A Newswise release delves back into the whys and wherefores of gender differences in life span and aging: "It has been widely assumed that men age earlier than women, as evidenced by their higher mortality rates and shorter average life spans. But [researchers] contend that the opposite is true. ... theory and data suggest that females begin to exhibit signs of physiological decline earlier than males, and that higher mortality figures are not necessarily correlated with the rate at which we age. ... There is also the physiological cost of testosterone, which suppresses the immune system and results in higher death rates among males due to infectious diseases and cancer. During the evolutionary history of our species, few individuals lived long enough to express genes that cause physiological deterioration late in life, so those genes had little effect on fitness and there was little natural selection to remove them. ... Getting old, falling apart and dying is not an adaptive trait. It evolves because of a lack of natural selection."