Some (weak) evidence that pessimists tend to be more accurate (and hence communities that value truth highly would tend to have more pessimistic people):

Pessimists are often right

Research shows that relative pessimists are more accurate at gauging success and failure rates at a simple laboratory task than optimists, who undercounted failures and overcounted successes, says Edward Chang, associate professor of psychology at the University of Michigan. Also, evidence shows that pessimism can be highly motivational, as what's called "defensive pessimism" drives people to achieve their goals.