The nation-state was new. So too were its citizens, as a result of two sweeping social changes. Beginning in the early 19th century, the landscape and society of the lower Chao Phraya basin was transformed by a frontier movement of peasant colonization. Uniquely in Asia, new land was being opened up faster than population growth from the mid-19th century right through to the 1970s. As a result of political decisions in the late 19th century, this frontier society was characterized not by landlords but by peasant smallholders. Until urbanization accelerated in the last quarter of the 20th century, this smallholder peasant society represented four-fifths of the population, and the main driving force of the economy.

Much of the urban population was also new, especially in the capital city of Bangkok. Continuous immigration from southern China made Chinese a dominant element in the city's economic life. Western merchants and advisers formed a new semicolonial segment of the elite. A fledgling commoner middle class began to form around the city's new role as capital of a nation-state.