Are the children of first Cousins necessarily worse quality？

Although marrying Cousins has become taboo in modern society, it was not uncommon in history.

The wife of Franklin Delano Roosevelt, the 32nd president of the United States, was his cousin, Eleanor. The wife of a great scientist like Darwin was his cousin who was a year older than him.

For a long time in human history, society did not consider it immoral or morally corrupt. For a long time, marrying a cousin was considered the best choice.

Yaniv Erlich, a data scientist at Columbia University, has published data in the journal Science on the genetic lineages of 130 million family members. Through the analysis, the researchers were able to find the specific time frame in which society discouraged marriage with Cousins and the degree of genetic correlation between married couples.

Why did ancient people marry Cousins because they couldn't get around?

According to experimental data provided by Erlich, between 1650 and 1850, one in four people eventually married a cousin or cousin.

Many even marry their first brother or sister. Within a century, however, this situation had changed markedly. By 1950, according to Erlich, the average married couple was seventh relative.

One common-sense explanation for the shift is improved transport. Convenient transportation means that single men and women can reach more potential marriageable partners more easily. There was a theory to this, because before 1950, most people lived in such a limited range that most people ended up marrying within a six-mile radius of where they were born, so most people married Cousins of close relatives.

But other factors may also be indirectly responsible.

Erlich said an extensive analysis of genetic data showed that one reason many people continued to marry their Cousins even after the Industrial Revolution's dramatic improvements in transportation mobility was that consolidating financial power and matching families was an important part of family marriage.

Erlich believed that the transformation of social form and the emergence of the taboo of cousin marriage finally pushed people to go beyond the inherent social range and family restrictions when choosing marriage objects. Other factors, including women's increasing autonomy and shrinking family sizes, have also contributed to the phenomenon.

Children who marry Cousins are more likely to have birth defects

The first is that siblings share one eighth of their genes. Children born from marriage with the first representative brother and sister will get more of the same genes, which is easy to cause the expression of recessive harmful genes that are detrimental to survival, resulting in the onset of many diseases.

In random marriage, because the couple is not related to each other, the same genes are few, they carry different recessive pathogenic genes, so it is not easy to form a recessive pathogenic gene homozygote.

In the case of consanguineous marriage, there is a high possibility that the couple will carry the same recessive pathogenic gene, which is easy to meet in the offspring, and increase the incidence of genetic diseases in the offspring.

It is estimated that 4 percent to 7 percent of children born of the first representative brother and sister marriage are likely to have birth defects, but this drops to 3 percent to 4 percent in random marriages.

While the odds are not particularly high, if the next generation of children also marries their Cousins, huge problems arise, resulting in more of the same DNA and a greater chance of birth defects.

History is full of such tragic examples, and the problem is still prevalent in some contemporary countries. In Iceland, for example, where almost all of the country's 330,000 people are concentrated in Reykjavik, the capital, and many people often marry close relatives without even realizing it, locals are opting to use mobile apps to prevent marriage with people who share too many of their genes.

The more distant the relatives, the lower the risk

Marrying a first sibling may carry some risks for offspring, but as the genetic gap between parents widens in each generation, marrying a cousin increases the likelihood of producing healthy offspring. The second represents only 6.25 percent of the genes shared between siblings, compared with 3 percent in the third generation. The average modern American spouse is the seventh closest relative, with no meaningful genetic connection at all.