We assessed the association and underlying genetic and environmental influences among intelligence (IQ) and personality in adolescent and young adult twins. Data on intelligence were obtained from psychometric IQ tests and personality was assessed with the short form of the NEO five factor inventory (NEO-FFI).IQ and personality data were available for 646 twins. There were an additional 1307 twins with NEO-FFI data, and 535 with IQ data. Multivariate genetic structural equation modeling was carried out.Significant positive phenotypic correlations with IQ were seen for agreeableness (r = 0.21) and openness to experience (r = 0.32). A negative correlation emerged for neuroticism and IQ (r = −0.10). Genetic factors explained (nearly) all of the covariance between personality traits and IQ. Genetic correlations were 0.3–0.4 between IQ and agreeableness and openness. The genetic correlation between IQ and neuroticism was around −0.18. Thus, personality and IQ did not appear to be independent dimensions, and low neuroticism, high agreeableness and high scores on openness all contributed to higher IQ scores.