Coolidge was the first American full-time professor of sociology. A statistician of women's poverty, she was also an authority on feminism, social welfare, Victorian sexuality, Chinese immigration to the United States, and Native Americans. Her major study Chinese Immigration (1909) was highly regarded in both America and China. This combined an analysis of both law and history to document race relations between this exploited minority group and the wider White community. A further study of a minority people, The Rain Makers: Indians of Arizona and New Mexico (1929), provided important photographic and other documentation of a group much neglected and oppressed at the time. In 1912 she published a systematic and comprehensive, but also controversial, study of gender—Why Women are So. In this she argues that women are socially constructed as a distinct group, and that their life-chances are seriously curtailed and constrained by dress, language, and the market-place—all factors which were subsequently addressed by second-wave feminism. Coolidge was an active leader of the San Francisco women's struggle for the vote, and believed that sociology could and should play an active part in documenting social problems and inequalities, which could then inform and encourage reform and improvement. Passionately concerned with women's poverty, she carried out many analyses of old age, poverty, catastrophic disasters, and the workings of the criminal courts.