In the early years of research on street children, the term "street child" included any child that worked on the street. From research, however, different categories of children on the streets have been distinguished, while still recognizing that children's complex experiences are difficult to define.[1] Mark W. Lusk, a prominent researcher of street children, developed four categories of children on the street from his research: children who work on the street but return to their families at night, children who work on the street but whose family ties are dwindling, children who live and work with their families on the street, and children who work and live on their own on the street.[5]