A bold claim (or maybe not), but evolutionary biologist Randolph Nesse thinks he can back it up.
[He] compares the human brain with race horses: Just as horse breeding has selected for long thin legs that increase speed but are prone to fracture, cognitive advances also increase fitness — to a point….
People with aggressive and narcissistic personalities are the easiest to understand evolutionarily; they look out for number one. But even if 16 million men today can trace their genes to Genghis Khan…very few potential despots achieve such heights. Perhaps to check selfish urges, in favor of more probable means to biological success, social lubricants such as empathy, guilt and mild anxiety arose….
But too much emotional acuity — when individuals overanalyze every grimace — can cause a motivational nervousness about one’s social value to morph into a relentless handicapping anxiety.
[Wingnut by Gibna Kebira]