Raichlen is a recreational runner, and he began to think about the runner’s high. No one had ever come up with a good explanation for why it exists. What if the high wasn’t some random physiological by-product of running long distances, but nature’s reward for persisting? Was it possible that evolution had found a way to harness the brain’s feel-good chemicals to make endurance exercise rewarding? Maybe, Raichlen mused, early humans got high when they ran so that they wouldn’t starve. He reasoned that such a neuro-reward would have to do two things: relieve pain and induce pleasure.
