Self-report psychiatric symptoms in twins and their nontwin relatives: are twins different?

While twin studies of psychiatric disorders have been conducted for over 65 years, surprisingly little is known about the comparability of psychiatric symptoms in adult twins and singletons. To address this question, we compared the means and variances of four-factor scores on the self-report Symptom Check List in twins and their relatives from the Virginia 30,000 twin-family study. The four factors were depression, panic-phobia, somatization, and insomnia. Twins had significantly higher scores on the panic-phobia factor than their relatives, by about one eighteenth of a standard deviation, and this was replicated in both subsamples. However, no consistent and significant mean differences between twins and their relatives were detected for the other three symptom factors. While some differences in variance were found between twins and their relatives, in no case were the differences replicated in both subsamples. With the possible exception of modestly elevated scores for panic-phobia, these results suggest that both the level and variability of common psychiatric symptoms reported by twins are similar to those found in the nontwin population.