The Northern Finland Birth Cohort (NFBC) is a population-based birth cohort comprised of 12 058 individuals born in the northernmost two Finnish provinces in 1966.19 In 1997, a temperament questionnaire was given to 5999 individuals who participated at the age of 31 in a follow-up assessment. Subjects were asked to complete the questionnaire and return it by mail; 5105 individuals returned the questionnaire,20 of whom 4508 were genotyped and passed quality control (QC, 55% female).
