Although infant attachment has been long seen as key for development, its long‐term effects may be complex. Attachment may be a catalyst or moderator of future developmental sequelae rather than a source of main effects. In 102 mothers, fathers, and infants, attachment was assessed at 15 months; children's negativity (rejection of parental rules and modeling attempts) at 25, 38, 52, and 67 months; and developmental outcomes (the child's parent‐rated externalizing problems and the parent–child observed relationship quality) at ages 10 and 12. In both mother–child and father–child relationships, children's higher negativity was associated with more detrimental outcomes but only in dyads with formerly insecure infants. Infant insecurity appears to amplify detrimental cascades, whereas infant security appears to defuse such risks.