Personality development in monozygotic twins of varying IQ discordance
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The present study sought to investigate the personality-intelligence association by using longitudinal MZ twin data to examine if intelligence and personality are developmentally linked. If twins discordant for IQ in childhood grew up to differ more in personality than twins of very similar intelligence, it would support theories of personality and intelligence that posit a developmental interaction between the two and a non-shared environmental inﬂuence can be inferred. It was also investigated if twins concordant for either low or high childhood IQ had personalities that on average differed more from their co-twins later in life. The sample consisted of 789 MZ twin pairs participating in the Minnesota Twin Family Study. The analysis used a series of one-way ANOVAs comparing the mean personality difference scores at ages 17 and 25 of groups of twins discordant for IQ to twins concordant for high or low IQ. Twins discordant for childhood IQ did not show a pattern of more discordance in personality at either age 17 or 25. When IQ was divided into performance IQ and verbal IQ conﬂicting results were obtained. The ﬁnal ﬁnding was that differences were greater between twins concordant for higher than for lower intelligence on the traits Wellbeing and Social Potency. This tentatively suggests that a positive, outgoing and social side to Extraversion is more variable at higher levels of intelligence. There is no straight forward interpretation of the overall lack of relation between early intelligence and subsequent personality. Interpretations related to limitations in the study design and an absence of a direct inﬂuence of intelligence on personality are discussed.