One of the first major studies of the middle class in America was "", published in 1951 by sociologist C. Wright Mills. Later sociologists such as Dennis Gilbert of Hamilton College commonly divide the middle class into two sub-groups. Constituting roughly 15% to 20% of households is the upper or professional middle class consisting of highly educated, salaried professionals and managers. Constituting roughly one third of households is the lower middle class 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.