Electrodermal hyporeactivity (or low skin conductance level, SCL) has been long established as a correlate of and diathesis for antisocial behavior, aggression, disregard for rules of conduct and feelings of others, and generally, externalizing behavior problems in children and adults. Much less is known, however, about how individual differences in children's SCL and qualities of their early experiences in relationships with parents interact to produce antisocial outcomes. In a community sample of 102 families (51 girls), we examined children's SCL, assessed in standard laboratory tasks at age 8 (N = 81), as a moderator of the links between parent-child socialization history and children's externalizing behavior problems at ages 8 and 10, reported by mothers and fathers in well-established instruments and by children in clinical interviews. Mother- and father-child socialization history was assessed in frequent, intensive observations. Parent-child mutually responsive orientation (MRO) was observed from infancy to age 10, parental power assertion was observed from 15 months to age 6 ½, and children reported their attachment security in interviews at age 8 and 10. For children with lower SCL, variations in mothers' power assertion and father-child MRO were associated with parent-rated externalizing problems. The former interaction was consistent with diathesis-stress, and the latter with differential susceptibility. For children with higher SCL, there were no links between socialization history and externalizing problems.