A shooting at a historic black church in Charleston, South Carolina, on Wednesday has shaken the country, leaving many reflecting on the state of race relations in the United States. Nine people, including Rev. Clementa Pinckney, the church's pastor and a South Carolina state senator, were shot to death by accused gunman Dylann Storm Roof at Emanuel African Methodist Episcopal Church. Authorities have called the shooting a hate crime. But how does one explain such a crime from a scientific perspective? What could lead someone to commit a racially motivated hate crime? What is racism -- and how can we...