Hur, Y., McGue, M. & Iacono, W.G. (1996). "Genetic and shared environmental influences on leisure-time interests in male adolescents." Individual Differences 21(5): 791-801.
Both genetics and shared environment (e.g., growing up in the same home) contribute to individual differences among adolescent twins' interests and leisure-time activities, but more than half of those differences are influenced by the twins' non-shared environment (e.g., having different friends or experiences). Shared environment seems to have a greater influence on activities such as TV viewing, dating, and social activities, than on skill-based activities, like sports, outdoor activities and crafts.
Zald, D. and W. G. Iacono (1998). "The development of spatial working memory abilities." Dev Neuropsychology 14(4): 563-578.
Between the ages of 14 and 20 an individual's memory for spatial locations improves significantly. Improvement in spatial working memory was not accounted for by improvement in seemingly related abilities (fine motor skills, prorated IQ),suggesting that spatial working memory provides new, unique information regarding the functioning and development of prefrontal regions brain regions.
Hur, Y-M., McGue, M.K., & Iacono, W.G. (1996). Genetic and shared environmental influences on leisure time interests in male adolescents. Personality & Individual Differences, 21, 791-801.
The present study explored genetic and environmental influences on individual differences in adolescent leisure-time interests. One hundred and ninety identical and 100 fraternal male twin pairs, aged 17 and 18 years, all participants in the ongoing Minnesota Twin Family Study, completed 120 items of the Leisure-Time Interests (LTI) inventory. A principal components analysis with varimax rotation of the 120 items of the LTI yielded nine interpretable factors (Intellectual Activities, Sports, Music and Artistic Activities, Handicrafts, Hunting and Outdoor Activities, Foreign Travel, TV Viewing, Dating and Social Activities, and Religious Activities). Correlation and biometrical analyses indicated that: (1) both genetic and shared environmental factors contributed to individual differences in adolescent leisure-time interests, although the relative magnitude of the contribution of the two factors varied across different leisure-time interests, and (2) approximately half of the variance in adolescent leisure-time interests was associated with nonshared environmental influences. The results are discussed in the context of developmental changes in genetic and shared environmental influences on leisure-time interests.
Johnson, W., Hicks, B.M., McGue, M., & Iacono, W.G. (2009). How intelligence and education contribute to substance use: Hints from the Minnesota Twin Family Study. Intelligence, 2009, 613-624. PMCID: PMC2799035
In old and even middle age, there are associations between physical health and both intelligence and education. This may occur because intelligence and/or education exert effects on lifestyle choices that, in turn, affect later health. Substance use is one aspect of lifestyle choice in young adulthood that could play such a role. The effects of intelligence and/or education on substance use could be direct and environmental, or indirect due to the presence of confounding genetic and shared family influences. We used the Minnesota Twin Family Study to distinguish these effects in males and females at age 24. In contrast to prevailing expectations, there were moderately negative direct nonshared environmental effects of both IQ and education on both smoking and drinking in both males and females. That is, controlling for positive family background effects in the form of both genetic and shared environmental influences, both higher IQ and greater education were associated with greater alcohol and nicotine use. These effects were accounted for by alcohol and nicotine use at age 17. Our results suggest that genetic and family-culture variables confound the associations between intelligence and education and substance use in young adults, rendering them indirect. Further research is needed to understand the roles of IQ and education in alcohol and nicotine use and their relative impacts on physical health throughout the lifespan.