Other things being equal, the latter will have an advantage. By selfishly refusing to give an alarm call, a monkey can reduce the chance that it will itself be attacked, while at the same time benefiting from the alarm calls of others.
So we should expect natural selection to favour those monkeys that do not give alarm calls over those that do. But this raises an immediate puzzle. How did the alarm-calling behaviour evolve in the first place, and why has it not been eliminated by natural selection? How can the existence of altruism be reconciled with basic Darwinian principles?
The problem of altruism is intimately connected with questions about the level at which natural selection acts. If selection acts exclusively at the individual level, favouring some individual organisms over others, then it seems that altruism cannot evolve, for behaving altruistically is disadvantageous for the individual organism itself, by definition.
However, it is possible that altruism may be advantageous at the group level. A group containing lots of altruists, each ready to subordinate their own selfish interests for the greater good of the group, may well have a survival advantage over a group composed mainly or exclusively of selfish organisms. A process of between-group selection may thus allow the altruistic behaviour to evolve.
Within each group, altruists will be at a selective disadvantage relative to their selfish colleagues, but the fitness of the group as a whole will be enhanced by the presence of altruists. Groups composed only or mainly of selfish organisms go extinct, leaving behind groups containing altruists. In the example of the Vervet monkeys, a group containing a high proportion of alarm-calling monkeys will have a survival advantage over a group containing a lower proportion.
So conceivably, the alarm-calling behaviour may evolve by between-group selection, even though within each group, selection favours monkeys that do not give alarm calls. The idea that group selection might explain the evolution of altruism was first broached by Darwin himself.
In The Descent of Man , Darwin discussed the origin of altruistic and self-sacrificial behaviour among humans. Darwin's suggestion is that the altruistic behaviour in question may have evolved by a process of between-group selection.
The concept of group selection has a chequered and controversial history in evolutionary biology. The founders of modern neo-Darwinism—R. Fisher, J. Haldane and S. Wright—were all aware that group selection could in principle permit altruistic behaviours to evolve, but they doubted the importance of this evolutionary mechanism.
Nonetheless, many mid-twentieth century ecologists and some ethologists, notably Konrad Lorenz, routinely assumed that natural selection would produce outcomes beneficial for the whole group or species, often without even realizing that individual-level selection guarantees no such thing. Williams and J. Maynard Smith These authors argued that group selection was an inherently weak evolutionary force, hence unlikely to promote interesting altruistic behaviours.
This conclusion was supported by a number of mathematical models, which apparently showed that group selection would only have significant effects for a limited range of parameter values. As a result, the notion of group selection fell into widespread disrepute in orthodox evolutionary circles; see Sober and Wilson , Segestrale , Okasha , Leigh and Sober for details of the history of this debate. These free-riders will have an obvious fitness advantage: they benefit from the altruism of others, but do not incur any of the costs.
So even if a group is composed exclusively of altruists, all behaving nicely towards each other, it only takes a single selfish mutant to bring an end to this happy idyll. By virtue of its relative fitness advantage within the group, the selfish mutant will out-reproduce the altruists, hence selfishness will eventually swamp altruism.
Since the generation time of individual organisms is likely to be much shorter than that of groups, the probability that a selfish mutant will arise and spread is very high, according to this line of argument. If group selection is not the correct explanation for how the altruistic behaviours found in nature evolved, then what is? This theory, discussed in detail below, apparently showed how altruistic behaviour could evolve without the need for group-level selection, and quickly gained prominence among biologists interested in the evolution of social behaviour; the empirical success of kin selection theory contributed to the demise of the group selection concept.
However, the precise relation between kin and group selection is a source of ongoing controversy see for example the recent exchange in Nature between Nowak, Tarnita and Wilson and Abbot et.
Sober and Wilson The basic idea of kin selection is simple. Imagine a gene which causes its bearer to behave altruistically towards other organisms, e. Organisms without the gene are selfish—they keep all their food for themselves, and sometimes get handouts from the altruists.
Clearly the altruists will be at a fitness disadvantage, so we should expect the altruistic gene to be eliminated from the population. However, suppose that altruists are discriminating in who they share food with.
They do not share with just anybody, but only with their relatives. This immediately changes things. For relatives are genetically similar—they share genes with one another. So when an organism carrying the altruistic gene shares his food, there is a certain probability that the recipients of the food will also carry copies of that gene.
How probable depends on how closely related they are. This means that the altruistic gene can in principle spread by natural selection. The gene causes an organism to behave in a way which reduces its own fitness but boosts the fitness of its relatives—who have a greater than average chance of carrying the gene themselves. So the overall effect of the behaviour may be to increase the number of copies of the altruistic gene found in the next generation, and thus the incidence of the altruistic behaviour itself.
Though this argument was hinted at by Haldane in the s, and to a lesser extent by Darwin in his discussion of sterile insect castes in The Origin of Species , it was first made explicit by William Hamilton in a pair of seminal papers.
