Thinking of behaviours an organism can adapt, at the very base an action can always be either selfish or altruistic (cooperative).

Usually, selfish behaviour is assumed to be the preferred choice and the question is how altruistic behaviour comes about.

But if reciprocal altruism can give both participants a higher benefit than their respective costs (which it usually does), why would selfish behaviour (in the sense of behaviour which reaps benefit at another entity's expense) be what we assume to happen? If evolution tends to optimise things, why does it not optimise this?

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    $\begingroup$ What about Symbiosis? In the end you can see every kind of cooperative behaviour as a selfish one if you want to... $\endgroup$
    – Mononess
    May 4, 2012 at 22:05
  • $\begingroup$ Thanks for the comment. You made me realise that the question needs more clarification and at the same time you helped me find the answer to it myself :) I will update the question and add my own answer. $\endgroup$
    – Armatus
    May 4, 2012 at 22:39

2 Answers 2


It basically comes down to a question of the unit of selection.

From the common viewpoint, in which natural selection is seen as acting on individual organisms, it's almost a tautology that the organisms favored by selection are those that maximize their own reproductive fitness. Thus, the possibility that some organisms might engage in acts that help another organism at their own expense may seem like a paradox, or at least a puzzle in need of explanation.

One solution to this puzzle is offered by the gene-centered view of evolution, where selection is viewed as acting on genes, with organisms being merely convenient (and usually, but not always, cooperative) collections of genes that (usually) reproduce together. From this viewpoint, it is not at all surprising that evolution might favor genes that cause an organism to help other organisms, provided that there's a statistical tendency for those other organisms to also be carrying the same gene.

Other mechanisms for the evolution of cooperation do also exist. For example, organisms with sufficiently advanced cognitive capabilities may indeed engage in reciprocal altruism, where they help others only if those others have shown themselves willing to help them in exchange. Such exchanges, being mutually beneficial, do indeed help both participating organisms, and are thus selected for even at the organism level. However, to persist (in the absence of gene or group level selection effects), they generally need some form of enforcement and/or learning mechanism — if cheaters can keep receiving help without ever helping anyone else, they'll do better than the cooperators and eventually displace them.

Also, in some situations, an organism acting simply to help itself (e.g. by modifying its environment to be more suitable for itself) may also end up helping other organisms that just happen to be nearby. In such a situation, selection indeed favors cooperative behavior in isolated organisms by default, with the evolution and persistence of "freeloaders" (who spend less effort on improving their surrounding, relying instead on others to do the work) being only possible as long as there are sufficiently many cooperators around.

Of course, none of these mechanisms are mutually exclusive. Indeed, the presence of some gene-level selection effects tends to be unavoidable in any realistic situation involving cooperative interactions; it's hard to come up with a setting in which no two interacting organisms ever share genes, and obviously the fact that two organisms share a common genetic predisposition to cooperate makes them more likely to do so when they meet.

(However, one not-so-realistic theoretical limit case where that does happen is that of an infinitesimally rare cooperative mutant invading an infinitely large and well mixed population. The popular use of this simplifying limit assumption in classical models of evolution may be one reasons why cooperative behavior is so often seen as something remarkable and hard to explain by evolutionary theorists.)

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    $\begingroup$ This is essentially what I was going to write myself: because there is such a thing as individuals which differ, and individuals who do better (variation and competition due to finite resources), individuals who do themselves best are successful. The biggest exception is probably kin selection since altruism towards relatives can benefit the own genes more than selfishness. $\endgroup$
    – Armatus
    May 4, 2012 at 22:57

Selfish behaviour is not necessarily preferred. It depends on the game (game theory). For example, in absence of relatedness and reciprocity, we would expect:

  • All the population defect in the Prisoner's dilemna
  • Some defect and some cooperate in the snow-drift game
  • Either all inds defect or all inds cooperate (depending on initial condition because it is an unstable equilibrium) in the stag hunt game.

When there is relatedness (genetic correlation) between individuals, Hamilton's theory of inclusive fitness says that:


where the letter d means partial derivative, the function w() is the fitness function and the parameters of the fitness function represent the trait of the focla individual (x), the mean trait in the subpopulation (y) and the mean trait in the populaiton (z). For more information This fitness function depends one the ratio of the fecundity function of cooperating over defecting.

These fecundity functions in the prisoner's dilemna equals the baseline fecundity (usually we write down 1) plus the payoff when the other cooperate times the probability that the interacting individual cooperate plus the payoff when the interacting individual defect times the probability that it defects. It gives:

$$f(cooperate) = 1 + (B-C)x + C(1-x)$$ if the individuals get the payoff B-C when the interacting individual also cooperates and Cwhen the interactzing individual defects. xis the probability that the other individual cooperates.

Usually, the first equation above is written under the form:


Instead of R, you might as well consider another correlation, a correlation of behaviour due to reciprocity. The equation becomes:


You might as well consider both, reciporcity and relatedness which give:



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