Let me start by recounting what Jean Decety, Inbal Ben-Ami Bartal, and I did in our study demonstrating empathic helping by rats, and what we found. We took pairs of rats who had lived in the same cage for a couple of weeks and placed them in an arena with a centrally located Plexiglas tube. In each hourlong session, one of the pair of rats was trapped in the tube and the cagemate was free to roam in the arena. The only way out of the tube is through a door that can be opened only from the outside and therefore only by the free rat. Sessions were repeated for 12 days.

Rats had to discover how to open the door by trial and error. This was possible because the free rat spent a lot of time climbing on, biting, and generally exploring the tube. After a mean of six daily sessions, the free rat opened the door for the trapped cagemate. Thereafter, the free rat opened the door on the following days. Additionally, after the first—accidental—opening, the free rat opened the door more and more quickly on subsequent days. When confronted with an empty tube, rats sometimes opened it once, but that typically did not get repeated, nor did rats open it more rapidly on subsequent days. Results from the trapped cagemate and empty conditions suggest to us that the free rat is sufficiently rewarded by the liberation of the trapped cagemate to endeavor to open the tube again and again, getting better and faster at it each day.

So with that background, let me explain the male-female differences that we observed. First, all six of the females became “openers,” whereas almost 30 percent of the males were “non-openers.” Second, the time that it took to open the door on days 7 to 12—in other words, after the rats learned to open the door—was greater for males than females. This difference was mostly due to males taking a day or two “off” from opening. In contrast, females opened the tube and liberated their cagemate every day. Finally, females were more active in the trapped condition than were males, although there was no sex difference in activity during the empty condition. In sum, females are more consistently prosocial than are males at both a population level (a greater proportion of females than males became openers and females showed more activity, indicative of more agitation in response to a trapped cagemate) and at the individual level (females opened more consistently day-to-day than did males).

What may underlie these differences? It is commonly thought that in mammals, empathic concern and helping behavior evolved in the context of parental care. Since mammalian offspring are so helpless at the start and need milk from mom, the mothers are, in essence, required for the survival of the offspring. This is a far cry from fish mothers, for example, who are unlikely to ever see their progeny. In order for the mammalian style system to work, a mother must understand the emotions and needs of her offspring, and respond appropriately to ensure their survival. We think that empathy and nurturing evolved or generalized beyond the mother-child bond to occur between two unrelated adults. This would be beneficial for all social animals since the ability to understand and respond to the well-being of another individual is crucial for successful navigation in the social arena.

If mammals had evolved so that only females showed empathic nurturing to unrelated others, then empathy would constitute a full-blown, neural example of a secondary sex characteristic, such as breasts or an Adam’s apple. This does not appear to have happened. Both male and female rats show empathically motivated helping behavior. I think that this tells us two things. First, the sex differences related to empathic helping are less than the sex similarities. Second, since females need empathic helping to successfully reproduce, there may be more selection pressure on females than on males to express this function. Therefore, it may be that prosocial abilities are absent, or inadequately expressed, in a larger proportion of males than females. This is entirely in line with what we see. Personally, I think the more important conclusion is the former: Typically, both female and male mammals will act empathically to help another in distress.

I look forward to hearing from you, the readers, on what this might have to do with religion.

Peggy Mason is a professor of neurobiology at the University of Chicago and the author of Medical Neurobiology.