In ancient feudal society, it is a very normal thing for close relatives to marry, sometimes people will marry for the sake of family interests and status, of which the combination of cousins and cousins is more, such as the original wife of the famous poet Lu You is cousin Tang Wan, in addition to many ancient film and television dramas, there are also many cousins and cousins in love, such as Jia Baoyu and Lin Daiyu in "Dream of the Red Chamber" are cousins.
Because the ancients did not have the awareness of eugenics and good parenting, so as long as it is not an immediate family marriage is not a big problem, and many family marriages are also for the sake of interests and status, such as political marriages for power, many of them are intermarriages of close relatives, but everyone found that there are few records in history of cousins and cousins who have given birth to mentally handicapped children after marriage.
First of all, we must understand the fact that cousins and cousins are not close relatives, they are only cousins, cousins are close relatives, so the blood relationship between cousins and cousins has exceeded three generations, although now the law and morality do not allow, but the cousins are not considered close relatives, and many political marriages do not have feelings, so cousins and cousins did not leave children after marriage.
In fact, even if the cousin and cousin gave birth to a problematic child after marriage, then there will be no record, because the ancients are very face-conscious, if it is found that the child has given birth to a mentally retarded child, then the child has not had time to grow up, it may be quickly dealt with, outsiders do not know at all, so it is even more unlikely in history that there are records of cousins marrying and giving birth to defective children, but the fact that history does not record does not mean that there is really no.