Does intelligence increase or decrease during adulthood? This question has plagued psychologists for decades. Cross‐sectional studies of IQ tend to show that young adults perform better than middle‐aged or older adults, while longitudinal studies of IQ appear to indicate that people increase in intelligence through the decades, at least until their 50s. But the issue of intellectual development in adulthood is not so straightforward or simple. The results of the cross‐sectional studies—younger adults, as a group, do better on IQ tests—may be due more to cohort influences,
such as longer schooling or greater exposure to television than that enjoyed by the previous generation, than to aging influences. The results of the longitudinal studies—over time, persons do better on IQ tests—may be due to the effects of practice, increased comfort taking such tests, or the tendency for those who remain in the studies to perform better than those who drop out.
Attempts to measure IQ are complicated by the fact that there are different types of intelligence. Crystallized intelligence is the ability to use learned information collected throughout a lifetime, and fluid intelligence is the ability to think abstractly and deal with novel situations. Young adults tend to score higher on tests of fluid intelligence, while middle adults tend to score higher on tests of crystallized intelligence. Variables unique to young, middle, and older adults complicate any comparison of IQs among the groups. All things considered, the results of traditional IQ tests suggest that intelligence usually continues at least at the same level through young and middle adulthood.
Young adult thinking, especially in a person's early 20s, resembles adolescent thinking in many ways. Many young people see life from an idealistic point of view, in which marriage is a fairy tale where lovers live happily ever after, political leaders never lie or distort the truth, and salespeople always have consumers' best interests in mind. People in their 20s have not always had the benefit of multiple life experiences, so they may still view the world from a naively trusting and black‐or‐white perspective. This is not to say that young adults do not question their world, challenge rules, or handle conflicts. These, and more, are normal developmental tasks that lead to realistic thinking and recognition of life's ambiguities. But until young adults reach that level of thinking, they may want absolute answers from absolute authorities.
Many young adults—particularly those who have attended college—develop the ability to reason logically, solve theoretical problems, and think abstractly. They have reached Piaget's formal operations stage of cognitive development. During this stage, individuals can also classify and compare objects and ideas, systematically seek solutions to problems, and consider future possibilities.
As young adults confront and work through the gray areas of life, some may go on to develop postformal thinking, or practical street smarts. Developing the wisdom associated with postformal thinking is a lifelong process, which begins in the teenage years and is fully realized in the older adult years.