It’s a little-known fact outside paleontology, but over the past 20,000 years—a span that has produced Plato, Shakespeare, Einstein, Mozart, and every piece of human technology not made from a chipped rock—the human brain has gotten smaller. The average male brain 20,000 years ago, during the late Stone Age, was 1,500 cubic centimeters. Now it’s 1,350. In other words, we’ve lost roughly a tennis ball of brain volume. Paleontologists and anthropologists aren’t sure why this has happened, though they offer a variety of explanations. Some point out that our bodies are smaller than those of our Cro-Magnon ancestors, so it stands to reason that our brains might be. Others suggest that our brains got smaller because they’ve grown more efficient.
A new book, The Domesticated Brain by University of Bristol psychologist Bruce Hood, endorses a third explanation: Our brains are smaller because we’ve become more social. The rationale, first advanced by Brian Hare of Duke University, is that the process by which we became better able to live and work together without killing each other also shrunk our brains.