I doubt there is a mean biological predisposition towards pessimism (assuming that's what you meant). For the vast majority of human history, the vast majority

I doubt there is a mean biological predisposition towards pessimism (assuming that's what you meant). For the vast majority of human history, the vast majority of people lived horrendous lives relative to our own. Imagine.. for hundreds of thousands of years, we had no idea how the universe operated, we feared death and the dark.. natural disasters were thought of as acts of god and most people died by 30 or 40. And unlike most other animals, we somewhat comprehend our predicament... that we and the people we care about aren't going to be here for long. Yet most humans still attempt to do something with their lives, and to leave a better world for their children. We never gave up as individuals and societies and persevered until we improved our material conditions significantly. That's a pretty strong indication of a species wide predisposition towards optimism.