Life expectancy is defined as a statistical measure of how long a person may live, based on demographic factors such as gender, current age, and most importantly the year of their birth. The most commonly used measure of life expectancy is life expectancy at birth or at age zero. The calculation is based on the assumption that mortality rates at each age were to remain constant in the future.
Life expectancy has changed drastically over time, especially during the past 200 years. In the early 20th century the average life expectancy at birth in the developed world stood at 31 years. It has grown to an average of 72 years worldwide until 2015, and is expected to keep on growing with advances in medical treatment and living standard continuing.
Life expectancy still varies greatly between different regions and countries of the world. The biggest impact on life expectancy have the quality of public health, medical care, and diet. As of 2015, the countries with the highest life expectancy were Hong Kong (84.1 years) and Japan (83.6 years). The following countries are mostly from Western Europe, but also from other industrialized parts of the world. Countries with the lowest life expectancy are mostly African countries. The ranking was led by Swaziland (48.9 years) in Southern Africa. The impact of AIDS on life expectancy in many African countries is notable in this list.
When compared inside a single country, difference in life expectancy are mostly the result of economic circumstances. With lower income comes lower life expectancy. Life expectancy of high income households averaged ad 79 years, while low income households had a life expectancy of 62 years as of 2013.