In ancient times, it was not uncommon for cousins to marry cousins, but genetic diseases, which are of common concern today, seem to have been less prominent at the time. For this phenomenon, we can learn from several aspects of the reasons behind it.
First, the issue of genetic diversity is key to understanding this phenomenon. In a small, relatively closed group, inbreeding does increase the risk of genetic disease, as this pattern tends to lead to the accumulation and expression of harmful genes. However, ancient societies were often extended family systems, with family members widely distributed among themselves in different regions. Even if cousins marry each other, their genes are not necessarily highly similar. In fact, due to inconvenient transportation and poor information, gene flow between members of the same family in different regions is relatively low, which may in turn increase gene diversity and reduce the concentration of disease-causing genes.
Second, the limited level of development of ancient medicine and the lack of understanding of genetic diseases have resulted in many malformed children or patients with genetic diseases not surviving to adulthood, or their symptoms are not recorded and paid attention to. In addition, infant mortality is generally high, and many health problems that are not genetically related to genesis may contribute to early child death. As a result, the percentage of offspring that survive and go down in history is relatively small, even if they are born by inbreeding.
Third, cultural and social structures also influence the outcome of consanguineous marriages to a certain extent. In ancient times, marriage was often not just a personal choice, but a means of alliance between families and even nations. Therefore, even if the marriage is a close relative, the two families will often screen and consider in advance, and try to avoid marriages between families with a known history of genetic diseases. This practice, while not based on the theories of modern genetics, inadvertently reduces the spread of certain genetic diseases.
Fourth, the living habits and environmental factors of ancient people cannot be ignored. Reproductive health can be affected by many factors, such as diet, living environment, and social stress. For example, ancient people ate more natural foods and were less exposed to chemicals and contaminants in industrial production processes. These environmental factors may reduce the probability of developing certain genetic disorders.
Finally, we have to take into account the bias of the historical record. Due to the limitations of ancient documentation, our understanding of the genetic status of ancient populations is incomplete. Misshapes or genetic diseases are likely to be overlooked or forgotten over history, leading to a biased understanding of the consequences of inbreeding in ancient times.
To sum up, the marriage of the elder brother marrying the cousin is not as easy to lead to the occurrence of genetic diseases as we think. The combination of genetic diversity, limitations of medical knowledge, socio-cultural factors, living environment, and discrepancies in historical records has resulted in a lower incidence of deformed children in ancient consanguineous marriages than in modern populations. However, this does not mean that inbreeding is harmless, and as humanity deepens our understanding of genetics, we know that such marriage patterns still hide potential risks. With the development of the times, people's understanding of genetic health has gradually improved, and modern society has become more and more inclined to avoid consanguineous marriages in order to ensure the health of offspring.