My grandpa married his second cousin, it was already frowned upon in 1935 but not unheard of. One improvement factor was transportation - the bicycle was invented not much before, and it was expensive. But a bike or a train ticket allowed you to know people beyond the rural village you lived. It's been even said that the inventor of the bicycle was the 2nd greatest feminist, because he improved the availability of suitable grooms, nobody needed to marry the violent moron next plantation anymore.