Hamilton demonstrated rigorously that an altruistic gene will be favoured by natural selection when a certain condition, known as Hamilton's rule , is satisfied. The costs and benefits are measured in terms of reproductive fitness. Two genes are identical by descent if they are copies of a single gene in a shared ancestor. The higher the value of r, the greater the probability that the recipient of the altruistic behaviour will also possess the gene for altruism.
So what Hamilton's rule tells us is that a gene for altruism can spread by natural selection, so long as the cost incurred by the altruist is offset by a sufficient amount of benefit to sufficiently closed related relatives. The proof of Hamilton's rule relies on certain non-trivial assumptions; see Frank , Grafen , , Queller a, b, Boyd and McIlreath and Birch forthcoming for details. Kin selection theory predicts that animals are more likely to behave altruistically towards their relatives than towards unrelated members of their species.
Moreover, it predicts that the degree of altruism will be greater, the closer the relationship. In the years since Hamilton's theory was devised, these predictions have been amply confirmed by empirical work.
Similarly, studies of Japanese macaques have shown that altruistic actions, such as defending others from attack, tend to be preferentially directed towards close kin. So a female may well be able to get more genes into the next generation by helping the queen reproduce, hence increasing the number of sisters she will have, rather than by having offspring of her own. Kin selection theory therefore provides a neat explanation of how sterility in the social insects may have evolved by Darwinian means.
Note, however, that the precise significance of haplodiploidy for the evolution of worker sterility is a controversial question; see Maynard Smith and Szathmary ch. The gene's eye-view is certainly the easiest way of understanding kin selection, and was employed by Hamilton himself in his papers. Altruism seems anomalous from the individual organism's point of view, but from the gene's point of view it makes good sense.
A gene wants to maximize the number of copies of itself that are found in the next generation; one way of doing that is to cause its host organism to behave altruistically towards other bearers of the gene, so long as the costs and benefits satisfy the Hamilton inequality.
But interestingly, Hamilton showed that kin selection can also be understood from the organism's point of view. Though an altruistic behaviour which spreads by kin selection reduces the organism's personal fitness by definition , it increases what Hamilton called the organism's inclusive fitness. An organism's inclusive fitness is defined as its personal fitness, plus the sum of its weighted effects on the fitness of every other organism in the population, the weights determined by the coefficient of relationship r.
Given this definition, natural selection will act to maximise the inclusive fitness of individuals in the population Grafen Instead of thinking in terms of selfish genes trying to maximize their future representation in the gene-pool, we can think in terms of organisms trying to maximize their inclusive fitness.
Contrary to what is sometimes thought, kin selection does not require that animals must have the ability to discriminate relatives from non-relatives, less still to calculate coefficients of relationship. Many animals can in fact recognize their kin, often by smell, but kin selection can operate in the absence of such an ability. Hamilton's inequality can be satisfied so long as an animal behaves altruistically towards other animals that are in fact its relatives.
The animal might achieve this by having the ability to tell relatives from non-relatives, but this is not the only possibility. An alternative is to use some proximal indicator of kinship. For example, if an animal behaves altruistically towards those in its immediate vicinity, then the recipients of the altruism are likely to be relatives, given that relatives tend to live near each other.
No ability to recognize kin is presupposed. Cuckoos exploit precisely this fact, free-riding on the innate tendency of birds to care for the young in their nests. Though some sociobiologists have made incautious remarks to this effect, evolutionary theories of behaviour, including kin selection, are not committed to it. So long as the behaviours in question have a genetical component , i. 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. Altruism is the unselfish concern for other people—doing things simply out of a desire to help, not because you feel obligated to out of duty, loyalty, or religious reasons. It involves acting out of concern for the well-being of other people.
In some cases, these acts of altruism lead people to jeopardize themselves to help others. Such behaviors are often performed unselfishly and without any expectations of reward. Other instances, known as reciprocal altruism, involve taking actions to help others with the expectation that they will offer help in return. Everyday life is filled with small acts of altruism, from holding the door for strangers to giving money to people in need.
News stories often focus on grander cases of altruism, such as a man who dives into an icy river to rescue a drowning stranger or a donor who gives thousands of dollars to a local charity. Some examples of altruism include:. Psychologists have identified several different types of altruistic behavior. These include:. While we may be familiar with altruism, social psychologists are interested in understanding why it occurs.
What inspires these acts of kindness? What motivates people to risk their own lives to save a complete stranger? Altruism is one aspect of what is known as prosocial behavior. Prosocial behavior refers to any action that benefits other people, no matter what the motive or how the giver benefits from the action. While all altruistic acts are prosocial, not all prosocial behaviors are completely altruistic.
