The population of the United Kingdom was estimated at over 67.0 million in 2020. It is the 21st most populated country in the world and has a population density of 270 people per square kilometre (700 people per square mile), with England having significantly greater density than Wales, Scotland, and Northern Ireland. Almost a third of the population lives in South East England, which is predominantly urban and suburban, with about 9 million in the capital city, London, whose population density is just over 5,200 per square kilometre (13,468 per sq mi). The population of the UK has undergone demographic transition—that is, the transition from a (typically) pre-industrial population, with high birth and mortality rates and slow population growth, through a stage of falling mortality and faster rates of population growth, to a stage of low birth and mortality rates with, again, lower rates of growth. This growth through 'natural change' has been accompanied in the past two decades by growth through net migration into the United Kingdom, which since 1999 has exceeded natural change. The United Kingdom's high literacy rate (99% at age 15 and above) is attributable to universal state education, introduced at the primary level in 1870 (Scotland 1872, free 1890) and at the secondary level in 1900. Parents are obliged to have their children educated from the ages of 5 to 16 years (18 in England from 2013), and can continue education free of charge in the form of A-Levels, vocational training, and apprenticeship until the age of 18. The United Kingdom's population is predominantly White British (81.88% at the 2011 Census), but due to migration from Commonwealth nations, Britain has become ethnically diverse. The second and third largest non-white racial groups are Asian British at 7% of the population, followed by Black British people at 3%. The main language of the country is British English. Some Celtic languages, namely Scottish Gaelic and Irish, are still spoken by minorities in Scotland and Northern Ireland, respectively, and Cornish has been revived to a limited degree in Cornwall; but the predominant language in all these areas is English. Welsh is widely spoken as a first language in parts of North and West Wales, and to lesser extent in South East Wales, where English is the dominant first language. History Before the census, 200–1800 Roman Britain had an estimated population between 2.8 million and 3 million at the end