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Not only was Einstein's inferior parietal region unusually bulky, the scientists found, but a feature called the Sylvian fissure was much smaller than average. Without this groove that normally slices through the tissue, the brain cells were packed close together, permitting more interconnections--which in principle can permit more cross-referencing of information and ideas, leading to great leaps of insight.
That's the idea, anyway. But while it's quite plausible according to current neurological theory, that doesn't necessarily make it true. We know Einstein was a genius, and we now know that his brain was physically different from the average. But none of this proves a cause-and-effect relationship. "What you really need," says McLean's Benes, "is to look at the brains of a number of mathematical geniuses to see if the same abnormalities are present."
Even if they are, it's possible that the bulked-up brains are a result of strenuous mental exercise, not an inherent feature that makes genius possible. Bottom line: we still don't know whether Einstein was born with an extraordinary mind or whether he earned it, one brilliant idea at a time.