Date of Award


Degree Type


Degree Name

Doctor of Philosophy


A behaviour genetic analysis of personality, liability to personality disorder, and the general environment of siblings was conducted using a classic twin study design. A sample of 138 same-sex young adult twin pairs (89 monozygotic pairs, 49 dizygotic pairs) was used to estimate trait variance attributable to direct gene action (h{dollar}\sp2\sb{lcub}\rm A{rcub}{dollar}), shared environmental experiences (c{dollar}\sp2{dollar}), and nonshared environmental experiences (e{dollar}\sp2{dollar}). Paralleling previously published results, model-fitting heritability analyses of the Personality Research Form (PRF; Jackson, 1986), and the Minnesota Multiphasic Personality Inventory, (MMPI, Hathaway & McKinley, 1983) showed that additive genetic and nonshared environmental factors could satisfactorily account for the trait variance in personality. Additional analyses revealed that genetic dominance effects (d{dollar}\sp2{dollar}) were present but are of a negligible magnitude. Multivariate genetic analyses also showed that there is evidence for a common genetic and environmental etiology to some dimensions of normal personality and liability to personality disorder.;Simple heritability analyses were also applied to four measures of the environment: Sibling Inventory of Differential Experience (SIDE; Daniels and Plomin, 1985), the Environmental Response Inventory (ERI; McKenchie, 1974), the Family Environment Scale (FES; Moos and Moos, 1986), and the Classroom Environment Scale (CES: Trickett and Moos, 1974). With the exception of the CES and specific scales from the SIDE, most of the remaining scales showed substantial additive genetic influence. However, the degree of genetic influence was found to be smaller than that reported in some previous studies.;Finally, a series of analyses was conducted with twins and an additional sample of 65 same-sex non-twin sibling pairs (51 sister-pairs, 14 brother-pairs) designed to identify sources of nonshared environmental influence related to differential personality and liability to personality disorder. Absolute differences in sibling personality as measured by the PRF and MMPI were regressed on absolute differences in sibling experience as measured by the SIDE, FES, CES, and ERI. Overall, only a few significant predictors of differential personality were found. Parental treatment and peer delinquency variables emerged as predictors of liability to personality disorder. However, this pattern is not consistent across kinship groups. These and other results are discussed.



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