A Darwinian puzzle is a trait that appears to reduce the fitness of individuals that possess it. Such traits attract the attention of evolutionary biologists. Several human traits pose challenges to evolutionary thinking, as they are relatively prevalent but are associated with lower reproductive success through reduced fertility and/or longevity. Some of the classic examples include: left handedness, menopause, and mental disorders. These traits are also found in animals, a peacock shows an example of a trait that may reduce its fitness. The bigger the tail, the easier it is seen by predators and it also may hinder the movement of the peacock. Darwin, in fact, solved this "puzzle" by explaining the peacock's tail as evidence of sexual selection; a bigger tail confers evolutionary fitness on the male by allowing it to attract more females than other males with shorter tails. The phrase "Darwinian puzzle" itself is rare and of unclear origin; it's typically talked about in the context of animal behavior.