[33] Michel Ferlus' work is based on some simple rules of phonetic change observable in the Sinosphere and studied for the most part by William H. Baxter (1992).

[34] Michel Ferlus notes that a deeply rooted belief in Thailand has it that the term "Thai" derives from the last syllables -daya in Sukhodaya/ Sukhothay (สุโขทัย), the name of the Sukhothai Kingdom.

[33] The word "Tai" (ไท) without the final letter ย is also used by Thai people to refer to themselves as an ethnicity, as historical texts such as "Mahachat Kham Luang", composed in 1482 during the reign of King Borommatrailokkanat.

[35] Similarly, "Yuan Phai", a historical epic poem written in the late 15th to early 16th century, also used the word "Tai" (ไท).

The Qin dynasty founded Guangdong in 214 BC, initiating varying successive waves of Han Chinese from the north for centuries to come.

[34] With dynastic Chinese political upheavals, cultural changes, and intensive Han migratory pressures from north that led the Tai peoples on the verge of being displaced, some of them migrated southwards[44] where they met the classical Indianized civilizations of Southeast Asia.

The Thai were called "Siam" by the Angkorians and they appeared on the bas relief at Angkor Wat as a part of the army of Lavo Kingdom.

Sometimes the Thai chiefdoms in the Chao Phraya valley were put under the Angkorian control under strong monarchs (including Suryavarman II and Jayavarman VII) but they were mostly independent.

A new city-state known as Ayutthaya covering the areas of central and southern Thailand, named after the Indian city of Ayodhya,[47] was founded by Ramathibodi and emerged as the center of the growing Thai empire starting in 1350.

Chinese, Malay, and British influences helped to further shape the Thai people who often assimilated foreign ideas, but managed to preserve much of their culture and resisted the European colonization that engulfed their neighbors.

The concept of a Thai nation was not developed until the beginning of the 20th century, under Prince Damrong and then King Rama VI (Vajiravudh).

King Rama VI also imposed the idea of "Thai-ness" (khwam-pen-thai) on his subjects and strictly defined what was "Thai" and "un-Thai".

Authors of this period re-wrote Thai history from an ethno-nationalist viewpoint,[49] disregarding the fact that the concept of ethnicity had not played an important role in Southeast Asia until the 19th century.

[50][51] This newly developed nationalism was the base of the policy of "Thaification" of Thailand which was intensified after the end of absolute monarchy in 1932 and especially under the rule of Field Marshal Plaek Phibunsongkhram (1938–1944).

Minorities were forced to assimilate and the regional differences of northern, northeastern and southern Thailand were repressed in favour of one homogenous "Thai" culture.

There is also evidence of bidirectional admixture between southern Thai and Nayu, who show close genetic relationships with Austronesian speaking groups from Island Southeast Asia.

[64] The modern Thai are predominantly Theravada Buddhist and strongly identify their ethnic identity with their religious practices that include aspects of ancestor worship, among other beliefs of the ancient folklore of Thailand.

[66] The belief in local, nature and household spirits, that influence secular issues like health or prosperity, as well as ghosts (Thai: phi, ผี) is widespread.

Some Thais worship Hindu gods like Ganesha, Shiva, Vishnu, or Brahma (e.g., at Bangkok's well-known Erawan Shrine).

This can be done mainly by food and in-kind donations to monks, contributions to the renovation and adornment of temples, releasing captive creatures (fish, birds), etc.