Last week a study revealed when Americans stopped marrying their cousins, a new study shows that it's okay to have children with first cousins despite being related. The new study shows when people stopped marrying their cousins, and the genetic effects of having children with cousins.

Many famous people have married their cousins.

Marrying Cousins

The paper published in the journal Science shows that from 1650 to 1850 people would, on average, marry their fourth cousins. The following century this changed, thanks to technological changes and the growth of cities, by 1950 people were marrying, on average, their seventh cousins.

Transportation played a large role in the change. Improved modes of transportation allowed people to travel further to look for a spouse. Before 1950, people tended to stay within a six-mile radius of where they were born and stayed just in the same place.

Other factors that contributed to the decrease in cousin marriage include changing social norms, shrinking family sizes, and the increasing autonomy of women. Cousin marriage became a taboo after it had been the norm for its usefulness in keeping money in the family.

Shrinking family sizes would have left fewer people to be related to. Family sizes were much larger increasing the chances of being related to someone else.

After the end of the Civil War, states began to outlaw marriage between cousins.

Cousin Genetics

People who are first cousins share 12.5 percent of their DNA. This drops the further away the relation is to the person. Sharing a number of genes is not good for preventing genetic diseases. Variety in genetics is good for preventing disease.

Research shows that children between first cousins have a 4 to 7 percent chance of being born with birth defects. Those that are more distantly related have a 3 to 4 percent chance of having children born with birth defects.

This would not be a large problem since the chances can be pretty similar. The problem would arise when the next generation of children would have children with their first cousins. Children from those unions would have a lot of genetic material in common, increasing the chance of birth defects.

Marrying cousins has become taboo in the world. Many states in the U.S. have outright outlawed marriage between first cousins. In states like Texas and Oklahoma, it is a criminal offense to marry cousins. It is legal to marry first cousins in states like California and New York.

Some of the famous persons known to have married their cousins were Charles Darwin, Albert Einstein, Franklin and Eleanor Roosevelt, Thomas Jefferson, Edgar Allan Poe, and Kevin Bacon to name a few.