Conservatism and cognitive ability are negatively correlated. The evidence is based on 1254 community college students and 1600 foreign students seeking entry to United States’ universities. At the individual level of analysis, conservatism scores correlate negatively with SAT, Vocabulary, and Analogy test scores. At the national level of analysis, conservatism scores correlate negatively with measures of education (e.g., gross enrollment at primary, secondary, and tertiary levels) and performance on mathematics and reading assessments from the PISA (Programme for International Student Assessment) project. They also correlate with components of the Failed States Index and several other measures of economic and political development of nations. Conservatism scores have higher correlations with economic and political measures than estimated IQ scores.
In other words, if you have low cognitive ability, a low SAT score, poor vocabulary, less than average education, are lacking in math or reading skills, or live in a place with a bad economy or politics, then you’re probably a conservative.
UPDATE: In fairness, here’s an article that points out some possible problems with this study.