Scientific evidence for exactly which kind of legislation would help stem gun violence is lacking, a situation that is in many ways of the government's own creation. Several congressional efforts have limited federal research vastly reducing the scientific data available for policymakers today. What's left is piecemeal and often small-scale research that fails to answer big questions about effective restrictions, the link between gun violence and mental health and cultural factors such as media, said Frank Farley, a psychologist at Temple’s College of Education.