Why has synesthesia survived throughout human evolutionary history?

An estimated 4% of the population have synesthesia, a condition where senses become confused. People with synesthesia may perceive that certain sounds have flavors, or that certain words have a specific color. Even though synesthesia is widely recognized, and at least vaguely understood, what we don't know is why it… » 11/25/11 7:00am 11/25/11 7:00am