I am now 59 years old. (I was born in 1960.) Like many people my age, I am terrified of cognitive decline with advancing age. In “Hints About What Can Be Done to Reduce Alzheimer's Risk,” I discussed what little can be done about one source of cognitive decline. But there are other sources of cognitive decline with advancing age. In their Journal of Economic Perspectives article “Mental Retirement,” Susann Rohwedder and Bob Willis provide the best available evidence for the welcome idea that continued, strenuous mental activity can stave off cognitive decline. They describe the hypothesis they are testing this way:

The phrase “use it or lose it” reflects a hypothesis contained in a large popular and scholarly literature to the effect that a person can stave off normal cognitive aging— the decline of reasoning ability and speed of mental processing with age—or even dementia by engaging in cognitively demanding activities that exercise the mind. Conversely, this hypothesis holds that an undemanding environment will fail to impede and may even accelerate the process of cognitive decline.

The big problem with trying to find evidence for the idea that strenuous mental activity can slow down cognitive decline in old age is that cognitive decline—or simply a lower level of cognitive acumen—can lead people to pursue less strenuous mental activity. That is, a positive correlation between strenuous mental activity and cognitive status doesn’t tell one whether strenuous mental activity improves cognitive status or good cognitive status leads to more mental activity. (Cue the phrase “reverse causality.”)

I know both Bob and Susann well. (Bob Willis is my coauthor for the paper “Utility and Happiness,” which I mention in “My Experiences with Gary Becker.”) The way they identify the effect of strenuous mental activity on cognitive status is to look at differences in age of retirement across countries that are due to differences in the social security/public pension systems in those countries. Working is more strenuous mental activity than many other things one might do. And there are many plausible political reasons social security/public pension systems differ across countries that have nothing to do with dealing with a country’s especially high or low level of cognitive decline in old age. In their abstract, they write:

We investigate the effect of retirement on cognition empirically using cross-nationally comparable surveys [the Health and Retirement Study (HRS) and its clones] of older persons in the United States, England, and 11 European countries in 2004. We find that early retirement has a significant negative impact on the cognitive ability of people in their early 60s that is both quantitatively important and causal. Identification is achieved using national pension policies as instruments for endogenous retirement.

Susann and Bob argue as follows. First, there is a correlation between early retirement and cognitive decline across countries. The horizontal axis of their graph below shows change in employment rate comparing ages 60-64 to ages 50-54 in each country, while the vertical axis shows change in “a cognitive test of immediate and delayed word recall” between those same two age ranges.

Second, they argue that most of the variation in the age-related change in employment on the horizontal axis is exogenous:

The literature on the determinants of cross-country differences in retirement behavior provides strong evidence that these differences are primarily driven by differences in national pension, tax, and disability policies. As a result, most of the cross-country variation in the relative labor supplies of 60-64 year-olds relative to 50-54 year-olds depicted in Figure 1 is a result of national policies. Because it is highly unlikely that these policies have been set in response to observed age patterns in cognitive performance in the respective country’s population, we argue that these policies provide valid instruments to identify the causal effect of retirement on cognition even in micro-data, where problems of reverse causation tend to have rendered identification difficult.

Third, they pursue an explicit instrumental variables strategy using the micro data in the HRS and its sister surveys:

Our instrumental variables estimation, as always, can be viewed as a two-stage process. In the first stage, we use retirement status as our dependent variable and national pension policies as the explanatory variables: whether the individual has reached the age of eligibility for early retirement benefits in the public pension system and whether the individual has reached the age of eligibility for full retirement benefits. We also investigate specifications that use as additional instruments the number of years to or since reaching the age of eligibility for early retirement benefits (eligibility age minus current age); and the number of years to or since reaching the age of eligibility for full retirement benefits.

Susann and Bob find large effects:

We find a large and significant effect of retirement (or more precisely of “not working for pay”) suggesting that retirement is associated with a reduction in the memory score of about 4.7 points on a scale from 0 to 20 compared to those who continue working. The average score in the sample is just under 10 and the standard deviation is 3.3. So the estimated effect amounts to just under 1.5 times the standard deviation of the cognitive score in our analytical sample.

“Significant” in this case means a p-value of below .05%, which meets the standard I advocate in “Let's Set Half a Percent as the Standard for Statistical Significance.”

Note that this doesn’t mean that brain-exercise apps will do a lot of good. The dosage is much larger from having a job than from doing a brain app for 15 minutes a day. Susann and Bob make that point:

As we discussed at the beginning of the paper, Salthouse (2006) offers reasons to be skeptical of evidence purporting to demonstrate that mental exercise reduces the rate of cognitive decline. Nonetheless, we believe that the unengaged lifestyle hypothesis may be a plausible explanation for the mental retirement effect for several reasons. First, unlike many of the interventions discussed by Salthouse, like crossword puzzles and card games, retirement represents a major change in a person’s lifestyle and activities and thus affords the potential for a large effect. Second, the range of cross-country variation in age of retirement due to differences in policy is also large. Finally, the ten-year span between ages 50-54 and 60-64 in Figure 1 is long enough to indicate that the mental retirement effect represents a change in the rate of cognitive decline, rather than a short-term effect of retirement itself.

I don’t think it has to be a job to be strenuous enough mental activity to keep one mentally sharp, but it has to be something big.

Susann and Bob propose two mechanisms for why having a job or something just as cognitively taxing might help stave off cognitive decline [numbering added]:

1. For many people retirement leads to a less stimulating daily environment. In addition, …

2. … the prospect of retirement reduces the incentive to engage in mentally stimulating activities on the job.

Susann and Bob end on a cheery note:

There is evidence that older Americans have reversed a century-long trend toward early retirement and, during the past decade, have been increasing their labor force participation rates, especially beyond age 65. This is good news for the standard of living of elderly Americans, as well as for the fiscal balance of the Social Security and Medicare systems. Our paper suggests that it may also be good news for the cognitive capacities of our aging nation.

I should say that there are other interesting things in the paper. For one thing, distinction between “fluid intelligence” and “crystallized intelligence” and their different time paths is worth knowing. For another, Susann and Bob have a nice summary of Kerwin Charles’s paper on retirement. (Kerwin Charles is a former University of Michigan colleague of mine.) Here is their description of Kerwin’s paper:

The problem of endogenous retirement makes it difficult to interpret many of the published estimates of the effect of retirement on various measures of health. In an interesting exception, Charles (2002) addresses the question “Is Retirement Depressing?” Reviewing the literature, he notes that there is often a positive correlation in cross-sectional data between symptoms of depression and retirement status—the retired are more depressed. However, using longitudinal data from the HRS and instrumental variable techniques, Charles finds that people who are depressed tend to select into retirement and, more surprisingly, once retired, their depression lessens, a causal effect that is opposite in sign to the correlation. His identification strategy uses policy variations concerning mandatory retirement and Social Security benefits that influence retirement incentives by age and cohort.

Conclusion: I plan to do everything I can to stay cognitively sharp in old age. To begin with, I don’t plan to officially retire until around age 72, and maybe later. (This will help a lot financially, too.) Then, I plan to keep doing research, learning more cool math, reading, and blogging (and maybe writing a book or two) after I officially retire. I won’t give up on making mental exertions, even if I suffer the grief from outliving near and dear friends and loved ones.

For annotated links to other posts on diet and health, see:

• Miles Kimball on Diet and Health: A Reader's Guide