Dorchester. Dorchester is a neighborhood comprising more than 6 square miles in the City of Boston. Originally, Dorchester was a separate town, founded by Puritans who emigrated in 1630 from Dorchester, Dorset, England, to the Massachusetts Bay Colony. This dissolved municipality, Boston's largest neighborhood by far, is often divided by city planners in order to create two planning areas roughly equivalent in size and population to other Boston neighborhoods. The neighborhood is named after the town of Dorchester in Dorset, from which Puritans emigrated to the New World on the ship Mary and John, among others. Founded in 1630, just a few months before the founding of the city of Boston, Dorchester now covers a geographic area approximately equivalent to nearby Cambridge. When annexed to Boston in 1870, Dorchester was still a primarily rural town and had a population of 12,000. Construction of railroad and commuter streetcar lines brought rapid growth, increasing the population to 150,000 by 1920. In the 2010 United States Census, the neighborhood's population was 92,115. The Dorchester neighborhood has a very diverse population, which includes a large concentration of African Americans, European Americans. More numerous immigrants and their descendants since the later 20th century have come from the Caribbean Caribbean, Central and South America, and East and Southeast Asian Americans. Dorchester also has a significant LGBT population, with active political groups. It has the largest concentration of same-sex couples in Boston after the neighborhoods of South End and Jamaica Plain. Most of the people over the age of 25 have completed high school or obtained a GED.