who are concordant for sexual orientation to groups using the same criteria as that used for the twins discordant for sexual orientation. Although all of the cotwins in the control groups were homosexual, the Klein Grid scores are usually slightly different. However, in the event that a set of twins had identical Klein Grid scores, the Kinsey scores were used to determine the group into which the twin was placed.

Wilcoxon tests were computed for total finger ridge count (TFRC). To calculate the Wilcoxon test for TFRC, the sum of all the ridges on the finger tips of both hands was determined for each of the twins.

RESULTS

Initially, 15 sets of twins responded to the advertisements and were interviewed. The sample is described elsewhere (Hall, in press). Table 1 shows the results for TFRC for 7 sets of female twins discordant for sexual orientation (the study group). All of the twins with the higher Klein Grid scores (homosexual females) had lower ridge counts than the cotwins with the lower Klein Grid scores (heterosexual cotwins). The resulting Wilcoxon (T = 0) was statistically significant with an n of 7 (p [is less than or equal to] .01).

Table 1. Wilcoxon Rank Sum Analysis for Total Finger Ridge Count (TFRC) in Twins Discordant For Sexual Orientation (Study Group)

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

 43                61           -            18          3
133               153           -            20          4
142               196           -            54          7
 46                76           -            30          5
 98               135           -            37          6
 60                65           -             5          1.5
162               167           -             5          1.5

Note. TFRC: [SR.sub.+] = 0; [SR.sub.-] = 28; T = 0; n = 7; (p < .01).

Table 2 shows the results for TFRC for 5 sets of female twins concordant for sexual orientation (the control group). In one set, the twin with the higher Klein Grid score had a lower TFRC than the cotwin with the lower Klein Grid score. In the other 4 sets, the twin with higher Klein Grid score had a higher TFRC than the cotwin with the lower Klein Grid score. Although it is possible to get significant results with an n of 5, in this case the resulting Wilcoxon (T = 4) was not statistically significant.

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).

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.

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Manuscript accepted March 16, 2000

This project was funded by a National Science Foundation Dissertation Improvement Award. The author wishes to thank Dr. Charles A. Weitz, Temple University Department of Anthropology, Dr. Jonathan Friedlaender, Temple University Department of Anthropology, Dr. Laurie Tompkins, National Institutes of Health and Dr. Norris Durham, University of Northern Iowa for their input on this project.

Address correspondence to Lynn S. Hall, Ph.D., Brown University, Department of Biology and Medicine, Center for Alcohol and Addiction Studies, Box G-BH, Providence, RI 02906, [e-mail].