Another cognitive innovation made it possible to compare potential futures. While other animals focus on the present, only humans, said Geary, "sit and worry about what will happen three years from now if I do that or this." Our ability to think things over, and over, can be counterproductive and lead to obsessive tendencies.

Certain types of depression, however, Geary continued, may be advantageous. The lethargy and disrupted mental state can help us disengage from unattainable goals — whether it is an unrequited love or an exalted social position. Evolution likely favored individuals who pause and reassess ambitions, instead of wasting energy being blindly optimistic.

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."