Though the American middle class does not have a definitive definition, contemporary social scientists have put forward several ostensibly congruent theories on it. Depending on the class model used, the middle class constitutes anywhere from 25% to 75% of households.
One of the first major studies of the middle class in America was White Collar: The American Middle Classes, published in 1951 by sociologist C. Wright Mills. Later sociologists such as Dennis Gilbert commonly divide the middle class into two sub-groups: the professional or upper middle class (~15-20% of all households) consisting of highly educated, salaried professionals and managers, and the lower middle class (~33% of all households) consisting mostly of semi-professionals, skilled craftsmen and lower-level management. Middle-class persons commonly have a comfortable standard of living, significant economic security, considerable work autonomy and rely on their expertise to sustain themselves.