Natural selection may be behind many psychiatric problems. Mental illnesses, many researchers claim, are the byproducts of an over-functional brain.

As humans improved their gathering, hunting and cooking techniques, population size increased and resources became more limited (in part because we hunted or ate some species to extinction).

As a result, not everyone could get enough to eat. Cooperative relationships were critical to ensuring access to food, whether through farming or more strategic hunting, and those with blunt social skills were unlikely to survive, explained David Geary, a researcher at the University of Missouri. And thus, a diversity of new mental abilities, and disabilities, unfurled.

It might seem as though modern man should have evolved to be happy and harmonious. But nature cares about genes, not joy, Geary is quoted as saying by LiveScience .

To explain our susceptibility to poor mental health, Randolph Nesse in 'The Handbook of Evolutionary Psychology' 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 (nature's definition of uber-success can be measured by his prolific paternity), 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.

For example, the first of our ancestors to empathize and read facial expressions had a striking advantage. They could confirm their own social status and convince others to share food and shelter.

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.

Natural selection also likely held the door open for disorders such as attention deficit. Quickly abandoning a low stimulus situation was more helpful for male hunters than female gatherers, writes Nesse, which may explain why boys are five times more likely than girls to be hyperactive.

Similarly, in its mildest form, bipolar disorder can increase productivity and creativity. Bipolar individuals (and their relatives) also often have more sex than average people, Geary noted.

Sex, and survival of one's kids, is the whole point - as far as nature is concerned. Sometimes unpleasant mental states lead to greater reproductive success, said Geary, "so these genes stay in the gene pool".