To correct these errors the team developed computer programs that "pruned" the tree, removing invalid relationships. After doing that, they generated 5.3 million disjointed family trees -- the largest of which included 13 million individuals.
By comparing people in the system with 80,000 death records from Vermont spanning from 1985 to 2000, the authors also found that the people included in their family tree were not any more likely to be rich or poor than the general population. They were, however, much more likely to be white.
"We have much more representation of Western populations, mostly from Europe and the U.S.," Erlich said. "And from the U.S. it is mostly from Caucasians rather than other ethnicities."
He added that he hopes more nonwhite people will soon add their families to the site.
After getting a handle on their sprawling data set, the research team came up with a set of questions that only a mega-family tree dating back hundreds of years could answer.
For example, after studying migration patterns in the tree they found that women leave their hometown more than men, but when men move, they tend to move much farther. This pattern has continued for a long time. It was true 300 years ago, and continues to be true today, the authors said.
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In another line of inquiry, the data were used to determine when people stopped marrying close relations.
The researchers found that prior to 1750, most marriages in their data set occurred between people born about 6 miles from each other. After the start of the Industrial Revolution in 1870, however, that distance rapidly increased to about 60 miles.
You might think that as people traveled farther to find a spouse, they would marry people who were more distantly related to them. And indeed, that was true. Eventually.
The authors report that between 1650 and 1850 the average genetic relationship of married couples was on the order of fourth cousins. After 1850 it was on the order of seventh cousins.