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Twin structural equation model for social contact (SC), additive genetic effects (A), shared environmental effects (C), and nonshared environmental effects (E) on BMI change in twins 1 and 2 within a pair. MZ, monozygotic twin pair; DZ, dizygotic twin pair; f, frequency of contact between twins by phone, mail, or in person (0, less than yearly contact; 0.25, yearly contact; 0.50, monthly contact; 0.75, weekly contact; 1.0, daily contact).
