Mechanisms and Extrapolation in the Abortion-Crime Controversy

Abstract

John Donohue and Steven Levitt’s seminal and controversial article, The Impact of Legalized Abortion on Crime, famously argues that the legalization of abortion in 1973 in the Unites States is a significant factor explaining the surprising decline in crime rates that occurred there in the 1990s. In this chapter, I examine the role of extrapolation in Donohue and Levitt’s study and draw three main philosophical conclusions. First, several different types of causal claims might be at issue in an extrapolation—including claims about mechanisms and probabilistic causal effects—and these distinctions matter for methodology because different conditions may be required to support extrapolation in each case. Secondly, scientific study of a phenomenon typically generates evidence at a variety of levels of aggregation, and this has important implications for extrapolation. The third and final point follows on the heels of the second. Like almost all other scientific inferences, extrapolations are normally components of a complex web of interrelated evidence that must be considered together in assessing a hypothesis.