Twins and virtual twins: bases of relative body weight revisited

OBJECTIVE: Application of a new investigative strategy to assess genetic and environmental influences on relative body weight. DESIGN: Covariance structure analysis of body mass index (BMI) using genetically informative samples. SUBJECTS: One-hundred and fourteen monozygotic (MZ) twin pairs (age 5.04-22.93y), 81 dizygotic (DZ) twin pairs (age 6.05 -16.52 y), and 98 virtual twin (VT) pairs, ie same-age unrelated siblings (3.68 -54.75 y). MEASUREMENTS: Height, weight, BMI variously obtained by direct measurement and self-report. RESULTS: In contrast with most previous studies, significant common environmental influence on BMI was observed. CONCLUSION: Much past research may have underestimated common environmental effects on BMI because the designs lacked the power or ability to detect them.