The commonly held notion of sex differences in aggression among mammals, that is males are more aggressive than females, is a convenient myth. It is derived from a culture in which overt, physical aggression is suppressed in girls from birth, while encouraging the same behaviors in boys. The counter argument to this social rearing hypothesis is that these same elements are not part of the social development of animals, yet the same sex difference exists for these species. Or does it?
The common way of assessing aggression in animals is what is called “the resident-intruder paradigm”. Here, an unfamiliar individual is introduced into another animal’s home cage. For rodents, the standard methodology is to place a male intruder into the cage of another singly-housed male. After a relatively brief period of time the resident animal will initiate an attack on the intruder. When similar studies were done using a female resident, the female rarely attacked the male intruder. Hence the basis for the sex difference in animals.
The problem here is that there are a lot of costs to being aggressive, including the potential for injury or death. So there needs to be a clear benefit to engage in aggression. That benefit is usually acquiring or defending key resources. One of those resources is access to potential mates. A situation that is familiar to all high school girls and boys.
The ecological question surrounding the construction of the resident-intruder test is what advantage is gained by attacking the intruding male? For a resident male, the intruder is a competitor for resources, for example, potentially a female mating partner. For the resident female there is no advantage to attacking a potential mate. Scenarios that again are familiar to all high schoolers.
Let’s turn this scenario around. Instead of a male intruder, what would be the outcome of introducing a female intruder? A male would not attack the female, but a female might do so. In this case, we also have a sex difference in aggression, though the conclusion would be that females are aggressive and males are not. This is not an intellectual exercise as the experiment has been done in a somewhat modified form. Female hamsters, if they are not sexually responsive, will attack both male and female intruders. If they are sexually responsive they will attack female intruders, but not males. Male hamsters will attack male intruders, but will try to mate with females whether or not the females are sexually responsive. Rats are a social species and triggering aggression in female rats requires establishing a stable social group of males and females. If a male rat is introduced into this social structure, the males will attack the intruder. If a female rat is introduced the females will attack the intruder.
In this scenario there is indeed a sex difference in aggression, not the binary question of whether are males and females aggressive, but a more subtle distinction of differences in the contextual cues impacting the initiation of aggression. What triggers aggression in male and female rodents, or in boys and girls, or in men and women is quite different. The same is true for the physiological control of aggression in male and females, as well as the brain circuits controlling aggression. All this being true is quite different from the common notion that males are aggressive and females are not.
The Science of Sex and Aggression 