Such an explanation invokes cultural group selection and gene-culture coevolution rather than genetic group selection. Many accounts invoke both kinds of group selection. Sober and Wilson , p. In other words, cultural group selection yielded rules and regulations, which then brought about a process of genetic group selection. Similarly mixed accounts have been proposed by others e. This may have led sceptics such as Pinker to dismiss any form of group selection, including cultural group selection, at the outset while only offering arguments targeted at genetic group selection.
Sober and Wilson , see also Wilson argue that individual natural selection cannot select for altruistic behavior because such behavior decreases the relative fitness of individuals within the group and would be selected against.
Therefore, they conclude, it must have been naturally selected at the level of groups. Altruistic behavioral dispositions, by this rationale, evolved because natural selective pressure at the level of the group outweighed selective pressure at the level of the individual. Simply put, advocates of explanations of altruism in terms of genetic group selection claim that altruistic dispositions evolved because altruistic individuals making up altruistic groups had greater reproductive success than less altruistic individuals making up less altruistic groups.
Group selection in this explanation is acting directly on the genome. Such a position is not only championed by Sober and Wilson , others followed in their wake e. Okasha ; Fletcher and Doebeli ; Bravetti and Padilla The obvious challenge to genetic group selection accounts of the evolution of human altruism is that individual selection is a prominent driver of evolution. It is hard to imagine that altruistic groups would not be invaded by free-riders outcompeting them and driving them to extinction.
In response, genetic group selectionists invoke assortative interaction Sober and Wilson , p. Such altruistic clusters would then have a marked evolutionary advantage over less altruistic groups and their genetic endowment would spread in the human genepool. This would have protected altruists against the exploitation of free-riders and explains why reciprocal altruism occurs not only in human groups but also in groups of other species such as certain bird species, vampire bats and meerkats, as pointed out above.
As critics have pointed out, however, postulating that there was genetic group selection of human traits requires us to make a series of additional assumptions that are problematic. First and foremost, it assumes that there was substantial genetic variation between human groups and that there was limited migration between groups which is necessary to sustain genetic variation between groups. Moreover, it assumes that there was a considerable rate of group extinction and that successful groups split up to form more groups reproducing or replicating as organisms and genes do Maynard Smith ; Pinker ; Richerson et al Therefore, the majority of evolutionary scientists are highly skeptical of theories advocating genetic group selection of human traits.
When the famous biologist Edward Wilson not to be confused with David Wilson mentioned above wrote an article in which he defended genetic group selection with colleagues Nowak et al. Footnote 2 Reviewing the arguments and counter-arguments is beyond the scope of this paper. For our purposes, it suffices to say that—while the jury is still out—the majority of evolutionary scientists reject explanations of human altruism in terms of genetic group selection.
What I will argue below is that we do not need to invoke this controversial evolutionary mechanism to explain human altruism. Sober and Wilson are right—I believe—in claiming that between-group dynamics are the architect of certain remarkable human altruistic dispositions.
We cannot explain the evolution of human altruistic dispositions solely in terms of inclusive individual fitness given that it is often directed at non-kin and reciprocity given that it is often directed at people who cannot reciprocate.
This, however, does not entail that we need to go up a level of natural selection the group level. Contra Sober and Wilson , I will argue that it is standard individual natural selection that selected for altruistic dispositions in humans.
How is this possible? To answer this question, we must insert culture and cultural evolution into the equation. Evolutionary processes do not only shape the genome of organisms, they also shape features of human cultures such as beliefs, customs and norms.
In previous work Vlerick , a , b , I have developed a model of cultural evolution in which I identify within and between group dynamics as the main drivers of cultural selection.
Within group dynamics select for cultural features that are psychologically attractive or beliefs that are memorable and are therefore taken up and transmitted by group members. Between group dynamics select for cultural features that provide the group with an advantage over other groups that do not possess these cultural features or possess them to a lesser extent. Between group dynamics select—among other things—for prosocial norms and punishments.
This enhances the altruistic cooperation within the group Vlerick a , b. In particular, competition between groups selects for norms and punishments that reduce conflict and enable and protect altruistic cooperation within groups Aviles ; Boyd et al ; West et al.
