Obama and President Bush are 10th cousins

How do they know this?

Researchers there say Barack Obama is a distant cousin of actor Brad Pitt. […] Hillary Clinton is related to Pitt’s girlfriend, Angelina Jolie. Clinton, who is of French-Canadian descent on her mother’s side, is also a distant cousin of singers Madonna, Celine Dion and Alanis Morissette. Obama, the son of a white woman from Kansas and a black man from Kenya, can call six U.S. presidents, including President Bush and his father, former President George Bush, his cousins.

Boston Herald