ECONOMY

stephen ansolabehere 35
data to “test” the spatial theory. A number of odd results in the literature, such
as directional voting (Rabinowitz and MacDonald 1989) or asymmetries in spatial
perceptions (Brady and Sniderman 1985), may result from the assumed positions of
the candidates in survey research analyses or the assumption that the indifferent voter
is the one midway between the candidates.
Stokes’s assertion that valence issues matter, thus, seems correct and correctable.
Inclusion of a valence term can capture assessments of candidate competence, integrity,
and performance. This simple generalization of voter preferences, however,
surely changes the incentives facing candidates and parties, and, therefore, what
positions candidates ought to take in equilibrium. The question of candidate positioning
represents an even more significant empirical challenge to the spatial theory
of elections, and one to which I now turn.
2 Why Do Candidates Diverge?
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The median voter theorem is perhaps the single most important theoretical result in
modern political science. It is simple and elegant; it is widely applied to the study of
elections and government; and, by most accounts, it is false.
In a two-candidate election or two-party system, the simple spatial model predicts
that the competing candidates will promise the same policy—that preferred most by
the median voter. The settings most likely to approximate the assumptions of this
theory are the US Congress and presidency and the UK Parliament. Extensive studies
of British and American elections find that the parties in fact take divergent positions,
and neither party represents the median voter’s ideal policy. 7
Political economists take the median voter theorem the same way many biologists
view the Hardy–Weinberg Law. The strong empirical prediction is almost surely
wrong and, as a result, one of the basic assumptions is incorrect. The challenge is
to find the factor or factors that explain divergence.
Many explanations have been offered—including primary elections, incumbency
advantages, political parties, uncertainty and risk aversion, interest group contributions,
candidates’ personal preferences, election laws, and potential third parties. I
will discuss four of these explanations here.
⁷ An important literature in political science examines the policy positions of the parties and their
candidates in the US House of Representatives. One line examines the roll call votes of candidates who
have served in the House and run against each other, as happens when one candidate beats an
incumbent (Fiorina 1974; Ansolabehere, Snyder, and Stewart 2001). The second line examines surveys of
candidates for office (Erikson and Wright 1997; Ansolabehere, Snyder, and Stewart 2001). While both
face sample selection problems, they show very similar patterns. There is a large gap between the average
Democratic candidate and the average Republican candidate overall, and the candidates competing with
each other within a given district almost always take different positions, with the Democrat on the left
and the Republican on the right.