Characteristics such as age (in years), gender (boy vs. girl), ethnic background (Dutch vs. non-Dutch), school level (pre-vocational education vs. senior secondary & pre-university education), and living situation (living with both parents vs. not living with both parents) were entered as biological and social-cultural factors. Ethnic background was classified as Dutch or non-Dutch, by following the definition of Statistics Netherlands; adolescents with at least one parent born outside the Netherlands were classified as non-Dutch [25]. With regard to school level; in the Netherlands, three general levels of secondary education can be distinguished. The lowest level is pre-vocational education that lasts four years and prepares for intermediate vocational education. The next level is senior secondary education that lasts five years and prepares for the university of applied sciences. Lastly, the highest level is pre-university education that lasts six years and prepares for the university [26].