McCain's BrainHow might the senator's mind deteriorate over the next eight years?

John McCainJohn McCain is old and, as several news outlets have reported, he's only getting older. This wouldn't be a problem if his opponent in the general election were equally chronologically gifted. But Barack Obama is one of the youngest Democratic nominees ever. If he won, he'd be the third-youngest U.S. president. The contrast is stark. Standing side-by-side, the two candidates look like a still from Finding Forrester.

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McCain's age is already causing problems. His slip-ups are billed as "senior moments," while Obama's are innocent mistakes. In response, the McCain campaign has mastered the swift and stinging rebuke. When Obama backhandedly praised McCain's "50 years of service," McCain's camp cried foul. When Obama suggested McCain was "losing his bearings," strategist Mark Salter wrote a memo accusing Obama of ageism. To calm speculation, the McCain camp released 1,173 pages of McCain's medical records last month. Despite repeated operations to treat melanoma, he's quite fit. The pages did not include mental-health records, presumably because McCain has not had any mental evaluation in the past eight years. (The records McCain released during the 2000 campaign showed no lasting psychological wounds from his years as a prisoner of war.)

But that doesn't change the fact that a 72-year-old's brain is different from that of a 46-year-old. Even the most healthy human brains undergo chemical and anatomical changes in their sunset years—changes that can and do affect behavior. We don't know the details of McCain's mental health, but it's worth examining what happens to the minds of normal people at his age.

As everyone with a grandparent knows, certain types of memory are affected by aging. Episodic memory—the ability to remember things that happened to you—declines. Same for prospective memory, or the ability to remember lists or agendas. You could argue these skills are less essential for a president, who has speechwriters to produce anecdotes and handlers to keep his schedule. But age also affects working memory, which we use to process, sort, and recall information on the fly. Mental arithmetic, for example, requires a good working memory. Fortunately, presidents have calculators. But working memory also translates into debating skills—the better your short-term retention, the better you can rebut your opponent's arguments.

Oldsters show fewer deficits in semantic memory, which includes vocabulary and general knowledge. So while McCain's ability to synthesize facts into a compelling argument might degrade over time, his verbal arsenal is likely to grow. Semantic memory generally peaks in the sixth decade of life and doesn't decline until the ninth. Similarly, behaviors that are considered overlearned—actions you repeat over and over, like driving a car or saying that cutting taxes raises revenue—are less easily forgotten.

An elder president might show some other cognitive symptoms, too. Aging is known to affect our ability to suppress an instinctual response to a given stimulus. A famous example is the "Stroop interference" test. Imagine you're looking at the word blue written in red ink. Now say the name of the color of the ink. Your instinct is probably to say blue, but you force yourself to say red. The speed with which you complete the task is supposedly an indicator of mental flexibility; older people generally take longer to say red. Likewise, inhibition control declines over time. If you're at a party and your nemesis walks through the door, you might be inclined to sock him in the nose. Instead, you shake his hand. Older people aren't likely to feel any different—they're just less likely to suppress their first instinct.

This doesn't explain McCain's famous temper; if anything, he has mellowed of late. But it does suggest that we may see a further development of McCain's shoot-from-the-hip style. When a questioner in New Hampshire asked what he thought of "staying in Iraq for 50 years," McCain cut him off: "Make it a hundred." The line has dogged McCain since, largely because of its lack of context. If he had resisted the urge to jump in, it would have been harder for opponents to distort his words.

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Christopher Beam is a Slate political reporter. Follow him on Twitter.

Photograph of John McCain by Paul J. Richards/AFP/Getty Images.

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