Human intelligence
Process, personality, intelligence and knowledge theory (PPIK)

Developed by Ackerman, the PPIK (process, personality, intelligence, and knowledge) theory further develops the approach on intelligence as proposed by Cattell, the Investment theory, and Hebb, suggesting a distinction between intelligence as knowledge and intelligence as process (two concepts that are comparable and related to g and g respectively, but broader and closer to Hebb's notions of "Intelligence A" and "Intelligence B") and integrating these factors with elements such as personality, motivation, and interests.

Ackerman describes the difficulty of distinguishing process from knowledge, as content cannot be eliminated from any ability test.

Personality traits are not significantly correlated with the intelligence as process aspect except in the context of psychopathology. One exception to this generalization has been the finding of sex differences in cognitive abilities, specifically abilities in mathematical and spatial form.

On the other hand, the intelligence as knowledge factor has been associated with personality traits of Openness and Typical Intellectual Engagement, which also strongly correlate with verbal abilities (associated with crystallized intelligence).
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