Wilson, S., Elkins, I.J., Bair, J.L., Oleynick, V.C., Malone, S.M., McGue, M., & Iacono, W.G. (2018). Maladaptive Personality Traits and Romantic Relationship Satisfaction: A Monozygotic Co-Twin Control Analysis. Journal of Abnormal Psychology, 127(4), 339-347. PMCID:PMC5951396

Matteson, L., McGue, M., & Iacono, W.G. (2013). Is dispositional happiness contagious? The impact of the well-being of family members on individual well-being. Journal of Individual Differences, 34(2), 90-96. PMCID: PMC4193666

Results: There was no evidence of sex differences in well-being of targets or parents. Also, there was no evidence of important differences in means or variances of well-being among the family types, suggesting that restriction of range in adoptive families is not a problem for this phenotype.
Discussion: The present study tested the contagion hypothesis of happiness using a novel adoptive family study design. If happiness were contagious, then family members would have similar levels of happiness even when they are not genetically related. However, we found no support for the contagion hypothesis over a 3-year period. The happiness of family members, either singly or in aggregate, was unrelated to target happiness. Our finding of moderate familial correlations among genetically related family members, which is consistent with a larger behavioral genetic literature based on studies of twins (e.g., Finkel and McGue, 1997), suggests that our failure to observe resemblance in adoptive families does not owe to some inherent limitation of our measure. Both our consistent observation that target happiness is unrelated to the happiness of individuals with whom he or she has lived for an extended period of time but who are not genetically related or actively selected (i.e., homophily) and the fact that biometrical modeling produced a c2 estimate that was essentially zero presents challenges to the contagion hypothesis....
In our behavior genetic analyses, we found significant similarity of well-being scores among biological but not among adopted relatives and thus no evidence for shared environmental influences on happiness. Although shared environmental effects refer to those that make siblings more similar while a contagion effect refers to influence of one person on another regardless of relationship, families living together provide the strongest test of contagion as they are in very close physical proximity. Since adopted relatives are neither genetically related nor selected, our results suggest that the effects of genetic confounding and homophily may have been underestimated in previous social network research on happiness.

Elkins, I.J., King, S.M., McGue, M. & Iacono, W.G. (2006) Personality Traits and the Development of Nicotine, Alcohol, and Illicit Drug Disorders: Prospective Links from Adolescence to Young Adulthood. Journal of Abnormal Psychology, 115, 26-39.

Benning, S.D., Patrick, C.J., Blonigen, D.M., Hicks, B.M. & Iacono, W.G. (2005). Estimating Facets of Psychopathy from Normal Personality. Assessment, 12, 3-18.

Elkins, I.J., McGue, M., Malone, S. & Iacono, W.G. (2004). The effect of parental alcohol and drug disorder on adolescent personality. American Journal of Psychiatry, 161, 670-676.

Objective: The relationship of parental alcohol or drug diagnosis to offspring personality was examined in a population based sample of 17-year-old twins (568 girls and 479 boys) participating in the Minnesota Twin Family Study. Whether offspring personality characteristics 1) are specific to the type of substance use disorder in parents (alcohol versus drug) and 2) are found in high-risk offspring without substance use disorders as well as in offspring with substance use disorders was investigated.
Method: Personality was assessed with the Multidimensional Personality Questionnaire; substance use disorders were assessed in person through diagnostic interviews.
Results: In both male and female offspring, parental history of alcohol dependence was associated with greater negative emotionality, aggression, stress reaction, and alienation but lower wellbeing; parental history of drug disorders was associated with lower constraint, control, harm avoidance, and traditionalism but higher social potency. Excluding offspring with a substance use disorder had virtually no effect on the statistical significance of these findings.
Conclusions: In contrast to findings in some adult samples, personality characteristics associated with a family history of substance use disorders are found even in adolescent offspring who have not yet developed these disorders themselves, suggesting that personality might be one indicator of familial risk for substance use disorders during this developmental stage. Personality profiles of offspring of parents with substance use disorders also show some diagnostic specificity, with constraint associated with parental drug abuse and negative emotionality with parental alcoholism.
(Am J Psychiatry 2004; 161:670–676)

Burt, S.A., McGue, M., Iacono, W., Comings, D., & MacMurray, J. (2002). An examination of the association between DRD4 and DRD2 polymorphisms and personality traits. Personality & Individual Differences, 33, 849-859.

To date, evidence supporting a positive association between the 7-repeat allele of the D4 dopamine receptor (DRD4) exon III 48 bp polymorphism and Novelty Seeking (NS) has been mixed, with some studies confirming and others refuting the association. A positive association between NS and the minor Taq1 A allele (A1) of the D2 dopamine receptor (DRD2) has also been reported. In the present study, we sought to replicate the associations between the DRD4 and DRD2 polymorphisms and various personality traits, as measured by the Multidimensional Personality Questionnaire. The sample consisted of 137 families (n=348) assessed as part of the ongoing Minnesota Twin Family Study. The data were analyzed at both the individual-level, to maximize comparability with previous studies, and at the family-level, to control for population stratification. The DRD4 and DRD2 polymorphisms were not associated with MPQ measures related to NS, results that may cast doubt on the generalizability of previous positive findings.
©2002 Elsevier Science Ltd. All rights reserved.

Johnson, W., Krueger, R. F., Bouchard, T.J., & McGue, M. (2002) "The Personalities of Twins: Just Ordinary Folks." Twin Research, 5, 125-131.

Twin studies have demonstrated that personality traits show moderate genetic influence. The conclusions drawn from twin studies rely on the assumptions that twins are representative of the population at large and that monozygotic and dizygotic twins are comparable in every way that might have bearing on the traits being studied. To evaluate these assumptions, we used Multidimensional Personality Questionnaire (MPQ) data from three samples drawn from the Minnesota Twin Registry (totaling 12,971 respondents) to examine the effect sizes associated with mean differences on the 11 MPQ scales and 3 higher-order MPQ factors for singletons versus twins and MZ twins versus DZ twins. The singletons in the samples were family members of the participating twins. We also used ratios of scale variances to examine the significance of variance differences. The only mean or variance difference replicated across all three samples was greater Social Closeness (about .1 standard deviation) for twins than for singletons. This difference was obtained for both males and females. It would appear that, with respect to personality, twins are not systematically different from other people. Our results also highlight the importance of replication in psychological research because each of our large samples showed differences not replicated in other samples.

Billig, J. P., S. L. Hershberger, et al. (1996). "Life events and personality in late adolescence: genetic and environmental relations." Behavior Genetics 26(6): 543-54.

The relationship between life events and personality was investigated in the Minnesota Twin/Family Study, using 216 monozygotic and 114 dizygotic 17-year-old male twin pairs. Participants completed a life events interview designed for adolescents and the Multidimensional Personality Questionnaire. Life events were categorized into three types: life events to which all members of a family would be subject and those affecting an individual, which can be broadly construed as either nonindependent or independent. Univariate genetic model fitting indicated the presence of significant genetic effects (h2 = 49%) for nonindependent nonfamily life events but not for the other two types of life events. Bivariate genetic model fitting further confirmed that the significant phenotypic correlation between nonindependent life events and personality is in part genetically mediated. Specifically, the findings suggest that genetically influenced individual differences in constraint play a substantial role in life events whose occurrence is not independent of the individual's behavior.