A consequence of Mendel’s First Law is that siblings’ genetic relatedness varies randomly (with a mean of 50% and a standard deviation of ∼4%). We use molecular genetic data to compute the genetic relatedness of ∼80,000 sib pairs. We then compare the pairs’ genetic relatedness to their similarity on 15 outcomes in the cognitive and educational, labor market, risk taking, health, and anthropometric domains, to estimate the relative importance of genetic (i.e., heritability) and family environmental influences on each outcome. We find evidence of sizeable genetic influences on risk tolerance, subjective wellbeing, cognitive performance, height, and BMI, and robust evidence of family environmental influences on educational attainment and labor market outcomes.