The concept of urban sociology as a whole has often been challenged and criticized by sociologists through time. Several different aspects from race, land, resources, etc. have broadened the idea. Manuel Castells questioned if urban sociology even exists and devoted 40 years worth of research in order to redefine and reorganize the concept. With the growing population and majority of Americans living in suburbs, Castells believes that most researchers focus their work of urban sociology around cities, neglecting the other major communities of suburbs, towns, and rural areas. He also believes that urban sociologists have over complicated the term of urban sociology and should possibly create a more clear and organized explanation to their studies, arguing that a "Sociology of Settlements," would cover most issues around the term.