William Isaac Thomas (August 13, 1863 – December 5, 1947) was an American sociologist, understood today as a key figure behind the theory of symbolic interactionism.

"[3] This microsociological concept served as a theoretical foundation for the field of symbolic interactionism which was developed by Thomas's younger peers—primarily at the University of Chicago.

[4] William Isaac Thomas was born on a farm in the Elk Garden section[5] of Russell County, Virginia, on 13 August 1863.

Despite a biological bias that would nowadays be considered sexist by many (e.g. "Anthropologists…regard women as intermediate between the child and the man"), the book was progressive for its time.

For the next decade (1908–1918) Thomas conducted research and began collaborating with Florian Znaniecki in 1913 to write The Polish Peasant in Europe and America.

Though charges under the Mann Act were eventually dropped, Thomas' moral and academic reputation was permanently damaged and he was dismissed by the University of Chicago before his trial reached a verdict.

From 1923 to 1928, he lectured at the New School for Social Research, a progressive but marginally influential academic institution at that time, where he was able to make connections with a younger generation of sociologists who would eventually help Thomas restore his reputation.

In 1936, Pitirim A. Sorokin, chairman of the sociology department at Harvard University, invited Thomas to become a visiting lecturer.

My interests, as I have indicated, were in the marginal fields and not in sociology as it was organized and taught at that time, that is, the historical and methodological approach of Professor Small and the remedial and correctional interests of Professor Henderson.In 1908, Thomas received a substantial grant from Helen Culver through the Helen Culver Fund for Race Psychology.

Until 1918, Thomas utilized the grant to undertake several journeys to Europe in order to study the background of East European immigrant groups.

Initially planning to study several nationalities, he narrowed his topic down to immigrants from Poland, who formed the largest and most visible ethnic community in Chicago.

This chance occurrence serendipitously led Thomas to develop the biographical approach to empirical sociological research that would prove part of his lasting reputation in the field.

Znaniecki eventually became Thomas's co-author on their monumental work The Polish Peasant in Europe and America (1918–1919),[8] which Lewis Coser called "the earliest major landmark of American sociological research".

Furthermore, Thomas and Znaniecki's work developed an approach to understanding ethnicity in particular, which in many respects was ahead of its time and is currently being rediscovered in the context of transnational studies in migration.

Along with the ideas of George Herbert Mead, this concept later proved to be an important part of social constructionism, and of the rebellion of symbolic interactionism against structural functionalism.

Thomas' 1928 book, The Child in America, co-authored with Dorothy Swaine Thomas, includes a notion, drawing from his initial idea of the definition of the situation, that would become a fundamental law of sociology, known as the Thomas theorem: “If men define situations as real, they are real in their consequences.”[14] The Child in America specifically investigated the misbehavior of children can be attributed to communal expectations of behavior and how these children define their own situations is indicative of their own conceptions maturation and acceptable behavior.

[15] In 1918, the FBI arrested Thomas under the Mann Act, which prohibits "interstate transport of females for immoral purposes," while in the company of one Mrs. Granger, the wife of an army officer with the American forces in France.

[15][16] Under orders from Judson, The University of Chicago Press, which already had published the first two volumes of The Polish Peasant, terminated its contract with Thomas, including ceasing distribution.

Thus, in 1921, Old World Traits Transplanted appeared by authors Robert E. Park and Herbert A. Miller, who had contributed only minor parts to the book.

It was not until 1951 that the book's authorship was re-credited to Thomas by a committee of the Social Science Research Council and reissued with its author's actual name.