David Maraniss in the WaPo:
The line of polygamists in Obama’s family can be traced back generations in western Kenya, where it was an accepted practice within the Luo (pronounced LOO-oh) tribe. His great-grandfather, Obama Opiyo, had five wives, including two who were sisters. His grandfather, Hussein Onyango, had at least four wives, one of whom, Akumu, gave birth to the president’s father, Barack Obama, before fleeing her abusive husband. Obama Sr. was already married when he left Kenya to study at the University of Hawaii, where he married again. His American wife-to-be, Stanley Ann Dunham, was not yet 18 and unaware of his marital situation when she became pregnant with his namesake son in 1961. The line of polygamists in the Romney family traces back generations, when it was an accepted practice in the Church of Jesus Christ of Latter-day Saints. His paternal great-grandfathers, Miles Park Romney and Helaman Pratt, were born in the United States but lived for decades in Mexico. Pratt was a Mormon missionary there; Miles Park Romney left Utah for Mexico with a tribe of polygamous Mormons after the Mormon church broke with the practice of polygamy in 1890. Pratt had five wives. Miles Park Romney had four, and 30 children, one of whom was Gaskell Romney. The polygamy stopped at Gaskell, who had a single wife and seven children. One of the children, George, was born in a Mormon colony in the state of Chihuahua, Mexico, although he was nonetheless a U.S. citizen. He was Mitt’s father.