The population has increased since the 1800s due to the birth rate exceeding the death rate. The birth rate rose as contraception became more available but is now falling as people prioritize careers over starting families. The death rate is very low because of advances in medicine, sanitation, and healthcare for the elderly. Economic, social, and political factors like the cost of living, people marrying later in life, and postwar baby booms have also impacted population changes. Migration within and to the UK affects the age distribution and concentration of populations in urban areas.