This paradox or impossible situation, is explained by having shared ancestors, which is called pedigree collapse. Instead of every person today having a family tree of all unique ancestors, many of these ancestors occupy more than one position in the pedigree. In other words, one's 15th great-grandfather may have been their 15th great-grandfather more than once if the person today is descended from children of both or all of his marriages. So in a typical family tree, the same ancestor may pop up in more than one place in the pedigree. Pedigree collapse is generally caused by close relatives marrying in the past, which was common in many societies. Even today, because most people do not know their ancestry beyond their grandparents, they are inadvertently marrying their own distant cousins. On average, a typical European marriage today is between sixth cousins (those descended from a common sixth great-grandparent). In closed societies the relationship is usually much closer.