Twin studies suggest that both genes and environments influence the emergence and development of individual differences in self-esteem. However, different lines of research have emphasized either the role of genes or of environmental influences in shaping self-esteem, and the pathways through which genes and environments exert their influence on self-esteem remain largely unclear. In this study, we used nationally representative data from over 2000 German twin families and a nuclear twin family design (NTFD) to further our understanding of the genetic and environmental influences on individual differences in self-esteem. Compared with classical twin designs, NTFDs allow for finer-grained descriptions of the genetic and environmental influences on phenotypic variation, produce less biased estimates of those effects, and provide more information about different environmental influences and geneenvironment correlation that contribute to siblings' similarity. Our NTFD results suggested that additive and nonadditive genetic influences contributed to individual differences in self-esteem as well as environmental influences that are both shared and not shared by twins. The shared environmental component mostly reflected non-parental influences. These findings highlight the increased sensitivity afforded by NTFDs but also remaining limitations that need to be addressed by future behavioural genetic work on the sources of self-esteem.Note: a, additive genetic variance; i, non-additive genetic variance due to gene × gene interaction; c, environmental variance shared by twins; s, non-parental environmental variance shared by siblings; t, twin-specific shared environmental variance; e, environmental variance not shared by twins (incl. measurement error); μ, phenotypic assortative mating; m, effects due to maternal transmission; f, effects due to paternal transmission. 1 We did not test for non-additive genetic components due to allelic dominance within gene loci (d 2 ), because meta-analyses (Polderman et al., 2015;Vukasović & Bratko, 2015) found little evidence for these types of non-additive genetic influences on individual differences in complex human traits.Nuclear twin family study of self-esteem 223

The German Twin Family Panel (TwinLife) is a German longitudinal study of monozygotic and dizygotic same-sex twin pairs and their families that was designed to investigate the development of social inequalities over the life course. The study covers an observation period from approximately 2014 to 2023. The target population of the sample are reared-together twins of four different age cohorts that were born in 2009/ 4). In the first wave, the study included data on 4097 twin families. Families were recruited in all parts of Germany so that the sample comprises the whole range of the educational, occupational and income structure. As of 2019, two face-to-face, at-home interviews and two telephone interviews have been conducted. Data from the first home and telephone interviews are already available free of charge as a scientific use-file from the GESIS data archive. This report aims to provide an overview of the study sample and design as well as constructs that are unique in TwinLife in comparison with previous twin studiessuch as an assessment of cognitive abilities or information based on the children's medical records and report cards. In addition, major findings based on the data already released are displayed, and future directions of the study are presented and discussed.

Political orientation is often assumed to be shaped by socialization processes; however, previous studies have shown substantial genetic variance components in party affiliation, political attitudes and behaviors, or closely related personality traits. The majority of these studies have relied on the Classical Twin Design, which comes with restrictive assumptions, some of which are easily violated. Moreover, most analyses lack a perspective of age-group differences. In this study, we investigated political orientation in adolescents (age: 16–18) and young adults (age: 21–25) in a cross-sectional Nuclear Twin Family Design. We used data of the German TwinLife project, including data from same-sex twins reared together, their biological parents, and nontwin full siblings. We found genetic variation in political orientation, which was significant in the older cohort, possibly indicating an increasing importance of active gene-environment correlation from adolescence to adulthood. Individual differences in political orientation because of passive gene-environment correlation and shared environmental effects were larger in the younger cohort, substantiating the same theoretical consideration and the importance of shared socialization contexts for adolescents’ political views. By running Nuclear Twin Family model analyses, and considering age-group differences, as well as the relationship of political orientation with the Big Five personality traits, our study extended previous work, and resulted in more robust and fine-grained estimates of genetic and environmental sources of variance in political orientation. Therefore, it contributed to a better understanding of the complex nature–nurture interplay that forms political orientation in emerging adulthood.

Political participation (POP), social participation (SOP), and political interest (PI) are important indicators of social status and social inequality. Previous studies on related trait differences yielded genetic and environmental contributions. However, focusing on adult samples, classical twin designs, and convenience samples often restricts parameter estimation and generalizability, and limits the understanding of age differences. We investigated sources of variance in POP, SOP, and PI in late adolescence and early adulthood with an extended twin family design (ETFD). We analyzed data from over 2,000 representative German twin families. Individual environments not shared by family members reflected the major source of variance for all variables, but genetic influences were also pronounced. Genetic effects were mostly higher for young adults, whereas effects of twins’ shared environment were significant in adolescence. Our study deepens the understanding of the interplay between genetic and environmental factors in shaping differences in young persons’ integration in society.

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