As is known from the theory of human self-domestication, humans possess certain traits similar to those observed in domesticated animals. This includes brain development and behavioral aspects. In this context, it is worth highlighting some points regarding the development of a tendency toward non-aggressive behavior toward unfamiliar members of one’s own species, which will allow us to better understand why a normal human possesses a strongly expressed violence inhibition mechanism.
In one study, domesticated animals such as pigs, rabbits, or guinea pigs, as well as animals characterized by cooperative breeding, such as the red fox and some primates, were found to possess a significantly larger amygdala volume and higher neuronal density within it compared to many other animal species. This is also true for humans, whom this study classifies as a cooperatively breeding species. During the Pleistocene (Ice Age), it became critically necessary for human survival for children to be cared for not only by their biological mothers but also by other group members. This led to the emergence of corresponding adaptations, including a reduction in children’s fear of close contact with other people and a reduction in mothers’ fear of other people approaching and caring for their children. Furthermore, prosociality, replacing aggressiveness, was extremely necessary for the evolution of all forms of human cultural cognition, including language.
Such an adaptation is fully consistent with the theory of the violence inhibition mechanism, according to which an unconditional reflex is triggered in humans when observing distress signals in others, such as expressions of fear or sadness, which deters them from causing harm by evoking negative feelings. The development of the amygdala led to better recognition and understanding of other people’s fear, which helps prevent a person from behaving aggressively toward them due to their own fear. This part of the brain plays a primary role in regulating aggressive behavior, which also aligns with the concept of a violence inhibitor. In many animals, this works similarly toward members of their own species.
Let’s look at another example of self-domestication: the pygmy chimpanzee (bonobo). Their females prefer to breed with non-aggressive males and jointly defend themselves against aggressive ones, which also steers their selection toward non-aggressiveness. Compared to common chimpanzees, the serotonin neuron density in the amygdala of bonobos is twice as high. This can generally be observed in species that do not typically exhibit aggressive behavior due to social frustration. And as we already know, the serotonergic system is a key regulator of aggressive behavior in animals and humans; the activation of certain serotonin receptors can completely eliminate attacking aggressive behavior toward conspecifics, replacing it, as a rule, with non-aggressive prosocial behavior (while this process does not suppress defensive behavior if a conspecific decides to attack first).
All these data prove once again that a healthy person is characterized by the inhibition of violence due to certain evolutionary adaptations, and that it relies on the full functioning of the amygdala and the serotonergic system. These data are extremely important for the search and development of therapeutic methods aimed at correcting dysfunction of the violence inhibitor in people who exhibit violent behavior and psychopathic predispositions.
Sources:
Cerrito, P., Burkart, J.M. (2023). Human Amygdala Volumetric Patterns Convergently Evolved in Cooperatively Breeding and Domesticated Species. Hum Nat. doi:10.1007/s12110-023-09461-3;
Hare, B. (2017). Survival of the Friendliest: Homo sapiens Evolved via Selection for Prosociality. Annual Review of Psychology, 68(1), 155–186. doi:10.1146/annurev-psych-010416-044201.