The sample consisted of 3,230 participants (median year of birth 1967, range 1926–1994; 64.7% females) of North-European ancestry from the Netherlands Study of Depression and Anxiety (NESDA, n=1,846) and from the Netherlands Twin Register (NTR, n=1,384). Unrelated participants were selected applying a cut-off threshold of 0.025 (i.e., no closer relationships than third or fourth cousin) to a relationship matrix measuring genetic similarity calculated using GCTAv.1.24.1(21). Detailed descriptions of the rationale, design and methods for both studies are given elsewhere(22;23). Briefly, NESDA is an ongoing cohort study into the long-term course and consequences of depressive and anxiety disorders. In 2004–2007 2,981 participants aged 18 to 65 years were recruited from the community (19%), general practice (54%) and secondary mental health care (27%) and were followed-up during three biannual assessments. The NTR study has been collecting data on Dutch twin families since 1991 and comprises longitudinal data on nearly 40,000 adult participants. The research protocols from both studies were approved by the ethical committee of participating universities, and all participants provided written informed consent.
