“If I had any desire to lead a life of indolent ease, I would wish to be an identical
twin, separated at birth from my brother and raised in a different social class. We
could hire ourselves out to a host of social scientists and practically name our fee.
For we would be exceedingly rare representatives of the only really adequate natural
experiment for separating genetic from environmental effects in humans – genetically
identical individuals raised in disparate environments.”
-- Stephen Jay Gould (1996: 264)
I.
Introduction
Social scientists are increasingly exploiting natural experiments in their research. A
recent search on “natural experiment” using “Google Scholar” (scholar.google.com)
turned up more than one million hits; an interocular test of the results appearing on the
first dozen pages suggests that economics and epidemiology are the leading fields to use
the term, but political science is also represented. A search in JSTOR for articles in
political science journals with “natural experiment” in the title or the abstract turned up
only two articles published between 1960 and 1990 inclusive, but it returned two articles
published between 1991 and 1995 and six between 1996 and 2000. A similar search in
economics journals revealed eight articles published between 1960 and 1990 inclusive,
thirteen between 1991 and 1995, and fifteen between 1996 and 2000. Table I presents a
non-exhaustive list of recent studies in political science that claim to exploit a “natural
experiment.”
[TABLE I HERE]
As the name suggests, natural experiments take their inspiration from the
experimental approach. A randomized controlled experiment (Freedman et al. 1997: 4-8)
has three hallmarks. First, the response of experimental subjects to a “treatment” (or a
series of treatments) is compared to the response of other subjects to a “control,” often
Convention