Several important factors underlie the cultural selection or proliferation of group beneficial social norms and punishments. In direct conflict between groups, other things being equal, the most cooperative group is more likely to be victorious and conquer the other group. In competition between groups over scarce resources, cooperative groups are likely to outcompete less cooperative groups and survive while the other groups perish. More cooperative groups are also more likely to produce more wealth which throughout human history until very recently correlated with demographic expansion and can lead to the demographic swamping of less successful groups.
Finally, individuals from less wealthy groups often migrate to wealthier groups and the customs and norms of successful groups are often imitated by less successful neighboring groups Bowles and Gintis , p. For all of these reasons, social norms underlying extensive and altruistic in-group cooperation are likely to proliferate. Ethnographic analogues suggest that Pleistocene hunter-gatherer groups possessed such complex sets of rules regulating the interaction of individuals within the group, that there were substantial differences with respect to these sets of rules between different groups and that there was frequent competitive interaction between groups Hill et al.
In such a context, between group dynamics must have been a prominent driver of cultural evolution. Because of this cultural evolutionary process driven by between group competition, societies emerged that were increasingly governed by prosocial norms and punishments. Such a social environment did in turn have a strong effect on the biological evolution of ancestral humans. It naturally selected for cooperative, norm-abiding and altruistic individuals.
Prosocial norms and punishment in ancestral societies did not only ensure that free-riders did not get away with their cooperation eroding behavior they are being punished and that consequently altruism could be sustained within groups Vlerick , a , over time they also shaped the genome of the individuals inhabiting those societies.
Because with such a normative framework in place, the egoists and the sociopaths are reliably punished which included banishment and murder for their anti-social behavior and would be less likely than norm abiding altruists to spread their antisocial genes.
In short, a culturally evolved highly cooperative niche radically changed the social environment in which human genetic evolution took place. It produced what Henrich , p. Humans did not only domesticate animal species e. A culturally evolved social environment steered human genetic evolution. Human culture and biology co-evolved, leading to ever more altruistic humans. The key to explaining the strong altruistic dispositions of many people—inciting to them to behave altruistically towards strangers without expecting anything in return—lies in the uniqueness of this behavior in the animal kingdom.
It evolved in response to an equally unique feature of human life: complex culture with prosocial norms and punishments, which in turn had the power to shape the human genome. Human altruistic dispositions, I have argued, were naturally selected in social environments characterized by ever more stringent prosocial norms, extensive monitoring of group members and harsh punishment of those not abiding by these norms harsh enough to decrease their reproductive success.
In this culturally evolved context, norm abiding altruists had an evolutionary advantage over their more selfish and unruly peers. Therefore, in this section, I will discuss the evidence supporting my hypothesis. While anthropological evidence for the universality of prosocial punishment is of course no guarantee that ancestral human hunter-gatherer societies would have possessed such prosocial punishments, it is nevertheless a good indication that they had. According to Boehm and Bowles and Gintis , p.
However, as Henrich , pp. Only as a last resort does it escalate to banishment, physical violence and coordinated group executions. Henrich finds support for the universality of such prosocial punishments in small-scale societies in studies on a wide range of different ethnic groups see Boehm ; Chudek and Henrich ; Bowles et al. This is not surprising. Developmental research has brought to light that children are prone to punish rule breakers and free-riders at a very young age Melis et al.
This points at an innate human desire to punish rule breakers. Moreover, in all cultures people are socially reprimanded for violations of rules of conduct that do not actually harm anybody such as violating a dietary taboo or ignoring a social convention. Demanding that others conform to the social rules and punishing often in subtle ways those who do not, seems to be deeply ingrained in human nature.
The prevalence of such punishments combined with effective monitoring of social behavior would have reliably disadvantaged individuals less prone to follow social norms and individuals who repeatedly put their own interests before those of others.
Effective monitoring, in turn, is facilitated by reputation tracking and by exchanging social information. There is equally good evidence for the prevalence of these activities in all human societies. According to Trivers and Panchanathan and Boyd , reputation tracking is another human universal. According to Dunbar language evolved gradually in the human lineage for this very purpose.
Language enabled our ancestors to form close ties with a relatively large number of individuals about individuals according to Dunbar and enabled them to acquire and transmit information to others. This protected them against the exploitation of free-riders see also Enquist and Leimar A whopping sixty percent of casual human conversations are about other people Dunbar et al.
