Brain, environment, heredity, and personality.

Behavior genetics research, committed to a model of the origins of personality and intellectual development restricted to heredity and environment, consistently generates evidence for two types of environmental influences. One of these, termed the between-family or shared environment, creates no conceptual problems. The other, termed the nonshared or within-family or unique environment, does. The other, termed the nonshared or within-family or unique environment, does. The latter accounts for a substantial portion of the variance in personality and intellect, it must be unique to each individual, and its origin has not been determined. It is contended here that the brain itself, as an inherently indeterminate dynamic system, is the source of this unresolved developmental variability.