There was also the result of drawing a sharp line between the in-group and everyone else. If we have strict dietary laws but the people on the other side of the hill don’t share them, then it follows that they’re unspeakable barbarians, and that we should not trade or cooperate with them, let alone intermarry, and indeed, it’s acceptable to kill them and take their stuff, as surely such a barbarous people would do the same to us.