Over the past several decades, there has been considerable interest inthe theoretical causes of work–family conflict (WFC). Most studies havefocused on situational determinants, often ignoring the role of personalfactors such as disposition and heritable elements. We increase understandingof person versus situation influences on WFC through estimation of therelationship between role demands and WFC after controlling for geneticconfounding, measured personality traits, family confounds, and other stabledispositions. Based on twin data from the National Survey of MidlifeDevelopment in the United States (MIDUS), we examine the role of geneticfactors in explaining variation in WFC (both work interference with family[WIF] and family interference with work [FIW]). Results support WFC has anadditive genetic component, accounting for 31% [95% CI 18%, 45%] and 16% [95%CI 2%, 30%] of the variance in WIF and FIW, respectively. In addition, wetest two competing hypotheses with regard to the relationship between roledemands and WFC. Results support the phenotypic causal relationship for WIF,consistent with the notion the relationship between work demands and WIFreflect situational processes. However, results support the geneticconfounding hypothesis for FIW, indicating observed relationships betweenfamily demands and FIW are primarily due to genetic factors. Our resultsprovide new insights into the nature of WFC relationships and underscore thatignoring the influence of heritability can bias estimates of role demandeffects in WFC research.