New findings in neurology always seem to come with the caveat that there are subtleties that need to be explained. It is therefore refreshing to consider a big, fat unsubtlety: the size of our brains. At first glance, a big brain's function seems simple: to think big thoughts. And indeed, brain size does loosely correlate with intelligence, between species and, as recent MRI studies confirm, within our own. Yet some people who are missing brain parts remain just fine with what little they've got. The cases have multiplied since brain scans became routine.

Take the 50-something lawyer who, fearing Alzheimer's, came in for an MRI and got good news and bad news. He was fine, but his brain lacked a corpus callosum, the wrist-thick stalk that normally connects the brain's hemispheres. Still, he enjoyed a successful practice and had a verbal IQ of around 130 and a nonverbal IQ above 90.

The patient exhibited subtle signs of abnormal behavior, says Warren S. Brown, a neuropsychologist who studies mind-body questions at the Fuller Theological Seminary in Pasadena, Calif.: "He just seemed odd--not remarkable, but he missed the point of social interaction." Brown adds that patients without a corpus callosum often do not get the point of jokes or understand pictures.

Of course, most brain abnormalities are found because neurologists had reason to look for them. To get around such bias, Elliott Sherr, a neurologist at the University of San Francisco, decided to study all the MRIs his university's hospital had taken. One in several thousand turned out to lack the corpus callosum. "Most times you'd get the 'Aha!' reaction," Sherr says. "So-and-so has had behavioral difficulties, and until then nobody had been able to put a finger on it." But for at least one patient, he adds, the finding had come as a complete surprise.

Equally illuminating are the patients whose corpus callosum has been surgically severed in an effort to control epileptic seizures. Michael S. Gazzaniga, director of the Center for Cognitive Neuroscience at Dartmouth College, observes that their unimpaired intelligence is inexplicable. "How come after splitting the brain, the overall cognitive capacity of the left hemisphere doesn't diminish?" he asks. "After all, half of the cortex is no longer available to it."

Some evidence suggests that people born without a callosum benefit from enhanced development of subsidiary bridges between the hemispheres. Yet even this workaround cannot explain the competence of epileptic children who have had an entire hemisphere removed to control seizures. In one case, documented in 1975 by the late neuropsychologist Aaron Smith, a baby who underwent the operation grew up to finish college, get a responsible job, and score above average on intelligence tests. A study of many such children at the Johns Hopkins Children's Center found no intellectual deficits.

Then there are the people who just happen to have very small brains. Although a disproportionate number are mentally handicapped, many end up in the general school population, performing no worse than others on IQ tests. At least one celebrated intellectual, French writer Anatole France, had a brain just two-thirds the normal size. That puts him right in the middle of the range for Homo erectus, some 200,000 years ago.

Whatever benefit big brains provide, it has to be big itself to offset the huge metabolic cost of feeding the organ--some 20 to 25 percent of all calories consumed--as well as the dangers involved in giving birth to a big-headed baby. Two recent papers in Science conclude that two genes affecting brain size expanded through the population, under the influence of natural selection, well after modern humans emerged; one was under selection as recently as 5,000 years ago.

So if intellect isn't the point, what is a big brain for? Perhaps the extra capacity serves as backup in case we get hit on the head or lose too many neurons to the ravages of old age. Or maybe it archives the decades-long accumulation of knowledge that makes veteran hunter-gatherers more productive than novices. Another theory holds that the brain evolved to vent heat so we could hunt at high noon, when the lions slept. Finding the reason for humanity's swollen skull undoubtedly will take more brainpower.