Correlations are Strong Between Different Aspects of Cognitive Decline
The evidence from this study points to a tight coupling between age-related losses in different areas of cognitive function. This is more or less what one might expect to occur in a complex system that is undergoing chemical and small-scale structural damage. Such damage will tend to affect all emergent properties of the system. The goal of medical research and development should be to find ways to repair that damage and restore function, not attempt to otherwise compensate for ongoing losses.
At the age of 20, people usually find it easier to learn something new than at the age of 70. People aged 70, however, typically know more about the world than those aged 20. In lifespan psychology this is known as the difference between "fluid" and "crystallized" cognitive abilities. Fluid abilities primarily capture individual differences in brain integrity at the time of measurement, whereas crystallized abilities primarily capture individual differences in accumulated knowledge.
Accordingly, fluid and crystallized abilities differ in their average age trajectories. Fluid abilities like memory already start to decline in middle adulthood. In contrast. crystallized abilities such as vocabulary show increases until later adulthood and only evince decline in advanced old age. This divergence in the average trajectories of fluid and crystallized abilities has led to the assumption that people can compensate for fluid losses with crystallized gains. A study now shows that this compensation hypothesis has more limits than previously claimed.
The correlations between the two types of changes observed were very high. Thus, individual differences in cognitive development are, to a large extent, domain-general and do not follow the fluid-crystallized divide. What this means is that individuals who show greater losses in fluid abilities simultaneously show smaller gains in crystallized abilities, and persons whose fluid abilities hardly decline show large gains in crystallized abilities. These findings are in accordance with the everyday observation that some people remain mentally fit in many areas into very old age while others' cognitive functioning declines across the board. 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 crystallized gains are less likely to be in need of relying on compensatory processes to begin with.