From these strands of evidence emerges a picture of the societal context in which our recent evolutionary history took place: a context characterized by demanding pro social norms, incessant monitoring whether or not individuals abide by these norms and hard to escape punishments for those breaking the rules. Tooby and Cosmides , p. Therefore, they argue, human altruistic dispositions were adaptive in our ancestral context which is why they evolved , but are actually maladaptive in the modern context.
They no longer increase the long-term, inclusive fitness of the individuals engaging in altruistic behavior but decrease it, since they lead people to behave altruistically towards total strangers with no chance of reciprocation.
While this was adaptive in ancestral times, it is maladaptive in modern times. Compare it with our craving for sweet tasting food and drinks. These cravings were adaptive in ancestral times, where they motivated humans to consume ripe fruit containing the necessary carbohydrates and vitamin C, but are maladaptive in modern environments filled with cheap and unhealthy candy and soft drinks.
Under scrutiny, however, the mismatch hypothesis to explain human altruism does not hold up. Firstly, as Hill and colleagues , have pointed out, hunter-gatherer societies are relatively open social systems. In all likelihood, our ancestors would have interacted with an important number of people outside of their tribe e. In other words, paleo anthropological evidence seems to refute the premise that our ancestors only interacted with kin and people to whom they were closely acquainted.
Moreover, I discern two important problems with Tooby and Cosmides , p. The first is that it seems to assume that people evolved to be indiscriminate altruists leading them to behave altruistically towards non-reciprocating strangers today. This is not the case. They typically do so because they empathize with these strangers and decide it is the morally right thing to do.
This brings me to the second problem with the mismatch hypothesis and most other evolutionary explanations of human altruism. As I will argue in the next section, underlying actual altruistic behavior are not merely evolved intuition and emotion-based dispositions but also conscious and voluntary reasoning processes.
Many scientific accounts of human altruism ignore Footnote 3 the important role of these reasoning processes or at least, the causal role of these reasoning processes remains underdeveloped in said accounts. They often look no further than the evolutionary rationale underlying altruistic behavior and miss a very important piece of the puzzle.
They also involve reasoning processes. In a landmark experimental study subjecting participants to brain scans while presenting them with moral dilemmas, Greene and colleagues found that next to an emotional cognitive subsystem, we employ a reason-based cognitive subsystem in moral evaluation and decision-making. Whereas the emotional system often floods our moral thinking automatically and subconsciously, the reasoning system can in some cases override its output and generally takes over when presented with moral problems for which we have no ready-made, automatic, intuition or emotion-based response see also Greene and Vlerick Therefore, if we want to explain human altruistic behavior we should not only take into account the evolution of the intuition and emotion-based psychological dispositions which I have described in Sect.
We must also take into account conscious and voluntary reasoning processes involved in moral decision-making. These reasoning processes, I will argue below, have a major impact on moral behavior in general and altruism towards out-group strangers in particular. The moral behavior some people engage in is far-removed from the kind of behavior we would expect given the adaptive rationale of the psychological dispositions underlying this behavior.
Our moral psychology, as argued above, evolved as an adaptation to a highly cooperative niche characterized by strong prosocial norms and punishments that orchestrated in-group interaction.
In other words, our moral psychology evolved for altruistic cooperation within the groups in which we live. Yet humans routinely engage in altruistic acts directed at obvious out-group members and even go so far as to behave altruistically towards non-human animals and future, unborn generations.
This is puzzling. They are not fooled by a confusing modern context, but consciously decide to help those in need, regardless of their culture or ethnicity Vlerick This kind of moral behavior is not rooted in intuition or emotion-based psychological mechanisms which evolved for in-group altruistic cooperation.
It is the outcome of conscious reasoning processes. Peter Singer , p. Reasoning about morality can lead to behavior and moral norms that are far-removed from the behavior for which our moral psychology evolved. These moral actions are not merely the output of hardwired psychological dispositions which explains why many people do not engage in these altruistic acts.
They often involve moral reasoning. Interestingly in this regard, a study has brought to light that altruistic behavior correlates with level of education Westlake et al. The authors of the study surmise that people who benefited from a higher level of education might be better at internalizing prosocial norms. I would add that people who benefited from a higher level of education might also be better trained in reasoning about moral issues and reflecting on their moral behavior.
A rival explanation for altruistic behavior that goes beyond the kind of behavior we would expect from an evolutionary perspective is that people just follow social norms that happen to impose or at least encourage this kind of altruistic behavior.
