Thirty families, consisting of two parents and two adolescent children, were tested on a high-processing load Continuous Performance Test, the CPT-IP, which required identification of identical stimulus pairs within a continuously presented series of stimuli. The purpose of this study was to provide normative data for research concerned with the role of attention in psychopathology, especially schizophrenia and major affective disorder. Retest data collected from 23 of the 30 families showed the CPT-IP to be a reliable measure of attention. A major developmental effect was found in capacity to sustain attention to spatial vs. verbal stimuli, which suggested that spatial skills are most developed during childhood and adolescence, while verbal attentional skills tend to peak in adulthood. Factor analysis and family transmission patterns further suggested that the two types of attention (spatial and verbal) were independent and that each was heritable to some degree. Experimental distraction did not disrupt performance in any of the subjects and, in fact, tended to improve it in the adolescents, especially for spatial stimuli. We conclude that the CPT-IP is appropriate for use with families containing members differing widely in age and processing skills.