The consequences of early-onset brain damage for the development of cognition and behavior have recently been identified as top research priorities by the NINDS. This special issue presents a series of new empirical studies that address this issue in depth, from several different perspectives and in both human and animal participants. The focus is on the development of personality, social behavior, and related "executive functions," in subjects who suffered early damage to prefrontal brain regions. A consistent theme throughout is that early-onset prefrontal lesions can frequently lead to severe, intractable deficits in social behavior and moral reasoning and to impairments of executive functions, such as planning, judgment, and decision making.