The Netherlands Twin Register: Longitudinal Research Based on Twin and Twin-Family Designs

The Netherlands Twin Register (NTR) is a national register in which twins, multiples and their parents, siblings, spouses and other family members participate. Here we describe the NTR resources that were created from more than 30 years of data collections; the development and maintenance of the newly developed database systems, and the possibilities these resources create for future research. Since the early 1980s, the NTR has enrolled around 120,000 twins and a roughly equal number of their relatives. The majority of twin families have participated in survey studies, and subsamples took part in biomaterial collection (e.g., DNA) and dedicated projects, for example, for neuropsychological, biomarker and behavioral traits. The recruitment into the NTR is all inclusive without any restrictions on enrollment. These resources — the longitudinal phenotyping, the extended pedigree structures and the multigeneration genotyping — allow for future twin-family research that will contribute to gene discovery, causality modeling, and studies of genetic and cultural inheritance.

The German Twin Family Panel (TwinLife)

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 2009/2010 (cohort 1), in 2003/2004 (cohort 2), in 1997/1998 (cohort 3) and between 1990 and 1993 (cohort 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 studies — such 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.

The Oslo University Adolescent and Young Adult Twin Project: Recruitment and Attrition

The Oslo University Adolescent and Young Adult Twin Project started in 2006 with the first of three questionnaire data collection waves, 2 years apart. All twins from the birth cohorts 1988–1994 were invited to participate, and both the twins and their parents were asked to sign consent forms. The twins were 12–18 years old at Wave 1, at which time parents were asked to complete similar questionnaires. The parents’ questionnaire enquired about the parents’ ratings of their twin’s traits. In addition, the parents answered questions regarding their own education, demographics and socioeconomic situation. When the twins were 18 years old, they were invited to a face-to-face interview and two new questionnaires were presented. The questionnaires for the waves included a number of personality scales, internalization and externalization traits, affective and behavioral problems, as well as measures of environment and coping. The most common DSM-IV mental disorders and all personality disorders were covered in the interview. Zygosity was established both by questionnaire and gene markers. The original sample consisted of 5374 twin families, and among these, 4668 pairs were alive and living in Norway. Of these, 2486 families (53.3%) consented to participate. Of these, again 1538 twin families (61.9%) actually participated in at least one wave and twins from 1422 pairs (57.3%) participated in the interview. Female gender, but not zygosity, predicted staying in the project. Moreover, having a planning, structured personality (being more conscientious, open to experience [i.e., curious and interested in learning], having higher resilience and better school habits) increased the chance of carrying on in the project. Interestingly, the attrition did not seem to bias the heritability estimates.

Familial factors and the risk of borderline personality pathology: genetic and environmental transmission

BackgroundParental characteristics and practices predict borderline personality disorder (BPD) symptoms in children. However, it is difficult to disentangle whether these effects are genetically or environmentally mediated. The present study examines the contributions of genetic and environmental influences by comparing the effects of familial risk factors (i.e. parental psychopathology and borderline traits, maladaptive parenting, marital discord) on child BPD traits in genetically related (biological) and non-related (adoptive) families.MethodsData are from 409 adoptive and 208 biological families who participated in the Siblings Interaction and Behavior Study (SIBS) and 580 twin families the Minnesota Twin Family Study (MTFS). Parent characteristics and practices included parental psychopathology (measured via structured clinical interviews), parental BPD traits, parenting behaviors, and marital discord. A series of multi-level regression models were estimated to examine the relationship of familial risk factors to child BPD traits and to test whether children's adoptive status moderated the association.ResultsSymptom counts of parents' conduct disorder, adult antisocial behavior, nicotine, alcohol, and illicit drug dependence, and paternal BPD traits substantially predicted child BPD traits only in biological offspring, implying genetic transmission. Maternal BPD traits and both maternal and paternal conflict, lack of regard, and lack of involvement predicted offspring BPD traits regardless of the adoptive status, implying environmental transmission.ConclusionsParental externalizing psychopathology and father's BPD traits contribute genetic risk for offspring BPD traits, but mothers' BPD traits and parents' poor parenting constitute environmental risks for the development of these offspring traits.

Unravelling the Interplay between Genetic and Environmental Contributions in the Unfolding of Personality Differences from Early Adolescence to Young Adulthood

In two studies, we examined the genetic and environmental sources of the unfolding of personality trait differences from childhood to emerging adulthood. Using self–reports from over 3000 representative German twin pairs of three birth cohorts, we could replicate previous findings on the primary role of genetic sources accounting for the unfolding of inter–individual differences in personality traits and stabilizing trait differences during adolescence. More specifically, the genetic variance increased between early (ages 10–12 years) and late (ages 16–18 years) adolescence and stabilized between late adolescence and young adulthood (ages 21–25 years). This trend could be confirmed in a second three–wave longitudinal study of adolescents’ personality self–reports and parent ratings from about 1400 Norwegian twin families (average ages between 15 and 20 years). Moreover, the longitudinal study provided evidence for increasing genetic differences being primarily due to accumulation of novel genetic influences instead of an amplification of initial genetic variation. This is in line with cumulative interaction effects between twins’ correlated genetic makeups and environmental circumstances shared by adolescent twins reared together. In other words, nature × nurture interactions rather than transactions can account for increases in genetic variance and thus personality variance during adolescence. © 2019 European Association of Personality Psychology

Does sibling and twin similarity in cognitive ability differ by parents’ education?

Stratification scholars predominantly investigate how differences among children from different families emerge and tend to neglect differences among children from the same family. I study sibling similarity in cognitive ability and examine whether their similarity varies by parents’ education. Although economic approaches and their extensions argue that disadvantaged parents reinforce differences while advantaged parents compensate for differences, I argue that parents may also make equal investments and thus accept differences among their children. I refer to the literature on stratified parenting that demonstrates that parents are engaged differently in child-rearing and their children’s skill formation processes. Because advantaged parents foster children’s talents more individually compared with disadvantaged parents, I propose that sibling similarity is lower in advantaged than in disadvantaged families. Previous studies based on sibling correlations provide conflicting evidence. To account for observable and unobservable differences among siblings, I extend the established sibling correlation approach and study dizygotic and monozygotic twins in addition to siblings. The analyses draw on novel data from a population register-based study of twin families. I find that young adult siblings and twins are less alike in cognitive ability in highly educated families than in less educated families. Hence, my results support the hypothesis concerning equal investments and indicate that stratified parenting has a long-lasting influence on children’s cognitive ability.

A Nuclear Twin Family Study of Self–Esteem

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 gene–environment correlation that contribute to siblings’ similarity. Our NTFD results suggested that additive and non–additive 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. Copyright © 2018 European Association of Personality Psychology