We examine the relationship between the social organization of communities - their emphasis on unilineal descent - and fertility. In unilineal societies, descent is traced primarily through either the mother's or the father's side, which strongly determines social identity, increases cohesion, and reinforces interdependence within the lineage. Numerous children are valued as reinforcing the position of a continuous lineage, conferring strength and prestige on its members. We show that, across countries, unilineal societies have higher fertility levels and experience slower fertility decline. These results are validated within sub-Saharan Africa using a spatial regression discontinuity design along ancestral ethnic boundaries. Furthermore, we show that the positive influence of unilineal descent on fertility is driven by patrilineal ethnic groups. In terms of mechanisms, we find suggestive evidence that the stronger motive to continue one's lineage, higher marital stability, the prevalence of extended households and polygyny, and the lower bargaining power of women in patrilineal societies drive fertility upward.