Trucking, after all, is grueling work: long shifts, night hours and lots of solitary work. It’s also a traditionally male-dominated industry — according to the Institute for Women’s Policy Research, less than 5% of truck drivers are female.

Occupations like carpentry, construction and automotive services are also among some of the most male-dominated jobs. In these roles, there are sometimes so few women workers, researchers are unable to estimate women’s median weekly earnings, according to the Institute for Women’s Policy Research.