From the New York Times:

By CARL ZIMMER
Published: May 6, 2008

“Why are humans so smart?” is a question that fascinates scientists. Tadeusz Kawecki, an evolutionary biologist at the University of Fribourg, likes to turn around the question.

“If it’s so great to be smart,” Dr. Kawecki asks, “why have most animals remained dumb?”

Dr. Kawecki and like-minded scientists are trying to figure out why animals learn and why some have evolved to be better at learning than others. One reason for the difference, their research finds, is that being smart can be bad for an animal’s health."

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