Background: Cognitive and family factors are implicated in the origins of behavioural problems, but little is known about their independence or interplay. Methods: We present data from 127 two-year-olds from predominantly disadvantaged families who completed tests of "theory of mind" (ToM), executive function (EF) and verbal ability. Researchers' home-visit ratings and detailed video-based coding of mother-child interactions were combined to give an aggregate measure of harsh parenting, while behavioural problems were indexed by a multi-informant, multi-setting, multi-measure aggregate. Results: Harsh parenting and deficits in ToM and verbal ability each predicted unique variance in behavioural problems; independent effects of EF were only marginally significant. Harsh parenting and ToM interacted significantly in their effects on behavioural problems. Conclusions: Child and family influences on behavioural problems should be considered in tandem, as they show significant interplay; in particular, advanced ToM skills appear to buffer young children against effects of harsh parenting.