Genetic and environmental contributions to stability and change of ADHD symptoms between 8 and 13 years of age: a longitudinal twin study.

To study the genetic and environmental contributions to stability and change of attention-deficit/hyperactivity disorder (ADHD) symptoms between 8 and 9 and 13 and 14 years of age. The sample included 1,480 twin pairs born in Sweden between May 1985 and December 1986. At wave 1 in 1994, when twins were 8-9 years old, 1,106 (75%) of the parents responded to a mailed questionnaire, and at wave 2 when the twins were 13-14 years old, 1,063 (73%) responded. A checklist with 14 items based on the 14 DSM-III-R symptoms for ADHD was completed. Structural equation modeling was used to analyze the data. A relatively high stability of ADHD symptoms over this 5-year period was found. This continuity was mainly due to the same genetic effects operating at both points in time. Change in symptoms between childhood and early adolescence was to a large extent due to new genetic effects in early adolescence but also due to new nonshared environmental effects that became important during adolescence. The genetic stability highlights the importance of the continuing search for genes and endophenotypes of ADHD.