This article studies the relationship between the “big five” personality traits and political ideology in a large U.S. representative sample (N = 14,672). In line with research in political psychology, “openness to experience” is found to predict liberal ideology, and “conscientiousness” predicts conservative ideology. The availability of family clusters in the data is leveraged to show that these results are robust to a sibling fixed-effects specification. The way that personality might interact with environmental influences in the development of ideology is also explored. A variety of childhood experiences are studied that may have a differential effect on political ideology based on a respondent's personality profile. Childhood trauma is found to interact with “openness” in predicting ideology, and this complex relationship is investigated using mediation analysis. These findings provide new evidence for the idea that differences in political ideology are deeply intertwined with variation in the nature and nurture of individual personalities.