From the paper:
Second, there was also much evidence to support Hypothesis 3, the notion of a curvilinear relationship with political extremists commanding greater cognitive resources than those in the political center
Not too surprising for anyone who pays attention to political campaigns. Also:
First, there was substantial support for Hypothesis 1 concerning a negative, linear relationship between conservatism and cognitive ability: conservative gender role attitudes and self-descriptions predicted SAT-V and ACT.and:
Although conducted at a different level of analysis than Study 1, Study 2 yielded some support for Hypothesis 1: In states with high political involvement, there was a linear and positive relationship between state-IQ and the proportion of Democrats in the state legislature. This finding represents a conceptual replication of important aspects of Study 1. At the same time, Study 2 also replicated an unanticipated finding in Study 1, namely, that some conservative leanings were related to higher cognitive ability. In Study 2, this finding was quite robust, but was confined to states with comparatively low political involvement. Unfortunately, Study 2 did not allow the examination of different kinds of conservative leanings, but relied on the proportion of Democratic lawmakers in a state’s legislatures. Similarly, Study 2 did not permit the examination of different aspects of cognitive ability; thus, it remains open whether observed effects were largely driven by verbal ability, as suggested by Study 1.
Personality and Individual Differences doi: doi:10.1016/j.paid.2008.08.003
Is there a relationship between political orientation and cognitive ability? A test of three hypotheses in two studies
Two studies tested one linear and two curvilinear hypotheses concerning the relationship between political conservatism-liberalism and cognitive ability. Study 1, focusing on students at a selective US university (n = 7279), found support for the idea that some dimensions of conservatism are linked to lower verbal ability, whereas other dimensions are linked to higher verbal ability. There was also strong support for political extremists both on the left and right being higher in verbal ability than centrists. Study 2 employed aggregate data pertaining to the 50 US states and demonstrated that conservatism was linked to lower cognitive ability in states with high political involvement, but found conservatism to be correlated with higher average ability in states with low political involvement. The discussion addresses potential implications and criticisms of this research.