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.