Definition
Assimilation is the process by which migrants and their descendants adopt features of the destination culture — language, dress, civic participation, religion, social networks — until they become indistinguishable, in those dimensions, from the surrounding population.
The classical "straight-line" version of the theory predicted that all migrants would eventually merge into the host society over three generations. Empirically, that pattern is the exception rather than the rule. Whether assimilation happens, how fast, and in which domains depends heavily on whether the receiving society will let it.
Why it matters
How it works
The clearest way to think about assimilation is as a vector with several components rather than a single number. A migrant family can score high on labor-market assimilation (working at the same firms, in the same occupations, at competitive wages) while scoring low on residential assimilation (living in the same neighborhoods as the receiving group). The components can also move in opposite directions: language assimilation often accelerates as residential assimilation stalls.
Wilkerson's central argument about the Great Migration turns on exactly this point. Black migrants from the South assimilated economically into the industrial workforce — Ida Mae Brandon Gladney into Chicago, George Swanson Starling onto the rail lines, Robert Joseph Pershing Foster into Los Angeles medicine — and yet they and their children were prevented from assimilating residentially through restrictive covenants, redlining, and white flight. They became Americans by every measure the country said mattered, and were nonetheless held apart spatially for generations.
The theory has evolved accordingly. Segmented assimilation recognizes that migrants assimilate into specific strata of the receiving society, not the society as a whole — and which stratum depends on race, education, neighborhood, and the labor market they enter. Selective acculturation notes that the second generation often keeps the family language and ties while adopting the destination's civic forms; this group tends to do better, not worse, than peers who shed the heritage entirely.
The deepest lesson is that assimilation is not something migrants do alone. It is a contract whose other party — the receiving society — sets the terms. When the receiving society refuses, you get not assimilation but stratification, which is the story of much of American urban history.