Can the trade-off between quantity and quality of children explain modern fertility decline?

David W Lawson, London School of Hygiene and Tropical Medicine
Ruth Mace, University College London

Modern industrialised populations lack the strong positive correlations between wealth and fertility that characterise most traditional societies. While modernisation has brought about substantial increases in personal wealth, fertility in many developed countries has plummeted to the lowest levels in recorded human history. These phenomena contradict evolutionary and economic models of the family that assume increasing wealth reduces resource competition between offspring, favouring high fertility norms. Here we review the current evidence behind this assumption and present new data on trade-offs between family size, parental investment and a range of child outcomes in contemporary British families. Contrary to traditional evolutionary and economic models, we find that higher levels of wealth often increase rather than alleviate trade-offs between the quantity and quality of children. On the basis of these findings, we argue that fertility decline remains a rational and quite possibly adaptive transition in the face of increasing costs of raising offspring.