David: I just briefly wanted to go back to that, that experiment of the monkey that starved itself. That seems to me an extraordinary thing for a monkey to do. I mean, did it not surprise them when they did that in the 50s?

FdW: I did that together with Sarah Brosnan, and we were doing experiments on economics in monkeys. Sort of like, how much food do you need for a task? How willing are you to work with a partner? How willing are you to share with a partner who helped you or did not help you? … and all of these things. And in that context, by accident, we discovered that the monkeys were very sensitive to what the partner was getting compared to what they were getting. It doesn't make any sense, because if you read the literature on rats pressing levers and things – all that animal learning theory – there's never any talk about this rat thinking about what the other rat is getting.
So we didn't know what to do with that, and we started to test it out systematically. And so we developed a very simple task. We would give a monkey a pebble in his cage; we would hold up a hand and he would have to give it back to us, and as soon as he gave it back, he got a reward – a very simple task.

Now, if you do that with one monkey and you use little pieces of cucumber, you can do that thirty times in a row, and he will eat a lot of cucumber. If you put a partner next to him and you give that partner cucumber, they will both do it thirty times in a row and they're perfectly fine. But if you give the partner grapes, and grapes are ten times better than cucumber, then the one who gets the cucumber still is going to refuse. And not only is he going to refuse, he's going to get agitated, and he's going to shake the cage and he's going to throw the cucumber out, and he gets agitated. He becomes very upset by the whole situation, which is irrational because the cucumber was good before, why is it not good anymore?

We called it inequity aversion, but the media immediately talked about fairness and saying the monkeys have a sense of fairness. Then a philosopher wrote to us, and said it’s impossible for monkeys to have a sense of fairness, because fairness was discovered during the French Revolution! Basically, that tells us morality comes from a bunch of old guys in Paris who sit around and say, ‘Well fairness would be a good idea, you know.’

David: And everyone else in the world went, ‘Oh my God, what a good idea.’

FdW: Yeah, ‘let’s spread the word on fairness; it's a good thing, you know.’ And so that's how they think we arrive, through reasoning and logic, at a point where we say fairness is a good thing and we implement it in society. But it is completely the other way around. Young children, two-year-old children, already have a sense of fairness, just like my monkeys do. And so it's an emotional process: you compare what you get with what somebody else gets.
People have done it with dogs; they've done it with crows, now, because it's a little up and coming field of inequity aversion in animals. And it's basically in many cooperative species we find this now. Our prediction is that in uncooperative species, like solitary animals – like, let's say, the domestic cat is a bit of a solitary animal – you will not find it as much. So it is related, we think, to animals needing cooperation partners with whom they need to share things, and if you're not willing to share, you're not a good partner.

David: In that experiment, did the monkey who was getting the cucumber sometimes just look at the one with the grapes and say, ‘Hey’?

FdW: No, chimpanzees will do that.

David: They do? They'll say, ‘Give me the grapes, come on hand them over’?

FdW: The monkeys have a sense of fairness that is at the level of a two or three-year-old child, which also don't look for hand-outs and things like that. But the chimpanzees, yes. In chimpanzees you may have a situation that the one who gets the grape will refuse the grape until the other one also gets the grape.  People have said, why would you call it a sense of fairness? You could call it resentment. I resent that you get more than me.

Ard Yeah, sure.

FdW: But then you still wonder, why do I resent that you get more than me and under what kind of circumstances? And certainly, when chimpanzees go further than that and share with the one who gets less – because the monkeys don't do that but the chimps do that – they have an understanding that that's also probably necessary for cooperative relationships. And so it's much than just resentment. I think it is a sense of fairness and a sense that they understand that if things are not equally divided, then cooperation’s going to fall apart. So there's a self-interested component to it. That is, for my cooperative relationships I need this kind of sense of fairness, and that's what we're seeing for humans also. Humans don't have a sense of fairness for no reason at all: they have a sense of fairness because our cooperative societies rely on it.