Research found that optimism is negative for old individuals. Why?
I suggest that optimism brings people to make more effort and get stressed (research by Segerstorm and Solberg Nes). WHile stress is negative, the positive results of these actions surpasses the cost, and makes optimism a positive trait.
The old, we know, are slow to recover form stress. It implies that hte cost of short term stress is much higher. This may make optimism an overally negative trait then.