Hodson and Busseri (2012, Bright Minds and Dark Attitudes: Lower Cognitive Ability Predicts Greater Prejudice Through Right-Wing Ideology and Low Intergroup Contact, Psychological Science, Vol. 23, pp. 187-195) in a British cohort study are able to conclude:
...our investigation establishes that cognitive ability is a reliable predictor of prejudice. Understanding the causes of intergroup bias is the first step toward ultimately addressing social inequalities and negativity toward outgroups. Exposing right-wing conservative ideology and intergroup contact as mechanisms through which personal intelligence may influence prejudice represents a fundamental advance in developing such an understanding.
In summary: it is argued that right wingers are typically less intelligent and people with low childhood intelligence tend to grow up to be prejudiced. Thoughts?