in children. It is somewhat less surprising that humans would have chosen to have fewer children as their incomes increased when one recognizes that the decrease in numbers of children was offset by enormous increases in the amount of resources invested in each child. Quality-quantity trade-offs have been central to both economic and evolutionary models of fertility and arguably offer the most plausible way for reconciling both types of models with the observed empirical patterns.

This chapter discusses the intersection between rational choice models of fertility—especially models developed by economists—and evolutionary biology models of human fertility. The focus is primarily on the issue of quantity-quality trade-offs, an area in which there is considerable theoretical overlap in the two approaches. A central motivating question is whether evolutionary perspectives on the biodemography of fertility can help us understand the negative relationship between income and fertility and how these evolutionary approaches compare to economic models of fertility behavior. The chapter begins with the presentation of a generic economic model of fertility that includes both quality-quantity trade-offs and the time cost of children. Particular emphasis is placed on the effect that increased productivity of parental time may have on choices about the quantity and quality of children and on time allocation. Then empirical evidence is presented from Brazil and South Africa that supports the view that quality-quantity trade-offs are fundamentally important in understanding fertility decline. The relationship between economic models and models drawn from evolutionary biology is then discussed. The paper points out the potential contributions that an evolutionary approach can make to economic models and considers potential differences in predictions coming from evolutionary and economic models.

THE RELATIONSHIP BETWEEN INCOME AND FERTILITY

The relationship between fertility and income has always been a fundamental issue for economists interested in fertility and the family. Becker’s seminal first paper on the economics of fertility (1960) heavily focused on the question of why we do not observe a stronger positive relationship between income and fertility. Becker found the observed lack of a positive relationship curious enough that he partially attempted to explain it as a statistical anomaly, suggesting that more complete data would in fact reveal a stronger positive association. But he also introduced the notion that quantity-quality trade-offs might be at the heart of the issue, noting that in the case of many goods that have an important quality dimension, higher incomes lead to much larger increases in quality than quantity. As will be discussed below, Becker would later appeal to evolutionary biology as part of the motivation for quality-quantity trade-offs in human fertility. In his



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