This paper tests whether parents reinforce or compensate for child endowments. The authors employ birth weight as a proxy for endowments and estimate how the difference in birth weight across siblings impacts specific parental investments, including breastfeeding initiation and duration, well-baby visits, immunizations, preschool attendance, and kindergarten entry age. They also examine whether parental investment in a child is impacted by her siblings’ endowments. Their results indicate that heavier birth weight children receive higher levels of most parental investment than their lower birth weight siblings suggesting that parental investments in infancy and early childhood reinforce differences in endowments. In one exception, they find weak evidence that lower birth weight children enter kindergarten slightly later than their normal birth weight siblings, which could be interpreted as a compensating parental investment. Presence of a low birth weight sibling in the household increases the likelihood of investments such as well-baby visits and immunizations.