Professor MacDonald studies crime and violence, race and ethnic disparities in criminal justice, and impact of public policy on safety. A current focus of his work is on examining how the science of urban planning can make our cities healthier, safer, and livable. The National Institutes of Health, the Centers for Disease Control and Prevention, and private foundations helped support the research on changing places. He is also active studying racial disparities in criminal justice, and ways to reduce these disparities through policy and program reforms.