These intelligences must be distinct since crystallized intellect grows with age—older folks can do crossword puzzles as well as or better than young people—while fluid intelligence declines with age (Horn, Donaldson, & Engstrom, 1981; Salthouse, 2004). The finding that crystallized intelligence increases with age while fluid intelligence decreases is known as "the old-folks' advantage" (Sternberg, 1983).
It has been suggested that the increase in crystallized intelligence with age may be due to experience and education playing a role in forming concepts and making judgments. These two factors would lead to improved memory performance because information stored in memory will be used when making decisions or solving problems. The old-folks' advantage also has been attributed to seniors using their increased knowledge about life to fill in gaps in their memory banks. For example, studies have shown that older adults are less likely to remember things that didn't matter much to them, such as what they had for breakfast this morning. However, older adults are more likely to remember important events from their past lives and use this information to make good decisions today.
The finding that fluid intelligence decreases with age is not limited to cognitive research. Scientists also report that older individuals suffer from a loss of agility and dexterity which doesn't appear to be related to how many cells remain in their brain. There are several possible explanations for this phenomenon.
How consistent are IQ scores over the lifespan? Cross-sectional research (comparing persons of different ages) and longitudinal studies (testing the same cohort over time) have demonstrated that fluid intelligence diminishes in older adults, owing in part to slower brain processing. However there is also good evidence that cognitive ability remains relatively stable from childhood through old age.
Longitudinal studies have shown that IQ scores are highly correlated across time points for individuals who do not experience major changes in their lifestyle factors such as education or occupational complexity. This indicates that individual differences in cognitive ability are largely stable over time.
Cross-sectional studies have also shown that people with similar lifestyles to which they were born into tend to score similarly on neuropsychological tests. For example, American children who were tested between the ages of 4 and 7 years of age showed a strong correlation between their first-grade IQ scores and their eighth-grade IQ scores (r = 0.74). There was no evidence that these children became dumber over time.
These findings were confirmed by another study which followed a large sample of Canadians aged 14 to 91 years old over 10 years.
While crystallized intelligence would continue to improve throughout adulthood, fluid intelligence would peak in the early 20s and begin to diminish between the ages of 30 and 40. Since then, this has been the dominant intelligence hypothesis. However, some recent studies have suggested that fluid intelligence may actually increase after 60 years of age.
The most widely accepted theory on the decline of fluid intelligence comes from Jaeggi et al. (2008). They argued that the main factor responsible for the decline in fluid intelligence is the increasing number of concepts that need to be maintained in memory. This leads to a trade-off between memory capacity and processing speed, with memory capacity being limited by neurophysiological constraints. Because both cognitive abilities increase with age, older people are able to maintain high levels of performance despite having to process more information than younger people.
However, there are some studies that have found evidence supporting an alternative theory on the decline of fluid intelligence. For example, Oberauer and Sander (2007) argued that while crystallized intelligence shows large individual differences, fluid intelligence only varies slightly across the population. They proposed that because of this fact, older people will start off with higher levels of fluid intelligence than young people, but this difference will decrease over time due to aging processes.
Both forms of intelligence develop during childhood and youth. Throughout maturity, crystallized intellect grows. Many components of fluid intelligence peak in adolescence and begin to deteriorate gradually around the age of 30 or 40. Some studies have even shown that it declines at a rate of 1% per year.
There are several factors that may lead to a decline in fluid intelligence. The most obvious is aging, but depression, stress, disease, and also alcohol use can also contribute to decreased fluid intelligence.
Although aging is the most obvious factor behind fluid intelligence, it is not the only one. Other factors such as disease, depression, and stress can also play a role in the development of dementia. These other factors will be discussed in more detail below.
It is important to remember that everyone experiences age-related changes to their brain function. There are many aspects of cognition that begin to decline with age, including memory, reasoning, perception, language, judgment, and planning.
The good news is that these losses can sometimes be prevented or at least delayed through exercise, nutrition, learning, and engagement with life. The best part is that there are many ways you can keep your mind agile and healthy into old age.