Expanding the Nomological Net of Personality Psychology

Abstract

Although the taxonomies of personality, including the Big Five and the Multidimensional Personality Questionnaire, possess many markers of psychometric validity, many researchers have called for the further substantiation of personality taxonomies—not just through psychometric validity, but through biology. To that end, this dissertation presents three studies that expand the nomological net of personality psychology by further exploring the biological basis of personality. In Study 1, I investigated the pattern and sources of personality stability and change as it develops from adolescence to middle age in a sample of twins. I found that both stability and change were influenced by genetic and nonshared environmental sources, with little influence from the shared environment. The heritability of the personality traits changed significantly across waves, ranging from 40-60%, but no clear pattern emerged. For several traits, I discovered that the genetic influence included nonadditivity. In Study 2, I explored the issue of genetic confounding in the association between parenting and self-control. While research into self-control holds that parenting is an important causal factor, the majority of this research bases this claim on study designs that do not use genetically informative samples. Using a longitudinal sample of nonadoptive and adoptive siblings, I investigated this issue further. While I did not find evidence in favor of a causal relationship, I simultaneously did not find evidence of genetic confounding. In Study 3, I examined the structure of the Openness/Intellect domain from the Big Five Aspects Scale and how its aspects (Openness and Intellect) differentially relate to intelligence and to reaction time. This pioneering study provided evidence that Intellect and Openness are distinct subfactors that may arise from independent biological sources. Altogether, these studies expand the nomological net by measuring a twin sample over a long duration of time, by using an adoptive sample to investigate the issue of genetic confounding, and by utilizing reaction time in a pioneering study. This dissertation takes a path towards further establishing the validity of personality as a construct, as well as a path towards better understanding the psychology and biology of personality.