This is how a musical air of Giuseppe Verdi’s Rigoletto recites:

Sì, vendetta, tremenda vendetta di quest’anima è solo desio...

Revenge is an integral part of our life and culture. Many times a day, we retaliate against someone who has wronged us, big or small. It is an idea that haunts us and, in the most serious cases, even at night and does not make us sleep.

Now we must ask ourselves what this feeling is real and from what is as old as the history of our humanity and of animals too (surprisingly also animal). It arises from a long psychological process in which we remove, in the short or long term, a negative fact that happened to us in the past. At first glance we might judge it as a negative feeling, but as we shall see it is not always so. It has an adaptive origin for the purpose of our survival. It may sound strange, but it helps us move forward every day. It is not only linked to a recent negative fact, but also in the distant past, childhood or adolescence. Who among us hasn't been wronged when we didn't have a chance to defend ourselves? Perhaps, this is also why Italian melodrama, and beyond, abounds with this feeling. It is a feeling that comes from a bad experience on which we develop a strategy based on metacognition to avenge a wrong right away. We can be avenged for being betrayed unexpectedly in times of need or for being deceived and offended in our loyalty.

In man, we could talk for entire pages of vengeance, but we'd never get anywhere. In human, it is a feeling very difficult to investigate, because psychologically very complex. But we could understand more by studying it in animals, monkeys in particular, only that in this case it is necessary to know their whole life, their present social relationships, past and all their experiences. There have been some behavioural scholars who have followed animals from their birth to their death, and contrary to what you might think, they have found that there are a lot of vengeful behaviours in them from as little as from the big ones, also linked to jealousy and envy1,2. We must always think that animals have been present on the face of the Earth for a long time before us and that they have a brain that has normalized for millions and millions of years, when we, Homo sapiens, for no more than 125-150 thousand years: a big difference (for example, it is more than 7 million years that the brain of a chimpanzee has a volume of about 350 cc, an orangutan of 405 cc, a gorilla of 495 cc; that of a man on average is 1250 cc). In essence, vengeful behaviours in animals have cultural roots older than ours. Another surprising fact is that monkeys never throw themselves into the wild, that is, without reflecting on what could be the consequences of their actions. Monkeys prove to be weighted and more calculating than we are. They make in-depth assessments for a long time. You cannot recklessly throw at someone, even if they are justified by a serious wrong suffered in the past. One cannot avenge against a well-coaled group or against a very important individual, or against someone who is protected, strong, rich and powerful. You have to think about it very carefully before you get revenge. We need to put in place a strategy that guarantees good results, even if it is not easy.

Revenge as a fundamental purpose should have to not backfire on those who implement it. You have to be smart, to know how to choose the right time when the individual on whom it has to fall is weaker. You have to have, and the monkeys, almost all of them, have a high-level representative capacity, because, revenge often has to be associated with deception and therefore you must have the ability to perpetrate it well and properly. Monkeys know how to process long-term retaliatory feelings, even after years. Let us give two emblematic examples observed in chimpanzees, namely infanticide and child abuse. Some might wonder what these two abominable revenge behaviours have to do with. Infanticide in chimpanzees is often in fact a vengeful act towards an "infidel" female who has gone to breed in other groups with males not allowed.

We must always remember that chimpanzees live in societies structured basically to a harem, so with an adult and dominant male at the centre of the group who claims to have at their disposal his harem of females who are allowed to reproduce only with him, never with another individual, especially if submissive. Child abuse can also be considered a vengeful act, for the same reasons, that is, because the leader has the “suspicious” that the child's mother betrayed him, that she was with another male. Someone here might raise objections: how does a male chimpanzee mature these suspicions? It's not like the male can question the female about her frequent absences from the group and ask her who she was with. These animals don't talk! Then, if they could talk, they could always lie. But many of them, chimpanzees in particular, may have a set of elements that make them suspicious about the motherhood of a female. The fact is that the adult male may keep in mind that this female has often strayed from the group and during the oestrous cycle and that the mounts have been infrequent. In chimpanzees, but in many other species of monkeys, the mountains are always short but repetitive and it is not that the ejaculations are equal to the number of the mountains themselves, indeed. The frequency of sexual intercourse could be a very important indicator to give the dominant male the guarantee of his paternity.

Child maltreatment is frequent in all animals, including humans, despite the fact that the great ethologist Konrad Lorenz told us about the "childish patterns" inhibiting the aggressiveness of adults towards children, in this case, by those who doubt his paternity. These social inhibitors also apply to humans: for example, large, round head of the small, larger than the body, sweet look, big, beautiful, shiny eyes, etc. Who has never felt these emotions in front of a newborn baby? They are characteristics that make tenderness, especially towards the individuals who are the fathers of these children. In spite of these patterns, the males who feel betrayed, mistreat the offspring they believe to be others, basically take revenge. What is certain is that these behaviours in animals, despite everything, are less frequent than in humans. It would seem far-fetched, but it is so, because we must not only refer to the case where a man feels betrayed by his woman for having conceived a child with another man, but also to cross-revenge or even when killing the children of others without many pretexts. This is the case of child abuse and the suppression of children in wars, famines, epidemics and many other misfortunes of this kind. Actually, if we take these last examples, revenge in animals is less violent and less widespread than in humans. When a child dies in the world every 6 seconds (8.640 a day) from hunger, disease and malnutrition, it is first of all child maltreatment, then infanticide by those who materially carry it out (soldiers, terrorists, mercenaries, etc.), but also by those who ordered it and others who, faced with these facts, turn their heads the other way.

There is another example often observed in monkeys, but it could also apply to man, in which revenge is associated with a behaviour which, at first sight, would seem to have nothing to do with it, that is, food selfishness (if we want also the economic one).

In monkeys, those who are too selfish in the end pay for their greed. Those who hide too much food for themselves can suffer retaliation from the excluded. Those who do not like to divide resources, in this case food, out of selfishness, may eventually suffer revenge, may be removed from society or may even be suppressed. There is, in fact, a collective memory that cannot be so easily erased. Then, those who want to take revenge, in these cases, almost never do so alone, but in a coalition, and never do the avengers present themselves scattered before the selfish. They form alliances, which we could call morals, which try to bring back or restore situations in which there must be no more doubt about the behaviour of these individuals on whom one often depends. All the revolutions of our times, but also those of the past (see for all of them the French Revolution), have had these motivations. In recent times another emblematic example comes to mind, namely the shooting, without trial, without anything, of the dictator of Romania, Nicolae Ceausescu who, until a moment before, was the supreme head of the State. The executors, in this case, did not spare his wife either. They eliminated them together sitting in their seats. As the well-known ethologist Frans de Waal3 wrote: in these cases, it must be clear that selfishness never pays because we are all naturally good. I would add, however, that unfortunately it is not always so, especially in human!

1 Tartabini, A. La coscienza negli animali. Mimesis Edizioni, Milano, 2020.
2 Aaron, B. Envy and jealousy. Canadian Journal of Philosophy, 20(4): 487-516, 1990.
3 de Waal, F.B.M. Good natured: the origins of right and wrong in humans and other animals. Harvard University Press, Cambridge, MA., 1996 (tr. It. Naturalmente buoni. Garzanti Editore, Milano 1997).