Parents raising children rely heavily on related helpers (“alloparents”), who are mainly the mother’s kin rather than the father’s. This “matrilateral bias” is cross-culturally ubiquitous and requires explanation. The dominant evolutionary view is that it serves as a defense against investing resources in unrelated children whose paternity has been misattributed. The evolutionary model proposed here includes paternity uncertainty as one component of the explanation, but adds additional elements derived from the insight that alloparenting constitutes an investment in the mother as well as in her children, raising her subsequent capacity to invest in other relatives. This model lays a foundation for future research on how the receipt of alloparental help changes the lives of mothers.