sasint / Pixabay
Source: sasint / Pixabay

The most common form of helping found in the animal world - including the human world - is the helping of kin (Hamilton, 1964). In other words, people tend to help those with whom they share large proportions of genetic materials (e.g., mothers helping their infants). This makes good evolutionary sense, as helping kin helps your genes as they exist in the bodies of others.

But kin-based altruism is only a slice of the helping behavior we see in humans (see Geher, 2014). If you’ve lived long enough to be reading this article, then you’re no stranger to an irrefutable fact about our kind: We often help non-kin. Some of us, such as social workers, teachers, and counselors, do this professionally – helping non-kin all day long for five days a week. All of us help non-kin with some level of regularity. Consider the following examples:

  • Helping a neighbor move a piece of furniture.
  • Holding the door for someone who’s holding a bag of groceries.
  • Letting a fellow driver enter your lane.
  • Making a nice dish that you bring to a pot-luck meal at work to share with others.
  • Sharing your notes from a lecture to a colleague who was sick during the last meeting.

We help non-kin all the time! As an evolutionist, I'd suggest that we can think of this fact in a few ways. First, to the extent that the helping of non-kin is species-typical, then this feature of our species must be rooted in our ancestral past. Second, if the helping of non-kin is so central to our nature, then there must be something adaptive about this phenomenon. It must, somehow, end up paying back the helper in the end.

Humans are not the only primates that show non-kin altruism. In fact, there are many species that demonstrate this phenomenon. In the evolutionary literature, this phenomenon is often referred to as reciprocal altruism, a concept made famous by the renowned evolutionary biologist, Robert Trivers (1971).

Put simply, reciprocal altruism is the helping of another with an implicit (unstated) agreement of having the help paid back. If you scratch my back, you can rest assured that I’ll scratch yours when called upon. People don’t help all non-kin equally – we are discriminating. We are more likely to help non-kin who have helped us in the past. In using such an algorithm in helping non-kin, this form of altruistic behavior, then, becomes adaptive for the helper. And it is partly for this reason that helping of non-kin is so prevalent in our species.

Famous for having a knack of articulating basic evolutionary truths that characterize species of all shapes and sized, Trivers demarcated three basic features that must be present in a species if reciprocal altruism is to be evolvable (i.e., capable of producing adaptive benefits to individuals) in that species:

1. The members of the species must have relatively long lifespans. If the lifespan of the organism in question is terribly short, then helping another with the implicit expectation of receiving help back is not likely to pay off – your would-be altruistic friend is too likely to die young so as to be of any use.

2. The members of the species must be able to identify one another. If you implicitly expect any kind of help in return for an altruistic act, then the individual whom you help had better be able to discriminate you from all the others! Not all species have this ability. And, as you can see, this ability is necessary for reciprocal altruism to be evolvable.

3. The members of the species must live in relatively stable groups. In some species, individuals are relatively solitary – perhaps except during mating season. If you live in such a species, helping out a conspecific (i.e., members of the same species) will not likely have much value to yourself. You’ll never see that one again!

Do these three qualities typify our kind? You bet. And this fact helps us understand the highly prevalent nature of non-kin altruism in critters like you and me.

Are humans the only species that fit these three criteria? Not by a long shot. A short list of other species that fit these three criteria include:

  • North American crows
  • Guppies in the freshwater lakes of Upstate New York
  • Olive baboons
  • Wolves
  • Vampire bats

And in demonstrating Trivers’ brilliance as an evolutionary biologist, guess what? Non-kin altruism exists in all of these species. Take vampire bats: The best-studied non-kin altruism in vampire bats takes the form of food-sharing. Their mechanism of food-sharing seems pretty gross to us, but it’s really a great example: Not all vampire bats are successful in finding a blood meal each night. These unsuccessful bats, of course, get hungry. They are reliant on the successful hunters to go out, bite some livestock, get some blood, bring it back, and regurgitate in a helpful manner, so that less successful bats can get a meal. But the successful hunters don’t share with just any old bat. They are biased. They are much more likely to offer up this generous regurgitation for specific others who have provided just this same generosity to them in the past.

While our table manners may be more civilized, humans are like vampire bats: We show a bias toward helping others who have helped us in the past.

Bottom Line

While there are many features of our social ecologies that serve as foundations for who we are, the reality of reciprocal altruism is perhaps one of the most significant. The evolutionary history of reciprocal altruism in humans clearly goes back deep into our history. And it helps explain so much about who we help, when we help, and when we decline to help. While humans are many things, with his ideas on reciprocal altruism, Trivers clearly make the case for understanding us as the “you scratch my back, I’ll scratch yours” ape.

Note: Some of this content was initially written and developed as part of a book proposal for Ivy Press.


Geher, G. (2014). Evolutionary Psychology 101. New York: Springer.

Hamilton W.D. (1964). "The genetical evolution of social behaviour. I" J. Theor. Biol. 7, 1–16.

Trivers, R. L. (1971). The evolution of reciprocal altruism. Quarterly Review of Biology, 46, 35–57.

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