Collective guilt is all too common throughout history, regardless of whether punishment is meted out because of political, economic or religious differences. The Jews, cruelly oppressed by Pharaoh. The Christians, persecuted by Nero. The consequences of collective blame and punishment -- people leaving their homes en masse in search of freedom and safety -- are also familiar. We see them today as people flee Afghanistan, Iraq and Syria, and as refugees flood into Europe or knock at America's door. Can looking back inform our present and mitigate the problems ahead?...