"Mental Illnesses" as Survival Skills
Researchers at the Univeristy of Minnesota have shown that impulsive behavior, such as that shown by those with attention deficit disorder (ADD), gives animals a competitive edge. Delaying gratification is a sign of maturity in our culture, but in the wild, one cannot afford to wait for better opportunities. Plus, easily-distracted animals may be more aware of their surroundings and be better able to evade imminent danger. Such traits would have served our early ancestors well, yet they cause problems in our more structured world.
Similarly, behaviorist Nancy Etcoff has suggested that symptoms of depression may have aided our ancestors by allowing them to conserve resources and even show signs of submission to superiors. Anxiety and what are now considered panic disorders may assist in a "flight or fight" response when faced with a threat. University of Michigan psychiatrist Randolph Neese calls this the "smoke detector principle": it's better to send out a false alarm than to fail to detect a fire.