Andrew Hamilton, president of New York University, writes: Should the United States arm its school teachers, or ban the sale of assault weapons? Should we raise the age limit for buying rifles, or give police the power to remove guns from those they deem mentally ill?  What is the right answer to prevent another Parkland, another Sandy Hook, another Columbine? Americans want to know. Yet sadly, despite having world class researchers in public health and social sciences, we do not know. Why not?