The continuing economic doldrums have many of us casting a worried eye on our retirement accounts. But in order to assure ourselves of a comfortable old age, there’s another fund on which we should be keeping tabs—a mental one. Each of us should be asking ourselves: How deep is my cognitive reserve?

Cognitive reserve is the term scientists use to describe the extent of the brain’s capacity to resist aging and degenerative conditions like Alzheimer’s disease. The notion that such a capacity could exist originated in a surprising discovery made almost 25 years ago, when the brains of 137 elderly residents of a nursing home were dissected after their deaths.

Remarkably, researchers failed to find a direct relationship between the degree of Alzheimer’s disease detected in the residents’ brains (revealed by the presence of structures called plaques) and how impaired they had been while they were alive. In other words, some of these individuals were able to resist the ravages of the illness better than others—but how?

The neuroscientists from the University of California, San Diego, reported that the subjects whose abilities were less affected by Alzheimer’s were those with bigger brains and a greater number of neurons—suggestive evidence that keeping their brains active had built a bulwark against decline.

Since then, the idea that a deep cognitive reserve provides protection against mental aging has received ever more support. The latest study, to be published in the journal Neurology, was posted online this month. A team led by Robert Wilson of Rush University Medical Center in Chicago enrolled almost 300 elderly people, testing their thinking and memory skills each year as they grew older. The researchers also asked participants about how often they read, wrote, and engaged in other mentally stimulating activities—not just currently but in childhood, young adulthood and middle age.

Following each participant’s death his or her brain was examined, and after accounting for physical evidence of dementia, the scientists produced an amazing finding: people who made a lifelong habit of lots of reading and writing showed a rate of cognitive decline that was 32 percent slower than that experienced bythose who engaged in only average levels of these activities. Compared to the average folks, people who rarely read or wrote experienced a decline that was 48 percent faster.

Reading and writing aren’t the only ways to augment your cognitive reserve. Research on bilingualism by Ellen Bialystok of York University in Toronto, for example, has demonstrated that speaking more than one language is associated with an average delay of five years in the onset of Alzheimer’s symptoms. Bialystok theorizes that the lifelong mental exercise required to speak multiple tongues—remembering which word belongs to which language—helps bilinguals build up their mental storehouse.

And Nina Kraus of Northwestern University has found that people who spend many years practicing a musical instrument are better able to respond quickly and accurately to sounds that they hear. Kraus reported that middle-aged musicians outperformed not only their non-musician peers but also non-musicians many years their junior. The mental rigor required by the practice of music may have effectively acted as an antidote to aging, keeping their nervous systems youthful.

We’ve all been taught the importance of beginning early in saving money for retirement. Accumulating mental capital—by reading and writing, speaking a second language, or practicing a musical instrument—may well work the same way. If you want a generous cognitive reserve to see you through your golden years, you’d better start contributing now.

UPDATE: On Twitter, reader Caleb Crain wrote, “Correlation isn’t causation. Not reading might simply be an early sign of cognitive decline.” The 2013 Wilson study addresses this point:

“Participation in cognitively stimulating activities has been associated with reduced late-life cognitive decline in most although not all studies, but the mechanisms underlying the association are not well understood. One idea is that cognitive activity somehow helps delay the cognitive consequences of neuropathologic lesions (cognitive reserve hypothesis ), possibly because of activity-dependent changes in key cognitive systems in the brain.

Alternatively, cognitive inactivity may be a consequence of neuropathologic lesions rather than a risk factor (reverse causality hypothesis ). Establishing the direction of the association between change in cognitive activity and cognitive function over time could differentiate the hypotheses, but this has been difficult to do in observational studies. Determining whether the association between cognitive activity and cognitive decline is attributable to (reverse causality hypothesis) or independent of (cognitive reserve hypothesis) neuropathologic conditions could also differentiate the hypotheses, but little relevant data have been published. We used data from participants in the Rush Memory and Aging Project to test the hypothesis that cognitive activity has an association with cognitive decline that is independent of common neuropathologic conditions . . .

DISCUSSION: Older persons in a longitudinal clinical-pathologic study rated current and past frequency of participation in cognitively stimulating activities and then underwent annual cognitive function testing. During a mean of 5.8 years of follow-up, nearly 300 individuals died and underwent neuropathologic examination. After adjustment for plaques, tangles, infarcts, and Lewy bodies, higher levels of cognitive activity in childhood, middle age, and old age were associated with slower rate of cognitive decline, together accounting for nearly 15% of variability in cognitive decline not attributable to neuropathologic burden.

The finding that more frequent cognitive activity predicts reduced cognitive decline is consistent with most prior research. These results add to knowledge by showing that cognitive activity was related to residual decline in cognitive function after adjustment for the association of neuropathologic burden with cognitive decline. The fact that cognitive activity has an association with cognitive decline that is independent of neuropathologic burden shows that more frequent cognitive activity can counterbalance the cognitive loss associated with neuropathologic condi tions. This finding supports the cognitive reserve hypothesis and suggests that the association of cognitive activity with loss of cognition is not the result of reverse causality.”

SECOND UPDATE: Over the course of today I’ve fielded more comments about whether the 2013 Neurology study shows that cognitive activity across the lifetime causes a slowing of cognitive decline, or whether such activity is merely correlated with a slowing of decline (see the Facebook page of The Brain Cafe for an extended discussion among Christopher Chabris, John Kubie and others). I asked Robert Wilson, lead author of the study, to comment. Here is his response:

“It is nice to see the level of public interest in the issues raised by this research and the level of sophistication in the debate. I very much agree that a correlation between two variables does not mean that one causes the other and that the correlation is sometimes due to a third variable with which they are both correlated. Most longitudinal studies have found, like our article, that persons who report engaging more often in cognitively simulating activities are less likely to experience cognitive decline and dementia in old age compared to less cognitively active people, but a longstanding hypothesis has been that the correlation between cognitive activity and cognitive decline is due to their associations with a third variable, dementia-related pathology in the brain. 

In the article, we controlled for dementia related pathology and showed that there was still a strong correlation between cognitive activity and rate of cognitive decline in old age. I agree that some other “third variable” could still explain some or all of the association (between cognitive activity and cognitive decline), but after more than a decade of research on this topic, it is not clear what that variable is. (Past cognitive activity studies have controlled for a wide range of variables, including education, lifetime socioeconomic status, physical activity, social activity, depression, and measures of physical health.) 

We found that within-person variation in level of cognitive activity across the life span was related to rate of cognitive decline in old age, making it hard to conclude that the results simply reflect the fact that some people were born more cognitively active or with better brains than others. In the article, we review animal research and human longitudinal neuroimaging studies documenting changes in brain structure and function in conjunction with systematic variation in experience. 

So at this point, I think that the weight of the evidence supports the hypothesis that the association is causal, and we are currently using neuroimaging data from the same parent study to try to identify aspects of brain structure and function that are related to cognitive activity and account for some or all of its association with cognitive functioning.

I also agree that factors other than cognitive activity contribute to cognitive reserve, including personality traits and other lifestyle activities.”

As for myself, I wish I’d been more cautious in the way I presented the findings initially. I’ve added some caveats to the story above (they’re in bold). I’m grateful to my readers for holding me to a high standard!