gests peak levels of intelligence are found in the 18–25-year-old population
(e.g., see Raven, Court, & Raven, 1977; Wechsler, 1944, 1997).
From a trait-complex perspective, the PPIK approach views particular
complexes as supportive or impeding of the development of domain knowl-
edge, or as supportive of one domain over another; to the degree that the
complexes are associated with both the intensity and the direction of intellec-
tual investment over long time periods. Integrating the PPIK perspective and
the trait complexes found by Ackerman and Heggestad (1997), predictions
can be made for the patterns of influence and development of adult domain
knowledge. Figure 5.1 illustrates the framework and a set of predicted rela-
tions among traditional Gf and Gc ability measures, trait complexes, and
knowledge across domains of physical sciences–technology, civics, humani-
ties, current events, and business. Consistent with Cattell’s theory, traditional
128 ACKERMAN AND KANFER
FIG. 5.1. A conceptual representation of the PPIK theory, including tradi-
tional measures of fluid intelligence (Gf ), crystallized intelligence (Gc), four se-
lected trait complexes, and five knowledge domains. Dotted arrows indicate
negative or impeding influences, and solid arrows indicate positive or suppor-
tive influences.