So, rather than behaving altruistically after autonomous moral reasoning or reflection, people could simply be abiding by social norms or social expectations.
This is a valid point. Norm abidance is indeed a major cause of altruistic behavior see Sect. As pointed out, data gathered from behavioral game-theoretic experiments in different cultural contexts shows that people tend to follow the social norms that govern their societies in these experiments Gintis However, social norm following does not explain all altruistic deeds.
There is no social norm that requires people to donate blood in contemporary societies people are not socially reprimanded for not donating blood , yet some people regularly volunteer to do so.
While they might do so for a variety of reasons—including virtue signaling—moral reasoning is likely to be an important factor. Campaigns for blood donation typically try to persuade people to donate by presenting the public with arguments e.
In other words, these campaigns trigger moral reasoning processes in potential donators, hoping they will make a conscious moral decision to donate. Moreover, even if many people engage in altruistic acts directed at non-kin with no chance of reciprocation because they abide by social norms or expectations, conscious reasoning processes are still part of the explanation of these altruistic acts.
Most of these norms saw the light because individuals challenged the status quo through moral reasoning and because many others accepted the new moral imperative after evaluating the reasons offered in support of this imperative.
Even in cases of social norm following, reasoning processes albeit of others are therefore still part of the picture. They explain why these norms arose in the first place. So, in answering the question why humans routinely engage in altruistic behavior towards non-kin and with no chance of reciprocation, the evolution of altruistic dispositions only provides us with half of the explanation.
In addition to evolved moral intuitions and emotions such as empathy and norm abidance , we must take into account reasoning processes that underlie moral decisions and behavior. This however does not diminish the importance of the evolution of these altruistic dispositions in explanations of human altruism.
Reasoning processes—which are content-free—will not lead to moral behavior by themselves. Footnote 4. These evolved psychological dispositions provide our moral reasoning processes with a direction. Such a moral compass powered by reason—I have argued—is the driver of moral progress. Without reasoning processes there would be no way to challenge the moral status quo. Without an innate intuition and emotion-based moral compass, reasoning would not lead to moral or altruistic behavior.
In the absence of these prosocial dispositions, it is safe to assume that we would apply our reasoning processes in our self-interest and the interest of close kin. What explains the uniqueness of human altruism—the fact that it is often directed at non-kin with no chance of reciprocation—is precisely this powerful combination of a highly prosocial nature adapted to a highly cooperative social context and our ability to take our prosocial behavior to the next level by reflecting on moral norms, decisions and behavior.
Human altruism is exceptional in the animal kingdom. In no other species has widespread biological altruism directed at non-kin, with no chance of reciprocation, been observed. This remarkable behavior has puzzled evolutionists since Darwin and attempts to explain human altruism have created a lot of confusion and debate. It has led many scholars to develop group selection theories, which in turn have been heavily criticized.
Explanations of human altruism are still the subject of much and heated debate today, but often the debate suffers from a lack of clarity. In response, I set out to create some much needed clarity to this incendiary debate by clearly distinguishing genetic from cultural group selection. The latter does not face the difficulties associated with the former and together with gene-culture coevolution provides us with an empirically supported hypothesis of the evolution of the strong altruistic dispositions of humans.
Evolved psychological dispositions, however, do not suffice to explain many instances of actual human altruistic behavior. The final aim of this paper, therefore, was to complete extant scientific explanations of human altruism that have focused solely on its evolutionary underpinning.
Underlying the uniqueness of human altruism are two equally unique human attributes: the social norms and punishments that govern our societies and the reasoning processes we unleash on the evaluation of moral norms and decisions. See Bateson for an extensive account of how empathic concern produces altruistic motivation. The response paper by Abbot et al. Nowak et al. They believe that eusociality evolved primarily through genetic group selection. With the notable exception of Darwin , p.
Abbot, P. Inclusive fitness theory and eusociality. Nature, , E1—E4. Article Google Scholar. Ananth, M. Psychological altruism vs. Acta Biotheoretica, 53 , — Aviles, L. PNAS, 99 22 , — Bateson, C. Altruism in humans. Oxford: Oxford University Press.