We might help others for a variety of reasons such as guilt, obligation, duty, or even for rewards. We're not sure why altruism exists, but psychologists have suggested a number of different explanations. Psychologists have long debated whether some people are just born with a natural tendency to help others, a theory that suggests that altruism may be influenced by genetics. Kin selection is an evolutionary theory that proposes that people are more likely to help those who are blood relatives because it will increase the odds of gene transmission to future generations, thus ensuring the continuation of shared genes.
The more closely the individuals are related, the more likely people are to help. Prosocial behaviors such as altruism, cooperativeness, and empathy may also have a genetic basis. Altruism activates reward centers in the brain.
Neurobiologists have found that when a person behaves altruistically, the pleasure centers of their brain become more active. Engaging in compassionate actions activates the areas of the brain associated with the reward system. The positive feelings created by compassionate actions then reinforce altruistic behaviors. Interactions and relationships with others have a major influence on altruistic behavior, and socialization may have a significant impact on altruistic actions in young children.
In one study, children who observed simple reciprocal acts of altruism were far more likely to exhibit altruistic actions. On the other hand, friendly but non-altruistic actions did not inspire the same results. Modeling altruistic actions can be an important way to foster prosocial and compassionate actions in children. Observing prosocial behavior seems to lead to helping behavior among adults as well though the extent to which this occurs varies based on factors like gender, culture, and individual context.
Society's rules, norms, and expectations can also influence whether or not people engage in altruistic behavior. Thus, the framework of altruism would be more complete when we concern both long-term and immediate benefits, as well as both external and internal utilities. The saying the roses in her hand, the flavor in mine could reveal a real psychological activation effect, through which people could promote their personal physical states.
Thus, we named this psychological activation effect as a lingering fragrance effect. When facing a threat e. One is to cope with the threat immediately e. The other strategy is to activate a psychological process to change the perception of the threat.
During crisis, the adverse external conditions could hardly be changed because of insufficient support of food, water, and living conditions.
However, individual cognitions and behaviors are pliant and controllable, offering possibilities to attenuate the threats in crisis. Hence, psychological activations or subjective adjustments could become a prospective coping strategy under such circumstances.
Moreover, we found consistent lingering fragrance effects in spite of this shortcoming. Thus we expect in real life that the lingering fragrance effect of altruistic behaviors should be amplified to benefit individuals in a more extensive manner. This also implied that the psychological reactions may be faster and more flexible than physical reactions. However, it is possible that the physical states could get feedbacks from psychological states when the timeline is extended concerning the close interactions of physical and psychological states e.
In addition, we have mentioned in the introduction as well as in the discussion section that an increase in warmth feelings could be helpful for individuals who were coping with cold environment. Warmth feelings could serve as an easily accessible comfort for them especially in some crisis situations.
On the other hand, it could become a problem when individuals optimistically perceived the environment as warm but failed to respond to the important environmental cue about temperature. This indicates that a raise of warmth feelings may actually become a double-edge sword in some cases. And it is interesting and prospective to explore the other edge of the sword in future studies. In summary, the lingering fragrance effect is an important perspective for understanding the strategy in coping with a threat.
The interaction between physical and psychological systems makes it possible to change the physical state through the mobilization of internal psychological resources. Under the crisis circumstances with limited conditions, using a psychological resource to resist the bad impacts of a crisis could be a more reasonable or even a single possible strategy to cope with threats. As the results showed, active altruistic behavior is a significant way for people to resist the cold environment.
The exploration of the proposed immediate self-reward model of altruistic behaviors could be expanded from two aspects. First, the crisis situations and the experimental settings in the current research were all related to coldness. In some cases, coldness could even be a significant threat to survival e.
In such cases, increased feelings of ambient warmth were regarded as a reward for the individuals. The specific reward and the corresponding psychological processes might change with different adverse situations. For example, coolness could become a more comfortable state when the environment was extremely hot. Future research could help to explore different contents of the lingering fragrance effect and to offer more evidence for an immediate internal reward system.
Second, further studies could be conducted on the rewarding nature of altruism. For example, a positive cycle might be established in which increased warmth perception following altruistic behaviors result in future altruistic behaviors.
Moreover, de Quervain et al. Similarly, neuroscientific studies would be helpful in finding out whether performing an altruistic behavior would activate the same region or some other reward-related brain regions.
Studies 3a, 3b, and 4 involved helping disadvantaged groups i. All of the helping scenarios used in the studies including refining education materials, sending postcards, and donations were actually carried out. The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
We would like to appreciate Xiaoxiao Hu for helping us with data collection. Also, many thanks to Student Union of Psychology in Peking University, Jimunai Middle School and Qizhi Ning, who offered us opportunities to conduct Studies 3a, 3b, and 4 and to help those kids who were in need. Last but not least, we would like to extend our sincere gratitude to the members of Risk and Decision-making Lab of Psychology Department of Peking University for their helpful comments and support on the research.
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