The taboo behind first cousins not being allowed to marry is based on social norms, not on genetics. The basis of that statement is a genealogy study of 13 million relatives, many of whom married their cousins.

There is not much difference between marrying a first cousin and a distant cousin and having children. Scientists explained that a person's lifespan, after all, is determined only by 16 percent of his genes, WTOC reported.

Higher chance of birth defects

But a 2014 study in Pakistan said that marriage between first cousins double the risk of children being born with birth defects. The research was conducted because of the higher-than-expected rates of deaths and congenital abnormalities of babies born in Pakistani communities.

According to the study, 31 percent of all birth defects in babies of Pakistani origin was due to marriage to a blood relative. The risk of birth defects, often heart or nervous system problems which can be deadly, is, nevertheless, still small but it goes up from 3 percent in the general Pakistani population to 6 percent among blood relatives who married, The Guardian reported.

However, it is a different matter when first cousins marry and their children would also marry their first cousin. Tech Times explained that first cousins share 12.5 percent of their DNA which further goes down the more distant a relative is. The website stressed that sharing a number of genes is not good for preventing genetic diseases while having a variety of genetics is good for preventing ailments.

Because of the shared genes, first cousins have a 4 to 7 percent chance of being born with birth defects which goes down to 3 to 4 percent for offspring of people who are more distantly related. However, if the next generation of children would have offspring with their first cousins, the children of those unions would have a lot of DNA in common and would increase the chance of birth defects.

According to the study, published in the Science journal, from 1650 to 1850, on the average, people married their first cousins which changed in the following century because of technological changes and the growth of cities. By 1950, the marriages were, on the average, between seventh cousins.

A ban on cousin marriages

When the Civil War ended, many states started to make it illegal for cousins to marry, resulting in 24 states banning the practice, but 20 states still allowed it. However, the last six states placed various restrictions on the practice.

However, it is only in the US where there are restrictions against cousin marriages. There are no such restrictions in Europe, Canada, and Mexico. By marrying a first cousin, the increased risk for genetic defects goes up by 1.7 percent to 2.8 percent. It is about the same risk if a woman bears a child after 40 years old, according to the National Society of Genetics Counselors, reported.

Dr. Arno Motulsky, professor emeritus of medicine and genome sciences at the University of Washington, noted that although the increase represents a near doubling of the risk, it is not big enough to discourage cousins from having kids. He pointed out that in terms of general risks in life, it is not very high. At its worst, the risk is just 7 percent, while 93 percent of the time, nothing is going to happen, The New York Times reported.

Motulsky also noted that no one has questioned the rights of people with genetic disorders – such as Huntington's disease – to have children even if some of them have much higher levels of risks compared to first cousins. Huntington is a severe neurological disorder during adulthood which has a 50 percent chance of passing the disease to their children.

It was the changing social norms that pushed people to go beyond their surroundings and relatives for spouses, Yaniv Erlich, a data scientist at Columbia University who devised the study, said. The genealogy, which has 13 million members and is the biggest one in the world, shows how closely everyone is related since everyone is a 10th to the 12th cousin of each other, Erlich said.

Cousins who married

Tech Times named Charles Darwin, Albert Einstein, Franklin and Eleanor Roosevelt, Thomas Jefferson, Edgar Allan Poe, and Kevin Bacon as some of the famous people who married their cousins. In the case of Darwin, the union even resulted in exceptional children.

Even religious groups cannot cite biblical verses that ban cousins from marrying. While Leviticus 18 listed all forbidden sexual relationships, it excluded cousin relationships. God the Father even commanded many cousins to marry, including the five daughters of Zelophehad, the daughters of Eleazar, Jacob and his first cousins Rachel and Leah who are sisters, and Isaac and Rebekah.

The website also cited current studies which indicate that cousin couples have a lower ratio of miscarriages, likely because the body chemistry of cousins is more similar.

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