Cross-species studies of animal populations have shown that cubs who are isolated from their mothers within a month of birth often show strong aggression. For example, when looking for food, he is aggressive and fighting for food, while other cubs give in and he is still fighting for food. The hypothesis that explains this phenomenon is that the aggressive problem is due to the lack of social training guided by parents at the newborn stage.
Babies who are adopted in the first three months of life often behave aggressively in childhood would most strongly strengthen the above argument.