Co-occurring internalizing and externalizing disorders are moderately prevalent in children, adolescents, and adults (Anderson, Williams, McGee, & Silva, 1987; McConaughy & Skiba, 1994), but much remains to be understood regarding why some children show "pure" versus co-occurring internalizing and externalizing symptoms. One possible influence that has previously not been considered is the failure to attain socio-developmental milestones, which paradoxically may prevent the development of co-occurring symptoms for some children. The present study proposes a model in which failure to attain relevant socio-developmental milestones might explain why some children may not develop heterotypic co-occurring symptoms. Specifically, it is proposed that specific clusters of internalizing symptoms (i.e., high social anxiety, withdrawal, and inhibition) and externalizing symptoms (i.e., high impulsivity, hyperactivity, and emotional reactivity) may be associated with the failure to attain socio-developmental milestones (i.e., poor peer relations for anxious children, lack of self-reflection and evaluation for impulsive/reactive children) that, in turn, may prevent subgroups of children from developing co-occurring, heterotypic symptoms.