I have previously suggested a conceptual framework for the understanding of cohort differences in intelligence to identify those influences in the historical cultural context that might impact cohort differences in both the mean level and trajectory of mental abilities across adulthood (cf. Schaie et al., 2005 ). This conceptual framework, adapted from Bronfenbrenner (1986 ; Bronfenbrenner & Crouter, 1983 ), is designed for the study of the major domains of influence that would provide possible mechanisms for cohort or generational differences in intellectual performance. While Bronfenbrenner ’ s model is ordinarily presented as a series of concentric circles, our framework is presented as a matrix (see Table 3.1 ). This conceptual structure is necessary to make explicit multiple systems of influence at different developmental phases childhood, adolescence, young adulthood, middle age, and old age) across the lifespan. At the core of our framework are the physical and psychological characteristics of the individual.