Psychologists tell us that, if you experimentally separate children in any arbitrary way - say, dress half of them in green T-shirts and half in orange - they will develop in-group loyalty and outgroup prejudice. To continue the experiment, suppose that, when they grow up, greens only marry greens and oranges only marry oranges. Moreover, “green children” only go to green schools and “orange children” to orange schools. Carry on for 300 years and what have you got? Northern Ireland, or worse. Religion may not be the only divisive power that can propel dangerous prejudices down through many generations (language and race are other candidates) but religion is the only one that receives active government support in the form of schools.