Feeling Faint; August/September 2006; Scientific American Mind; by Rolf R. Diehl; 4 Page(s)

As soon as her instructor's dissecting knife cuts into the cadaver's skin, a medical student swoons, falling to the floor. Her fellow students pity her, thinking that she is simply too tenderhearted to be a doctor. They are wrong: her problem is not in her head.

Rather than being unable to endure life's occasional unpleasantness, otherwise healthy people who faint when they see a few drops of blood or if they stand in place too long are survivors. They are displaying a lifesaving mechanism bestowed on them by evolution.