Replication of asymmetry of a-b ridge count and behavioral discordance in monozygotic twins.

We have replicated an earlier study (Rose et al., Behav. Genet. 17, 125-140, 1987) relating dermatoglyphic asymmetry to intrapair discordance in a completely new sample of monozygotic (MZ) twins. Consistent results were observed for at least 31 of 37 Minnesota Multiphasic Personality Inventory (MMPI) scales tested in the two samples. Intrapair differences were confirmed to be greater in 28 MZ pairs with asymmetric palmar a-b ridge counts than in 29 MZ pairs with little or no a-b asymmetry for at least 15 scales. We also examined longitudinal stability over a 4 to 5-year period for six MMPI scales and found suggestive evidence in three scales for greater test-retest instability in twin pairs asymmetric for a-b ridge count. The results offer new evidence for our hypothesis that an asymmetric a-b ridge count may identify individuals who are poorly buffered from developmental noise, but analyses of symmetric and asymmetric a-b ridge counts in nontwin subjects are required to extend and test the hypothesis further.