ABSTRACT
This chapter suggests familial influences in the etiology of at least two minimal brain dysfunction (MBD) symptoms, learning difficulties and hyperkinetic-impulsive behavior. Patterns of familial resemblance in hyperkinetic-impulsive behavior are most consistent with a polygenic mode of inheritance. The chapter describes the results of familial analyses in a subsample of the MBD cohort. The results of the sibling analyses are consistent with the limited twin data that suggested a familial component for each individual MBD symptom. Estimated risks for abnormal MBD factor scores among siblings of affected children were compared to the risk among siblings of children with no abnormal scores. Because the genetic correlation between full siblings is approximately twice that for half-siblings, the phenotypic correlations or estimated risks for traits with a multifactorial (polygenic) basis should be roughly twice as high for full siblings as for half-siblings.