Google Scholar. Bicchieri, C. The grammar of society: The nature and dynamics of social norms. New York: Cambridge University Press. Book Google Scholar. Binmore, K. Natural justice. Boehm, C. Egalitarian behavior and reverse dominance hierarchy. Current Anthropology, 34 3 , — Impact of the human egalitarian syndrome on Darwinian selection mechanics. The American Naturalist, Suppl. Bowles, S. The punishment that sustains cooperation is often coordinated and costly. Behavioral and Brain Sciences, 35 1 , 20— A cooperative species: Human reciprocity and its evolution.
Princeton: Princeton University Press. Boyd, R. The evolution of altruistic punishment. PNAS, 6 , — Kin selection theory does not deny the truism that all traits are affected by both genes and environment. Nor does it deny that many interesting animal behaviours are transmitted through non-genetical means, such as imitation and social learning Avital and Jablonka The importance of kinship for the evolution of altruism is very widely accepted today, on both theoretical and empirical grounds.
However, kinship is really only a way of ensuring that altruists and recipients both carry copies of the altruistic gene, which is the fundamental requirement.
If altruism is to evolve, it must be the case that the recipients of altruistic actions have a greater than average probability of being altruists themselves. Kin-directed altruism is the most obvious way of satisfying this condition, but there are other possibilities too Hamilton , Sober and Wilson , Bowles and Gintis , Gardner and West For example, if the gene that causes altruism also causes animals to favour a particular feeding ground for whatever reason , then the required correlation between donor and recipient may be generated.
It is this correlation, however brought about, that is necessary for altruism to evolve. This point was noted by Hamilton himself in the s: he stressed that the coefficient of relationship of his papers should really be replaced with a more general correlation coefficient, which reflects the probability that altruist and recipient share genes, whether because of kinship or not Hamilton , , This point is theoretically important, and has not always been recognized; but in practice, kinship remains the most important source of statistical associations between altruists and recipients Maynard Smith , Okasha , West et al.
Consider a large population of organisms who engage in a social interaction in pairs; the interaction affects their biological fitness. Organisms are of two types: selfish S and altruistic A. The latter engage in pro-social behaviour, thus benefiting their partner but at a cost to themselves; the former do not. So in a mixed S,A pair, the selfish organism does better—he benefits from his partner's altruism without incurring any cost.
However, A,A pairs do better than S,S pairs—for the former work as a co-operative unit, while the latter do not. The interaction thus has the form of a one-shot Prisoner's dilemma, familiar from game theory. The question we are interested in is: which type will be favoured by selection?
To make the analysis tractable, we make two simplifying assumptions: that reproduction is asexual, and that type is perfectly inherited, i. Modulo these assumptions, the evolutionary dynamics can be determined very easily, simply by seeing whether the S or the A type has higher fitness, in the overall population.
The fitness of the S type, W S , is the weighted average of the payoff to an S when partnered with an S and the payoff to an S when partnered with an A , where the weights are determined by the probability of having the partner in question. The conditional probabilities in the above expression should be read as the probability of having a selfish altruistic partner, given that one is selfish oneself.
From these expressions for the fitnesses of the two types of organism, we can immediately deduce that the altruistic type will only be favoured by selection if there is a statistical correlation between partners, i. For suppose there is no such correlation—as would be the case if the pairs were formed by random sampling from the population. Then, the probability of having a selfish partner would be the same for both S and A types, i.
From these probabilistic equalities, it follows immediately that W S is greater than W A , as can be seen from the expressions for W S and W A above; so the selfish type will be favoured by natural selection, and will increase in frequency every generation until all the altruists are eliminated from the population.
Therefore, in the absence of correlation between partners, selfishness must win out cf. Skyrms This confirms the point noted in section 2—that altruism can only evolve if there is a statistical tendency for the beneficiaries of altruistic actions to be altruists themselves.
The easiest way to see this is to suppose that the correlation is perfect, i. This simple model also highlights the point made previously, that donor-recipient correlation, rather than genetic relatedness, is the key to the evolution of altruism. What is needed for altruism to evolve, in the model above, is for the probability of having a partner of the same type as oneself to be sufficiently larger than the probability of having a partner of opposite type; this ensures that the recipients of altruism have a greater than random chance of being fellow altruists, i.
Whether this correlation arises because partners tend to be relatives, or because altruists are able to seek out other altruists and choose them as partners, or for some other reason, makes no difference to the evolutionary dynamics, at least in this simple example.
