Does Cognitive Decline Start in Early Adulthood?

If you think you don't have to worry about your brain performance slipping until you are sixty, a new study says you may be off by four decades.

The University of Virginia's Timothy Salthouse writes in Neurobiology of Aging that there is an answer to a longstanding puzzle about why some studies show brain performance declining in the twenties, while other research doesn't show a drop until about sixty. And unfortunately, his answer isn't the one middle-aged people want to hear.

Salthouse's analysis comes down to the difference between cross-sectional and longitudinal studies.

A cross sectional study collects data about hundreds or thousands of subjects at one point in time, and then compares (say) people in their twenties to similar men and women in their forties, sixties, and so on. Problems in these studies occur if there are differences in the cohorts -- for instance, the generation that grew up in the Depression had significantly lower caloric intake than children twenty years later, and that may have influenced a wide variety of differences between the two groups. Apples, meet oranges.

A longitudinal study is generally thought to generate better results, because it follows the same people over time. This research is more difficult because of the time involved -- data might not be available for fifty or more years if you want to see what youthful behavior and environmental influences ultimately affect health in old age. Some ongoing studies have been nurtured by generations of researchers and grad students.

The argument that cognitive decline doesn't begin until about sixty rests on longitudinal studies, which would normally close the debate. Salthouse, however, says 'not so fast' -- cross sectional studies consistently show that brain performance declines from the twenties, so why the discrepancy?

What if the very act of taking the tests improves test-taking skill enough that it covers up an underlying dropoff? Salthouse retested subjects to measure this effect. The abstract of his recent paper "When does age-related cognitive decline begin?" concludes

Results from three methods of estimating retest effects ... together with results from studies comparing non-human animals raised in constant environments and from studies examining neurobiological variables not susceptible to retest effects, converge on a conclusion that some aspects of age-related cognitive decline begin in healthy educated adults when they are in their 20s and 30s.

Performance in some tests peaked at 22; declines in some aspects of cognitive speed, abstract reasoning, and puzzle solving were evident by 27.

Salthouse's conclusions are contested by other researchers, including three in the same issue of Neurobiology of Aging where his new paper appears.

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