Jamie Jones (Department of Anthropological Sciences, Stanford University)
10/13/2008, at noon in room 6050 ISR-Thompson.
Evolutionary models for reproductive decision-making have not fared well in explaining fertility transitions. Such models typically make the implicit assumption that all births are substitutable, however, this is only true under extremely restrictive demographic circumstances. We develop a method for measuring the marginal value of fertility based on an extension of the Leslie matrix to individual birth histories. The cumulative fitness function with parity is concave, indicating diminishing marginal returns to higher-order births. From this, we predict that women should be risk-averse decision-makers if a birth jeopardizes the well-being of existing children. We investigate the predictions of this model using a large historical database from 19th and early 20th century Utah. Employing discrete event analysis, we show that birth intervals increase in this natural-fertility population during economic crises. Furthermore, the changes in birth interval length follow from the predictions of our model of decreasing marginal value of fertility.