Which states of mind that are classified as mental illnesses might be a natural and useful part of our brains?
For example, some evolutionary biologists have suggested that some types of depression are a natural consequence of losing a fight for dominance. i.e. In order for the loser to fit into the social pecking order he must have feelings of low self worth.
Are there other types of "mental illnesses" that might be natural and useful? (By "natural" I simply mean evolutionary selected for.) And that if this illness was cured for all human beings it would in fact damage the social fabric of society?
Another example. A phobia of spiders. Most people would agree that this is not a mental illness but a common trait of most humans. A phobia of trees is probably not.