Does cognitive ability change together, or do they change independently of each other? A team of researchers investigated cognitive development in adulthood.
People usually find it easier to learn something new at age 20 than they do at age 70; however, older individuals are typically more knowledgeable than their younger counterparts. In lifespan psychology, this is known as the difference between “fluid” and “crystallised” cognitive ability. Fluid cognitive ability primarily captures individual differences in brain integrity at the time of measurement, whereas crystallised cognitive ability captures individual differences in accumulated knowledge.
An international research team from the USA, Sweden, and Germany, with the support of the Max Planck Institute for Human Development, have presented their findings in the publication Science Advances.
Fluid and crystallised ability
Fluid and crystallised cognitive abilities differ in their average trajectories. A fluid cognitive ability like memory starts to decline in middle adulthood; contrastingly, crystallised abilities such as vocabulary increase until later adulthood and only appear to decline in advanced old age.
The divergence in the average trajectories of fluid and crystallised has led to the assumption that people can compensate for fluid losses with crystallised gains. For example, if an individual’s memory declines can be compensated for by an increase in knowledge.
The researchers highlighted in their study that this compensation hypothesis is more limited than previously claimed. They analysed data from two longitudinal studies, the Virginia Cognitive Aging Project (VCAP) study from the USA and the Betula study from Sweden. In the VCAP study, 3633 female and 1933 male participants aged 18-99 years at the first occasion of measurement were followed for a period of up to 18 years and assessed up to eight times. The Betula study involved 1803 women and 1517 men who were aged between 25 and 95 years old at the first measurement occasion and examined up to four times over 18 years.
The research team used multivariate methods of change measurement to examine the extent to which individual differences in changes in crystallised cognitive ability are related to individual differences in fluid changes. The findings were clear: the correlations between the two types of changes observed in both studies were very high. The individual differences in cognitive ability development are, to a large extent, domain-general and do not follow the fluid-crystallised divide. This means that individuals who show greater losses in fluid abilities simultaneously show smaller gains in crystallised abilities, and persons whose fluid abilities hardly decline show large gains in crystallised abilities.
Maintaining cognitive ability
The findings follow the everyday observation that some people remain mentally fit in many areas into very old age whilst others’ cognitive ability functioning declines across the board.
“In intelligence research, people often talk about a general factor or g-factor of intelligence that expresses the commonality of different cognitive abilities,” said the lead author of the study, Elliot Tucker-Drob of the Department of Psychology and the Population Research Center at the University of Texas at Austin, USA. “In previous work, we have already demonstrated that not only individual differences in cognitive abilities at a given point in time can be captured by a general factor, but also changes of cognitive ability. Our new results confirm this finding and demonstrate that changes in crystallised abilities can indeed be subsumed under a general factor of common change.”
“Our findings call for a revision of textbook knowledge,” added Ulman Lindenberger, Director of the Center for Lifespan Psychology at the Max Planck Institute for Human Development in Berlin. “If those who show the largest fluid losses also show the smallest crystallised gains, then this places tighter limits on the compensatory power of knowledge than previously believed.”
For example, people whose memory is declining also show a low gain in knowledge, even though they are in most need of such gains. Conversely, individuals with small fluid losses and strong crystallised gains are less likely to need to rely on compensatory processes to begin with.
Overall, the results underscore the great importance of identifying and supporting modifiable influences that contribute to the general maintenance of cognitive ability in later adulthood and old age. An example is physical exercise that can prevent cardiovascular diseases and thereby help to maintain cognitive ability.