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Little is known about why people differ in their levels of academic motivation. This study explored the etiology of individual differences in enjoyment and self-perceived ability for several school subjects in nearly 13,000 twins aged 9–16 from 6 countries. The results showed a striking consistency across ages, school subjects, and cultures. Contrary to common belief, enjoyment of learning and children’s perceptions of their competence were no less heritable than cognitive ability. Genetic factors explained approximately 40% of the variance and all of the observed twins’ similarity in academic motivation. Shared environmental factors, such as home or classroom, did not contribute to the twin’s similarity in academic motivation. Environmental influences stemmed entirely from individual specific experiences.

The German twin family study 'TwinLife' was designed to enhance our understanding of the development of social inequalities over the life course. The interdisciplinary project investigates mechanisms of social inequalities across the lifespan by taking into account psychological as well as social mechanisms, and their genetic origin as well as the interaction and covariation between these factors. Main characteristics of the study are: (1) a multidimensional perspective on social inequalities, (2) the assessment of developmental trajectories in childhood, adolescence, and young adulthood in a longitudinal design by using (3) a combination of a multi-cohort cross-sequential and an extended twin family design, while (4) capturing a large variation of behavioral and environmental factors in a representative sample of about 4,000 German twin families. In the present article, we first introduce the theoretical and empirical background of the TwinLife study, and second, describe the design, content, and implementation of TwinLife. Since the data will be made available as scientific use file, we also illustrate research possibilities provided by this project to the scientific community.

The present study investigated the prevalence and heritability of nightmares and their comorbidity with psychopathology in a sample (N = 1,318) of children, adolescents, and child and adolescent twins ranging in age from 4 to 17 years old. The prevalence of terrible nightmares was estimated to be 6.4%, which is similar to previous studies. There were marginal gender differences in this rate (7.7% for boys; 5.1% in girls), contrary to previous studies that purport higher rates for girls. There was little evidence for prevalence changes across age. Nightmares were highly heritable and attributed to an additive genetic influence (51%) and nonshared environmental effects (49%). There was little evidence for a shared genetic correlation for nightmares and generalized waking anxiety (Overanxious Disorder of Childhood). There was also a substantial and pervasive comorbid psychopathology for those parents who reported Strongly True on Item 59: My child has terrible nightmares on the 200-item parent-as-respondent, Coolidge Personality and Neuropsychological Inventory. Issues in estimating prevalence rates of nightmares were identified.