Altruism is a well understood topic in evolutionary biology; the theoretical ideas explained above have been extensively analysed, empirically confirmed, and are widely accepted. Nonetheless, there are a number of conceptual ambiguities surrounding altruism and related concepts in the literature; some of these are purely semantic, others are more substantive.
Three such ambiguities are briefly discussed below; for further discussion, see West et al. According to the standard definition, a social behaviour counts as altruistic if it reduces the fitness of the organism performing the behaviour, but boosts the fitness of others.
This was the definition used by Hamilton , and by many subsequent authors. However, there is less consensus on how to describe behaviours that boost the fitness of others but also boost the fitness of the organism performing the behaviour. As West et al. To avoid this confusion, West et al. Whatever term is used, the important point is that behaviours that benefit both self and others can evolve much more easily than altruistic behaviours, and thus require no special mechanisms such as kinship.
The reason is clear: organisms performing such behaviours thereby increase their personal fitness, so are at a selective advantage vis-a-vis those not performing the behaviour. The fact that the behaviour has a beneficial effect on the fitness of others is a mere side-effect, or byproduct, and is not part of the explanation for why the behaviour evolves. For example, Sachs et al. Also indicative of the difference between altruistic behaviour and behaviour that benefit both self and others is the fact that in the latter case, though not the former, the beneficiary may be a member of a different species, without altering the evolutionary dynamics of the behaviour.
By contrast, in the case of altruism, it makes an enormous difference whether the beneficiary and the donor are con-specifics or not; for if not, then kin selection can play no role, and it is quite unclear how the altruistic behaviour can evolve. Unsurprisingly, virtually all the bona fide examples of biological altruism in the living world involve donors and recipients that are con-specifics.
A quite different ambiguity concerns the distinction between weak and strong altruism, in the terminology of D. Wilson , , This distinction is about whether the altruistic action entails an absolute or relative fitness reduction for the donor. To count as strongly altruistic, a behaviour must reduce the absolute fitness i. Strong altruism is the standard notion of altruism in the literature, and was assumed above. To count as weakly altruistic, an action need only reduce the relative fitness of the donor, i.
Thus for example, an action which causes an organism to leave an additional 10 offspring, but causes each organism s with which it interacts to leave an additional 20 offspring, is weakly but not strongly altruistic. Should weakly altruistic behaviours be classified as altruistic or selfish? This question is not merely semantic; for the real issue is whether the conditions under which weak altruism can evolve are relevantly similar to the conditions under which strong altruism can evolve, or not.
To appreciate this argument, consider a game-theoretic scenario similar to the one-shot Prisoner's dilemma of section 4, in which organisms engage in a pair-wise interaction that affects their fitness. Organisms are of two types, weakly altruistic W and non-altruistic N. W -types perform an action that boosts their own fitness by 10 units and the fitness of their partner by 20 units; N -types do not perform the action.
The payoff matrix is thus:. The payoff matrix highlights the fact that weak altruism is individually advantageous, and thus the oddity of thinking of it it as altruistic rather than selfish. To see this, assume for a moment that the game is being played by two rational agents, as in classical game theory. Clearly, the rational strategy for each individual is W , for W dominates N. Each individual gets a higher payoff from playing W than N , irrespective of what its opponent does —30 rather than 20 if the opponent plays W , 10 rather than 0 if the opponent plays N.
This captures a clear sense in which weak altruism is individually advantageous. In the context of evolutionary game theory, where the game is being played by pairs of organisms with hard-wired strategies, the counterpart of the fact that W dominates N is the fact that W can spread in the population even if pairs are formed at random cf.
Wilson To see this, consider the expressions for the overall population-wide fitnesses of W and N :. Therefore, weak altruism can evolve in the absence of donor-recipient correlation; as we saw, this is not true of strong altruism. So weak and strong altruism evolve by different evolutionary mechanisms, hence should not be co-classified, according to this argument.
However, there is a counter argument due to D. Wilson , , who maintains that weak altruism cannot evolve by individual selection alone; a component of group selection is needed. Wilson's argument stems from the fact that in a mixed W , N pair, the non-altruist is fitter than the weak altruist.
More generally, within a single group of any size containing weak altruists and non-altruists, the latter will be fitter.
