McCain's fossilized brain

John 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.

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.

All of these behavioral changes have their basis in anatomy. As the brain ages, it starts to look different. It loses about 2 percent of its weight and volume every decade, starting around age 20. Certain areas of the cortex get smaller and thinner, and its grooves and ridges become more pronounced. The spaces between the gyri—the wormy coils you might call brains—often get wider as well.

The brain's chemistry also changes with age. Levels of dopamine, a neurotransmitter associated with sensations of happiness, generally decline. (Some schizophrenics, who experience abnormally high dopamine levels, recover suddenly when they reach a certain age.) Same with hormones like testosterone; on average, men in their 80s have testosterone levels 40 percent lower than men in their 20s. (Forget McCain, just ask Bob Dole.) The behavioral effects of lowered testosterone aren't known, but it's correlated with weaker cognition. But as recent history has shown, a president with low testosterone isn't necessarily a bad thing.

Roberto Cabeza of Duke University has conducted studies showing older adults often perform just as well as their younger counterparts because of what he calls "compensation." If one area of the brain is weaker, the theory goes, other areas will pick up the slack. For example, while younger people use the right side of their prefrontal cortex for high-level cognitive processing, some elderly people use the left side as well, resulting in equal performance.