The Mormons unofficially do practise polygyny. This is a long story: not that long after the European immigrants started going to America, the East Coast became quite populated. Various colonies then encouraged the immigrants to move west. The Mormons were among the first adventurers cos the religion was losing popularity in the Continent. Unfortunately, many many people (esp men) dead on the journey owing to extreme coldness (women are in general more resistent to coldness). After they settled in the Mid-West, the religious leaders started to encourage polygyny (usually shared wives with brothers) so as to increase population and overall fertility. Few decades later, this had been forbidden by laws although it only goes underground.
