A model is present for the truncated trivariate normal distribution that arises in behavior genetic adoption designs that focus on sibling similarity to estimate share d environmental effects. The model estimates that point of truncation and shared environmentality. Both moment and maximum likelihood estimates are obtained numerically. Simulations indicate that the model and the numerical procedure perform well when they are most needed, that is, when shared environmentally is large, truncation is extensive or both. When applied to published data from the Texas Adoption Project, results indicate that the point of truncation is at about 63rd percentile of the family environmental quality distribution (i.e., the bottom 63% is missing) and shared environmentality is about 55%. Implications for current view on the importance of shared environment for child developmental outcomes such as antisocial behavior and IQ are discussed.