The YNTR survey 12 was emailed to both parents of twins around the 12th birthday of the twins. The survey targets the development of emotional and behavior problems, as assessed by the Child Behavior Checklist 6-18 (CBCL). From 2001 onwards, the Conners’ Parent Rating Scale — Revised (CPRS-R) was also included. In addition, these surveys included items on zygosity, medical conditions, demographic characteristics, growth, sport and leisure activity of both children and parents (from 2003), and additional items on well-being (from 2008). In 2017 and 2018 the length of the questionnaire was shortened, the ASEBA Brief Problem Monitor (BPM) was used in stead of the CBCL, and the 'cito survey' was added (see YS_CITO). From 2019, the questionnaire returned to its original length with CBCL and Conners items, and from 2020 the items on leisure time and on family and living situation were removed.
