It has been proposed that no fixed theoretical limit to human longevity is apparent today. Studies in the biodemography of human longevity indicate a "late-life mortality deceleration law": that death rates level off at advanced ages to a late-life mortality plateau. That is, there is no fixed upper limit to human longevity, or fixed maximal human lifespan. This law was first quantified in 1939, when researchers found that the one-year probability of death at advanced age asymptotically approaches a limit of 44% for women and 54% for men.