| − | Pessimism is very often the cause of [[depression]]. [http://www.shearonforschools.com/learned_optimism.htm] | + | Pessimism is very often the cause of [[depression]].<ref>[http://www.shearonforschools.com/learned_optimism.htm Learned Optimism] by Martin E.P. Seligman, Ph.D.<ref> |