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

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.