Objective: We studied monozygotic (MZ) twins who were discordant for bulimia nervosa in order to identify environmental factors that could have contributed to the expression of bulimia nervosa in only one member of these genetically identical twin pairs.
Method: Female twin pairs from the Virginia Twin Registry were identified. One twin met DSM-III-R criteria for bulimia nervosa (excluding the frequency and duration criterion) and the co-twin either reported some symptoms of bulimia nervosa but never met full diagnostic criteria (n = 20 pairs) or reported no symptoms of bulimia nervosa (n = 10). We compared the twins on demographic, developmental, personality, and psychiatric dimensions using self- and maternal report.
Results: Affected twins were more likely to report lifetime generalized anxiety disorder and were described by their mothers as more anxious and fearful as children. They also had significantly lower mastery, optimism, and self-esteem, and significantly greater obsessive and compulsive symptoms than their unaffected co-twins. Affected twins recalled greater discord in their families but viewed their parents as more warm toward them than toward their unaffected co-twin.
Discussion: Given that MZ twins are genetically identical and share a common family background, differences between them must be attributable to unique environmental factors. Although it is difficult to disentangle predisposing risk factors from sequelae of psychiatric illness in a retrospective co-twin control design, the affected twins were perceived to have been more anxious as children by their mothers-possibly reflecting a predisposing trait. More extensive interviews are required to determine the nature of environmental events that contribute to the expression of the observed personality, behavioral, anxious, and eating-related traits in the affected twins.