Population ageing is a phenomenon that occurs when the median age of a country or region rises due to rising life expectancy and/or declining fertility rates. There has been, initially in the more economically developed countries (MEDC) but also more recently in less economically developed countries (LEDC), an increase in life expectancy which causes the ageing of populations. This is the case for every country in the world except the 18 countries designated as "demographic outliers" by the UN. For the entirety of recorded human history, the world has never seen as aged a population as currently exists globally. The UN predicts the rate of population ageing in the 21st century will exceed that of the previous century. Countries vary significantly in terms of the degree, and the pace, of these changes, and the UN expects populations that began ageing later to have less time to adapt to the many implications of these changes.