Trait Complexes, Cognitive Investment, Domain Knowledge 11
example, chemistry, physics, technology). Conversely, individuals with
high scores on the Social trait complex showed lower levels of knowl-
edge in all domains – even in domain knowledge about business.
Study 2
A second study centered on a cross-sectional investigation of 228 adults
between the ages of twenty-one and sixty-two (Ackerman, 2000). The
subjects were administered a large battery of ability tests to specifically
assess Gf and Gc, a battery of personality and interest measures, and a set
of eighteen knowledge tests. The study was designed to address three
questions derived from Ackerman’s PPIK theory: (1) Are middle-aged
adults more knowledgeable than younger adults (or at least equally
knowledgeable)? Or more generally, what is the relationship between
age and individual differences in knowledge? (2) Are individual dif-
ferences in knowledge well accounted for by traditional measures of Gf
and/or Gc? and (3) What are the non-ability correlates of individual dif-
ferences in knowledge? The study generated the following conclusions:
1. Supporting evidence was found for a coherent view of adult
intelligence-as-knowledge, that is, in turn, quite different from
the extant data and discussion of adult intelligence as representing
only abstract reasoning or working memory abilities. First, there
were significant positive correlations between age and knowledge
scores in ten of the eighteen domains we investigated. Five of
the remaining correlations between age and knowledge showed
no significant relationship with age, and only the remaining three
knowledge scales showed significantly negative correlations with
age – all three were science tests (chemistry, physics, and biology)
that were also the most highly correlated with Gf (in contrast to
Gc). Overall, a single composite score computed across all the
knowledge scales yielded a correlation of .19 ( p < .01) between
age and knowledge, indicating that at least across the domains
and participants we sampled, older adults were on average more
knowledgeable than younger adults. For comparison purposes:
Gf yielded a correlation of −.39, ( p < .01) with age; and Gc yielded
a correlation of +.14, ( p < .05).
2. The results of the analyses to determine the respective contribu-
tions of Gf and Gc to predicting individual differences in knowl-
edge were differentiated by knowledge domain. Gf had a quite
12 Philip Ackerman and Margaret E. Beier
considerable explanatory power in predicting knowledge in the
science domain, accounting for 38.5 percent of the variance in the
Science composite scores. It had a much diminished role in ac-
counting for individual differences in any of the other areas we
tested, accounting for less than 15 percent of the variance. In con-
trast, Gc accounted for an additional 34 percent of the variance in
Civics knowledge and 42.8 percent of the variance in the Human-
ities, with a lesser role in Science and in Business/Law. A reason-
able conclusion from these results is that Gf is mostly related to
Science knowledge, Gc is mostly related to Civics and Humani-
ties knowledge, but there is much variance in knowledge that is
unaccounted for by these traditional intelligence assessments.
3. Selected personality traits of Social Closeness, Traditionalism, and
Typical Intellectual Engagement accounted for significant vari-
ance in knowledge in every domain except for Business/Law. In-
dividual differences in Realistic, Investigative, and Artistic inter-
ests accounted for significant amounts of variance in knowledge
for all the broad domains we assessed. After trait measures were
considered, individual differences in educational attainment and
age provided relatively little additional explanatory power to pre-
dicting knowledge, suggesting that age may only be a useful pre-
dictor of knowledge in the absence of measures of relevant traits.
As such, the influence of chronological age, in and of itself, on in-
dividual differences in knowledge may be substantially overem-
phasized. Personality/interest/self-concept trait complexes mea-
sures accounted for a substantially greater amount of variance in
domain knowledge than did age.
4. The coherence between trait complexes and cognitive investment
over time are illustrated by the pattern of trait complex scores
across college majors. Figure 1.5 provides a breakdown of mean
trait complex scores by college major, for domains of physical
sciences, social sciences, arts/humanities, and business. Partici-
pants who had majored in one of the physical sciences showed
higher levels of Science/Math-oriented personality/interest/
self-concept traits, but lower levels on Social and Intellectual/
Cultural trait complexes. In contrast, participants who majored in
arts and humanities fields had higher scores on the Intellectual/
Cultural trait complex and lower scores on the Science/Math trait
complex. Social science majors were much more differentiated,
in that they did not show a coherent pattern of different trait
