First of all, there is no evidence that all children born to close relatives are healthy. In ancient times, children born to close relatives had a high probability of illness or disability or direct death. The probability of close relatives carrying the same pathogenic gene increases, and the possibility of deformed children increases. If close relatives are carriers of excellent genes, naturally born children will be smarter.
The ancients said that? Inbreeding? Children born between cousins usually have albinism and heart disease. Due to the poor social and medical conditions, the congenital disabled can not be treated, and even within the clan, they will decide whether to extend their children's lives. Many children will die soon, so the pathogenic genes will not be passed on to the next generation. Relatively speaking, the chances of such children appearing in society are much smaller. In fact, in ancient times, the ancients were not as ignorant as we thought, and people had the consciousness of avoiding consanguineous marriage. Because experience told them that marriage between close relatives would be bad for future generations, it was later forbidden to marry between close relatives, such as cousins. But the combination between cousins is still relatively common.
With the progress of society, medical conditions are getting better and better, and technology is also developing. People gradually find out where the threshold of consanguineous marriage is, in fact, every three generations. The Marriage Law stipulates that close relatives separated by more than three generations can get married. Although this is the result of scientific calculation, science is not everything. There will inevitably be many diseases and accidents in medicine, which also shows that many intractable diseases are unexpected. In order to avoid unnecessary troubles, protect the health of future generations and try to avoid close relatives.