In Homelessness is a Housing Problem, housing scholar Gregg Colburn and data journalist Clayton Aldern seek to explain the substantial variation in rates of homelessness apparent in cities across the United States.  Using accessible statistics, the researchers test a range of conventional beliefs about what drives the prevalence of homelessness in a given city—including mental illness, drug use, poverty, weather, generosity of public assistance, and low-income mobility—and find that none explain why, for example, rates are so much higher in Seattle than in Chicago. Instead, housing market conditions, such as the cost and availability of rental housing, offer a more convincing explanation. Over the course of the book, the researchers illustrate how absolute rent levels and rental vacancy rates are associated with regional rates of homelessness. Many other common explanations—drug use, mental illness, poverty, or local political context—fail to account for regional variation.