Why do all kids take their dad's last name?

The reason for this is that the family name is still related to the society, the primitive society is 90% of the time the matriarchal clan, but at that time the family name is not well developed. Later, the transition to patrilineal clans, the offspring of course with the father's name. A child's family name is actually meant to define the child's rights and obligations to the paternal family. The child's family name identifies the family to which he or she is entitled to inherit, and the family to which he or she is responsible and liable for the affairs of. By taking the child's surname from the father, the flow of the inheritance is always in favor of the immediate male line, and it also ensures that the child will always be loyal to the male line. This is the role of the family name and the fundamental reason why it is important to take the father's name - to ensure the direction in which rights and duties are enforced in a male-dominated society. In fact it is most natural for a child to take the mother's surname. Because the mother never has to wonder if the child is biological, whereas the father has no such natural assurance. The tradition of taking the father's name is given by the social system, and since it has not always been there, it will not always be there. For these reasons, it is easy to understand why inverted marriages take the mother's surname. It is because the child's paternal line has no inheritance to enforce at all; the inheritance comes primarily from the maternal line, and in order to mark the child as having a maternal inheritance, the child must take the maternal sex. The vast majority of families in ancient times had inheritance from the paternal line, so the vast majority of children took their father's surname to mark their inheritance. Daughters were to take their husband's surname in the future, and with the loss of the surname came the loss of inheritance rights and obligations. In modern society, the amount of property in the paternal and maternal lines is getting closer and closer, and the generalization of the one-child policy has led to the uniqueness of the flow of property, so that the significance of the family name has gradually been lost, and only the concept of the so-called tradition that remains in people's minds will still exist for some time. But concepts without a social function don't last long.