Inbreeding. Couples who have a common ancestor in generations below their great-grandfather are considered close relatives. Cousin marriage is a relatively common consanguineous marriage. Most countries do not encourage consanguineous marriage, or even prohibit it. When close relatives get married, the mortality rate of offspring is high, and dementia, deformed children and genetic diseases often appear. This is because the husband and wife who get married by close relatives get more identical genes from the common ancestor, and the recessive harmful genes that are not conducive to survival are easy to meet in the offspring (that is, homozygous), so it is easy to give birth to children with poor quality. According to the estimation of the World Health Organization, everyone in the population carries about 5-6 recessive genetic diseases. In random marriage (non-consanguineous marriage), it is not easy to form homozygotes (patients) with recessive pathogenic genes because the husband and wife are not related by blood, have few identical genes and carry different recessive pathogenic genes. However, when close relatives get married, it is very likely that both husband and wife carry the same recessive pathogenic gene, so it is easy to meet in offspring, which will increase the incidence of genetic diseases in offspring. The following table lists several recessive genetic diseases, indicating that the incidence rate of offspring of consanguineous marriage is several times that of offspring of non-consanguineous marriage.

In addition, the incidence of polygenic genetic diseases is high, such as hypertension, schizophrenia, congenital heart disease, anencephaly, epilepsy and so on. The mortality rate of children born of consanguineous marriage is also significantly higher than that of children born of non-consanguineous marriage. To this end, many countries have passed laws prohibiting consanguineous marriage. Article 6 of China's Marriage Law stipulates: "lineal blood relatives and collateral blood relatives within three generations are forbidden to get married." Lineal consanguinity refers to the relationship between parents and children, grandparents and grandchildren, and grandparents and grandchildren. Collateral blood relatives within three generations include compatriots, uncles (aunts) nephews (female), uncles (aunts) nephews (female) and cousins. According to the survey, the rate of consanguineous marriage is about 0.7% in cities, 1.2% in rural areas, and some are as high as 2.8%. In some mountainous areas, rural areas and islands, due to underdeveloped transportation, the proportion of close relatives is high, so there are more genetic diseases. Therefore, forbidding consanguineous marriage is an effective measure to reduce or even eliminate the incidence of recessive genetic diseases and improve the quality of the population, which is one of the main contents of eugenics.