So weak altruism can only evolve, Wilson argues, in a multi-group setting—in which the within-group selection in favour of N , is counteracted by between-group selection in favour of W. On Wilson's view, the evolutionary game described above is a multi-group setting, involving a large number of groups of size two. Thus weak altruism, like strong altruism, in fact evolves because it is group-advantageous, Wilson argues. The dispute between those who regard weak altruism as individually advantageous, and those like Wilson who regard it as group advantageous, stems ultimately from differing conceptions of individual and group selection.
For Wilson, individual selection means within-group selection, so to determine which strategy is favoured by individual selection, one must compare the fitnesses of W and N types within a group, or pair. For other theorists, individual selection means selection based on differences in individual phenotype, rather than social context; so to determine which strategy is favoured by individual selection, one must compare the fitnesses of W and N types in the same social context, i.
These two comparisons yield different answers to the question of whether weak altruism is individually advantageous. Thus the debate over how to classify weak altruism is intimately connected to the broader levels of selection question; see Nunney , Okasha , , Fletcher and Doebeli , West et al. A further source of ambiguity in the definition of biological altruism concerns the time-scale over which fitness is measured. Conceivably, an animal might engage in a social behaviour which benefits another and reduces its own absolute fitness in the short-term; however, in the long-term, the behaviour might be to the animal's advantage.
So if we focus on short-term fitness effects, the behaviour will seem altruistic; but if we focus on lifetime fitness, the behaviour will seem selfish—the animal's lifetime fitness would be reduced if it did not perform the behaviour. Why might a social behaviour reduce an animal's short-term fitness but boost its lifetime fitness? Sachs et al. By performing the behaviour, and suffering the short-term cost, the animal thus ensures or raises the chance that it will receive return benefits in the future.
Similarly, in symbioses between members of different species, it may pay an organism to sacrifice resources for the benefit of a symbiont with which it has a long-term relationship, as its long-term welfare may be heavily dependent on the symbiont's welfare. From a theoretical point of view, the most satisfactory resolution of this ambiguity is to use lifetime fitness as the relevant parameter cf.
West et al. This stipulation makes sense, since it preserves the key idea that the evolution of altruism requires statistical association between donor and recipient; this would not be true if short-term fitness were used to define altruism, for behaviours which reduce short-term fitness but boost lifetime fitness can evolve with no component of kin selection, or donor-recipient correlation.
The theory of reciprocal altruism was originally developed by Trivers , as an attempt to explain cases of apparent altruism among unrelated organisms, including members of different species.
Clearly, kin selection cannot help explain altruism among non-relatives. Trivers' basic idea was straightforward: it may pay an organism to help another, if there is an expectation of the favour being returned in the future.
The cost of helping is offset by the likelihood of the return benefit, permitting the behaviour to evolve by natural selection. For reciprocal altruism to work, there is no need for the two individuals to be relatives, nor even to be members of the same species.
However, it is necessary that individuals should interact with each more than once, and have the ability to recognize other individuals with whom they have interacted in the past.
This evolutionary mechanism is most likely to work where animals live in relatively small groups, increasing the likelihood of multiple encounters. Where reciprocal altruism is referred to below, it should be remembered that the behaviours in question are only altruistic in the short-term.
The concept of reciprocal altruism is closely related to the Tit-for-Tat strategy in the iterated Prisoner's Dilemma IPD from game theory. In the IPD, players interact on multiple occasions, and are able to adjust their behaviour depending on what their opponent has done in previous rounds. There are two possible strategies, co-operate and defect; the payoff matrix per interaction is as in section 2. The fact that the game is iterated rather than one-shot obviously changes the optimal course of action; defecting is no longer necessarily the best option, so long as the probability of subsequent encounters is sufficiently high.
In their famous computer tournament in which a large number of strategies were pitted against each other in the IPD, Axelrod and Hamilton found that the Tit-for-Tat strategy yielded the highest payoff. In Tit-For-Tat, a player follows two basic rules: i on the first encounter, cooperate; ii on subsequent encounters, do what your opponent did on the previous encounter.
The success of Tit-for-Tat was widely taken to confirm the idea that with multiple encounters, natural selection could favour social behaviours that entail a short-term fitness cost. Subsequent work in evolutionary game theory, much of it inspired by Axelrod and Hamilton's ideas, has confirmed that repeated games permit the evolution of social behaviours that cannot evolve in one-shot situations cf.
