MDD cases were recruited through mental health care organizations, general practices and in the community setting (for details see Boomsma et al. 2007; in press). The inclusion criteria for the 1780 participants are: 1) a DSM-IV diagnosis of major depressive disorder as confirmed by the CIDI psychiatric interview, 2) an age between 18 through 65 years, 3) sufficient knowledge of the Dutch language, and 4) North-european ancestry. As the samples should be representative of patients seen in different settings, there are few a priori exclusion criteria. Excluded patients are: 1) those with a primary diagnosis of psychosis, bipolar disorder, obsessive compulsive disorder, severe addiction disorder and 2) those with insufficient knowledge of the Dutch language.

For age and gender matched control subjects are mainly derived from the Netherlands Twin Register, for which data collection in twin, their parents, spouses and siblings occurred in 1991, 1993, 1995, 1997, 2000, 2002/3 and 2004/5. A total of 1860 controls were selected (only one member from each family) with the following inclusion criteria: 1) age 18 through 65 years, 2) never scoring high (> 0.65) on a general factor score for anxious depression (a combined measure of neuroticism, anxiety and depressive symptoms via questionnaires, Boomsma et al., 2000), 3) never reported a history of MDD in any survey, and 4) North-european ancestry. Controls and their parents were born in the Netherlands or northwestern Europe.