A statistical model of dynamic intrafamily investment behavior incorporating endowment heterogeneity is estimated to evaluate alternative estimation procedures that have exploited family and kinship data. These procedures, which place alternative restrictions on the endowment structure and on behavior, include generalized least squares, instrumental-variables, fixed-effects based on the children of sisters, fixed-effects based on siblings, and sibling fixed-effects with instrumental variables. The framework is applied to data on birth outcomes, with focus on the effects of teen-age childbearing net of other maternal behavior. The empirical results imply that the least restrictive statistical formulation, consistent with dynamic behavior and heterogeneity among siblings, fits the data best. All of the estimation procedures that control for a family-specific endowment indicate, however, that the biological effect of having a birth at younger ages is to marginally increase birthweight and to increase fetal growth.