The TOPP study (Norwegian title: “Trivsel og OPPvekst i barndom og ungdomstid”) started in 1993. All families from 19 health care areas in Eastern Norway, who attended a child public health programme (i.e., an 18-month vaccination programme) during 1993 were invited to participate. Of the 1,081 invited families, 913 mothers (84%) participated with data on 921 children (including twins and triplets). The families completed questionnaires eight times, from when the children were 18  months old (time 1) to when they were 19 years old (time 8).