Many of the studies on age-related cognitive decline have been cross-sectional studies. These studies examine different groups of people who are different ages. Comparing different people has some drawbacks, largely due to the wide variations that can occur between people, even who are the same age. A better way of assessing cognitive changes with age is to follow the same group of people over their lifetime to see what happens. This is called a longitudinal study.

K. Warner Schaie is conducting this type of study, which has been going on for over 35 years. The Seattle Longitudinal Study of Adult Intelligence involves more than 5,000 people who have been examined for cognitive abilities every seven years starting in 1956. The study is examining whether intelligence changes uniformly throughout adulthood, at what age deficits in ability occur, what causes differences among individuals, and whether intellectual decline with age can be reversed.

The study measures intelligence using tests of verbal meaning (recognize and comprehend words), spatial ability (mentally rotate objects in space), reasoning (identify regularities and principles of rules), number skill (arithmetic computation), and word fluency (recalling words that fit into categories, such as names of flowers).

In general, Dr. Schaie has discovered that deficits in these abilities were not seen before age 60, and by age 74 a small decline was observed, on average, for all the abilities measured. However, even at age 81, fewer than half of the people tested showed significant declines during the previous seven years.

In addressing the last question (whether intellectual decline with age can be reversed), Dr. Schaie has found that use of an educational program can indeed help to reverse the cognitive effects of age. Two-thirds of the people given this program showed significant improvement, and 40% returned to pre-decline cognitive performance levels. The effects were maintained even after seven years, sometimes with periodic booster sessions.