I was led to this theory by the book Learned Optimism. I call it a theory because Dr. Seligman claims int he book to have come up with a hypotheses: (1) That you can measure a quality he labels optimism and pessimism, and that (2) optimism is a predictor for performance. He claims he then tested his theory and it was confirmed. That being said, confirmation by the theorist is much weaker than independent confirmation, so if you're a stickler for the scientific method, research this and see what current thinking has to say about it.

Seligman's second theory is even more interesting, he claims in his book to have cognitive therapy that improves optimism, and to have confirmed his second theory. Independent confirmation would be great for this one!