Mechanisms and Extrapolation in the Abortion-Crime Controversy - Mechanism and Causality in Biology and Economics - page 179

many American cities in the late 1980s and subsided in the 1990s. However, the
subsidence of the crack epidemic does not explain drops in crime in suburban and
rural areas in which crack was never a serious problem and fails to explain why the
crime would persistently fall below pre-crack levels (Donohue and Levitt 2004 ;
Fryer et al. 2005 ; Levitt 2005 ). In addition, although increased incarceration and
swelling ranks of police forces likely played a role in reducing the nationwide crime
rate, these factors predate the decline in crime in the 1990s by at least a decade
(Donohue and Levitt 2001 , p. 380; Levitt 2004 ). Since Donohue and Levitt's ( 2001 )
essay, a few new explanations of the surprising 1990s crime decline have been
proposed. One explanation that accounts for all three basic features of the decline
focuses on the phase out of leaded gasoline required by the 1970 Clean Air Act
(Reyes 2007 ). Early childhood lead exposure is known to cause a number of
cognitive deficits, some of which are related to aggressive and violent behavior in
adulthood. Thus, environmental regulations enacted by the federal government that
sharply reduced lead exposure could have had a crime-diminishing effect a genera-
tion later, in much the same manner as legalized abortion according to Donohue and
Levitt's hypothesis. The lead-crime hypothesis also has the advantage of explaining
rising crime rates in the two decades prior to the 1990s, which could have partially
resulted from increased lead exposure throughout the mid-twentieth century to the
early 1970s (Reyes 2007 , p. 33). Of course, the causes surveyed above are not
mutually exclusive, and the most likely scenario is that fluctuations in crime rates
are due to a variety of factors, including some not mentioned here. 1
The next part of Donohue and Levitt's case consists of tracing the mechanism
from legalized abortion to reduced crime (section III of their 2001 article). They in
fact propose two mechanisms through which legalized abortion could reduce the
crime rate: cohort-size reduction and selection. Cohort-size reduction is the idea
that legalized abortion would reduce the birth rate, which in turn would mean a
smaller cohort of individuals aging into the high-crime years of 18-24 in the early
1990s. Selection is a more interesting and controversial mechanism and is the one
that will occupy our attention here.
In general, the term “selection effect” refers to a situation in which a group of
individuals defined in an inquiry is not a random sample of the population but
instead differs in some further way that is relevant to topic under investigation. In
this context, the selection mechanism operates if children born to mothers who
wished to terminate their pregnancy are not a random sample of births generally but
are much more likely to be born into adverse family, social, or economic
circumstances that put them at greater risk for criminal activity later in life. The
selection mechanism can be helpfully further divided into two separate sub-
mechanisms. The first concerns the impacts of being born unwanted on the child's
1 Reyes ( 2007 , p. 36) regards both reduced lead exposure and legalized abortion as significant
factors. Levitt ( 2004 ) critically examines several further proposed explanations of the crime drop,
including demographic factors and improved economic conditions. Wadsworth ( 2010 ) proposes
that immigration may have contributed to the 1990s decline in crime rates.