Nowak ; this is closely related to the so-called 'folk theorem' of repeated game theory in economics cf. Bowles and Gintis For a useful discussion of social behaviour that evolves via reciprocation of benefits, see Sachs et al. If the wildebeests gather into a single group, then the risk of any single individual being eaten is reduced. Wildebeests do suffer social costs from aggregating in groups — grazing sites may not provide adequate food for every individual in the group, for example.
However, it is not difficult to imagine that the costs of social aggregation are much smaller than the benefits of the defense against predation. This is a simple example of how the costs and benefits of social behavior may evolve and be maintained. Figure 2: Wildebeests gathered into groups are more protected from predators than any solitary wildebeest.
Living in groups involves a balance of conflict and cooperation, which is mediated by the costs and benefits associated with living socially. When the benefits of living socially exceed the costs and risks of social life, scientists predict that social cooperation will be favored. The benefits of social life typically occur when one individual is the benefactor of an act of altruism.
An altruistic act is one that increases the welfare of another individual at an actual or potential cost of the individual who performs the act. An example of altruism comes from ground squirrels, who may warn other members of their group about a predatory hawk overhead.
Other examples of altruistic behavior include sharing nesting space and helping to raise offspring of an unrelated individual. Altruism by definition decreases the fitness of the individual, so how can this behavior persist? Vampire bats Figure 3 returning from an unsuccessful foraging bout will beg to share food from successful individuals. It is most directly in the interest of the solicited bat to keep its own food, as it requires the nutrients to survive and reproduce, and giving up part of its meal is in fact altruistic.
Figure 3: A vampire bat, an excellent model system to test theories regarding the altruistic sharing of food. So why would a vampire bat share its meal of blood?
The answer is reciprocity. In the early s, graduate student and researcher Gerald Wilkinson conducted a series of experiments to demonstrate that vampire bats in Costa Rica often shared blood with other bats sharing their roosts. He found, however, that bats did not share their meals with all other bats equally. Why would bats not share food equally? Based on long-term measurements of bat movements among roosts, Wilkinson found that some bats were more likely to interact with certain individuals more than others.
Bats were far more likely to share blood with bats they were more likely to encounter in the future. In other words, when there was a greater opportunity for reciprocation, the bats were more likely to share their meals.
Bats would not share blood meals with other bats if there was little chance that the other individual would be able to return the favor. Reciprocity enables the existence of altruism because — in the long term — the benefits of altruism can outweigh the costs of altruism.
In this particular example, the relative cost of sharing food, when available, is less than the potential future benefit of receiving food when hungry. The evolution of social behavior at its most intimate and complex degree is found in eusocial animals. Eusocial species live in colonies. Only a relatively small fraction of the animals in the colony reproduce; the non-reproductive colony members provide resources, defense, and collective care of the young.
The list of known eusocial animals includes ants, termites, some wasps, some bees, a small number of aphid and thrip species, two species of mammal the naked mole rat and the Damaraland mole rat , and multiple species of reef-dwelling shrimp. How can selection produce an organism that has no chance of reproducing independently, whose fitness is entirely invested into colony mates?
In other words, how can animals have no direct fitness and only indirect fitness? Individuals in colonies are usually related to one another, and relatedness can even be greater than 0. An ecological explanation for the evolution of eusociality is that colonies often produce a very large number of offspring, such that even when relatedness is low the indirect fitness of the non-reproductive workers may be greater than if they had the capacity to reproduce independently.
In eusocial animals, the high productivity resulting from communal life and the efficient division of labor among workers takes place in an environment which is usually well defended against natural enemies Figure 4.
In nearly all eusocial species, colonies are protected through structural means such as termite nests in wood, or shrimp in marine sponges , with venom of wasps, bees, and ants , or by both means. Figure 4: Social insects have well protected or defended nests, including termites a , wasps b , and bees c.
Social and altruistic behaviors require a broad view of Darwinian fitness and an understanding that animals can perform behaviors that are responsive to short-term and long-term consequences for their fitness. By conducting research into how organisms interact with their environment and how the environment is predictive of their survival and reproductive success, researchers are able to explain how social behavior has evolved via the mechanism of natural selection.
Hamilton, W. The genetical evolution of social behaviour. International Journal of Theoretical Biology 7 , Sherman, P.
Nepotism and the evolution of alarm calls. Science , Wilkinson, G.
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