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In Part III of this three-part series on multifactor-system theory, multivariate, life-span development is approached from the standpoint of a quantitative and qualitative analysis of the ontogenesis of factors in each of the six systems. The pattern of quantitative development (described via the Gompertz equation and three developmental parameters) involves growth, stability, and decline, and qualitative development involves changes in the organization of factors (e.g., factor differentiation and convergence). Hereditary and environmental sources of variation are analyzed via the factor gene model and the concept of heredity-dominant factors, and the factor-learning model and environment-dominant factors. It is hypothesized that the sensory and motor systems are heredity dominant, that the style and value systems are environment dominant, and that the cognitive and affective systems are partially heredity dominant.

Highly inbred mice of fast and slow conditioning strains (N = 228) were trained in avoidance conditioning and 180 were continued to extinction. The principal goal was to determine optimal stimulus conditions for fast learning, small variances, and maximal strain differences. Conclusions are: a) the major effect of change in CS-US and intertrial intervals occurs, respectively, for intervals less than 3 sec. and 60 sec.; b) change in amperage (150-400 pa) or voltage (250-400 v.) caused no significant differences; c) strain differences in extinction occurred after nearly optimal conditioning, and d) strain differences were altered considerably as stimulus conditions became optimal. Optimal levels of parameters are: 3 sec. CS-US interval, 120-sec. intertrial interval, and 400 v. (220 K ohm impedance circuit).