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World's Biggest Family Tree Reveals When Americans Stopped Marrying Their Cousins

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A team of scientists creates a huge family tree of 13 million people, spanning 11 generations. The family tree attempts to try and find answers to intriguing questions about the human population.

Family Tree

The branching lines of family trees have information tucked away, which can help people in knowing some answers to questions about the movement of their forefathers around the globe, their physical traits, and even their disease risk.

The family tree also reveals how longevity is inherited, whether men or women journeyed farther from home for matrimonial purposes, and when people stopped marrying their cousins.

Yaniv Erlich, who is a New York Genome Center computer biologist and a data scientist, said the massive family tree is the largest to be scientifically validated and is based on information that is publicly available.

“Through the hard work of many genealogists curious about their family history, we crowdsourced an enormous family tree and boom, came up with something unique,” Erlich stated. “We hope that this dataset can be useful to scientists researching a range of other topics.”

Erlich and his team published their study in the journal Science on March 1.

When Did Americans Stop Marrying Their Cousins

The experts discovered that before 1750, most U.S. marriages seen in the data set took place between people born about six miles from each other. The distance, however, increased rapidly to around 60 miles after the Industrial Revolution began in 1870.

"It became harder to find the love of your life," Erlich joked.

The researchers reported that during the period from 1650 to 1850, the average genetic relationship between married partners was that of 4th cousins. The genetic relationship widened to the order of 7th cousins after 1850.

The team also found that during 1800 to 1850, the distance partners traveled to marry each other became double, possibly due to the progress in transportation mode such as railways. It made travel faster in the United States and Europe.

The increase in distance traveled to get married to someone went along with an increase in genetic relatedness between partners. In simple terms, during these five decades, people journeyed farther to marry close relations.

"Families dispersed, and people started taking the train to marry their cousin," Erlich said.

The researchers noted that rather than easy access to rapid transit, the changing social norms were the main trigger for people to look for partners who were more distantly related to them than just 4th cousins.

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