A longitudinal investigation of the relationship between disordered eating attitudes and behaviors and parent-child conflict: a monozygotic twin differences design.

Twin studies suggest 50–60% of the variance in eating disorders is accounted for by genetic factors, with the remaining variance accounted for by nonshared environmental influences (Bulik, Sullivan, & Kendler, 1998; Bulik, Sullivan, Wade, & Kendler, 2000; Kendler et al., 1991; Keski-Rahkonen et al., 2005; Klump, Kaye, & Strober, 2001; Klump, Miller, Keel, McGue, & Iacono, 2001; Wade, Bulik, Neale, & Kendler, 2000). Indeed, a longitudinal study using twins from the Minnesota Twin Family Study (MTFS) (Klump, Burt, McGue, & Iacono, 2007) found nonshared environmental factors accounted for 44–54% of the variance in disordered eating across adolescence. Despite the large percentage of variance accounted for by the nonshared environment, specific nonshared environmental factors remain understudied (Klump, Wonderlich, Lehoux, Lilenfeld, & Bulik, 2002). The most powerful design for identifying nonshared environmental factors is the discordant monozygotic (MZ) twin study. This method compares environmental risk factors in MZ twins who are discordant (i.e., one twin is affected while the other is unaffected) for disordered eating. Discordant MZ twin studies provide an ideal opportunity to study nonshared environmental effects, as reared together MZ twins share 100% of their genes and shared environmental influences. Thus, co-twin discordance must be due to nonshared environmental factors (or measurement error) (Plomin, DeFries, McClearn, & McGuffin, 2001). Two discordant MZ twin studies have examined nonshared environment in eating disorders. In both studies, differential parental treatment was identified as an important nonshared environmental factor for eating pathology. Specifically, Bulik, Wade and Kendler (2001) found MZ twins with bulimia nervosa (BN) reported greater overall family conflict than their co-twins. Given the twins lived in the same household, perceptions of their family environment may differ. Similarly, Wade, Gillespie and Martin (2007) found MZ twins with BN reported higher levels of parent-child conflict than unaffected co-twins. Despite the consistency of results, these studies had key limitations. First, sample sizes in both studies were small (N = 14–20 twin pairs). Small sample sizes limit statistical power and increase the potential for sampling errors, limiting the generalizability of findings. Second, both studies examined cross-sectional data, therefore, it remains unclear whether differential parent-child relationships contribute to twin differences in eating disturbances, or result from twin differences in eating pathology (e.g., a twin’s repeated self-induced vomiting causes increased parent-child conflict). Finally, previous studies relied on retrospective reports in which adult twins were asked about their relationship with their parents during childhood and adolescence. This is problematic, given the decreased reliability of retrospective reports as a result of deterioration of memories, hindsight bias, and recall bias (Hardt & Rutter, 2004). In eating disorders, retrospective reports may be biased toward reporting more negative parent-child interactions as a result of a tendency to attribute current problems to past events (Hardt & Rutter, 2004). The present study sought to extend prior research via a prospective longitudinal MZ twin differences design, examining parent-child conflict as a nonshared environmental influence on disordered eating attitudes and behaviors. Using data from the MTFS (see Klump et al., 2007), we examined MZ twins who reported disordered eating attitudes and behaviors and conflict with parents at ages 11, 14 and 17. We hypothesized MZ twins with greater parent-child conflict would report more disordered eating than their co-twin over time.