The Seattle Longitudinal Study of Adult Intelligence has followed a group of more than 5000 people for well over four decades. The program began in 1956 and participants have been tested across a whole gamut of mental and physical abilities at seven year intervals since that date.
The study has found:
- no uniform pattern of age-related change across all intellectual abilities
- some support for the idea that abilities that are primarily genetically determined tend to decline earlier than abilities that are primarily acquired through schooling or experience (although there may be gender differences here)
- although abilities that are primarily genetic may decline earlier, abilities acquired through training decline more steeply after late 70s the change in perceptual speed begins in young adulthood and declines in a linear fashion (that is, the rate of decline is constant)
- the rate and magnitude changes in intelligence seen in those entering old age showed greater decline in the 1st 3 cycles (till 1970); at the same time, younger members are scoring lower on tests at the same age.
- a decline in psychometric abilities is not reliably observed before 60, but is reliably observed by 74. However, even by 81, fewer than half showed reliable decrements over the past seven years.
- the size of this decline however is significantly reduced when age changes in perceptual speed are taken into account.
- substantial cohort / generational differences have been observed. Later-born groups have attained successively higher scores at the same ages for inductive reasoning, verbal meaning, and spatial orientation; however, they’ve scored successively lower in number skill and word fluency (number skill peaked with the 1924 cohort). These changes presumably reflect educational changes.
- substantial similarity between parents and their adult children and between siblings has been found for virtually all mental abilities and measures of flexibility (the exceptions are the attitude measure of social responsibility, and a measure of perceptual speed). The magnitude of similarity varied for different abilities, and was closer between parent & child than between siblings.
- the following variables may reduce the risk of cognitive decline in old age:
- absence of chronic diseases
- a complex and intellectually stimulating environment
- a flexible personality style at mid-life
- high intellectual status of spouse
- maintenance of high levels of perceptual processing speed
- cognitive training studies suggested that the observed decline in many community-dwelling older people is probably a function of disuse and is often reversible. Some 2/3 of participants in a cognitive training program showed significant improvement, and 40% of those who had declined significantly were indeed returned to their earlier (pre-decline) level of cognitive functioning. These training gains were retained over seven years.