I seem to recall a reason along the lines of "to keep the peace in the families". Can't find that source. But in practice, I expect a paternity test to come in

I seem to recall a reason along the lines of "to keep the peace in the families". Can't find that source. But in practice, I expect a paternity test to come in an otherwise troubled relationship, so I don't know how often it happens that both parents would seek to get one.