The survey evidence presented in the concluding section suggests that participants were well aware of this. In order to evaluate the quality of hand hygiene amongst participants, we use three distinct measures from the WHO guidelines about HH.
First, we consider the time spent washing hands. Second, we study the quality of hand coverage, i. Third, we consider compliance with a rule prescribing participants use a tissue to switch off the tap, after finishing washing their hands. For simplicity and brevity, we present the means of these variables in the figures below; the distributions of these variables can be found in section K in S1 Appendix. Fig 2 shows that, across treatments, participants on average washed their hands for slightly more than 20 seconds.
While this may be more than the time spent by many adults washing their hands, it falls considerably short of the minimum recommended duration stated by WHO 40 seconds.
This implies that there are good conditions for our treatments to increase the quality of hand hygiene and, in this instance in particular, the time spent washing. However, we find no statistically significant differences across treatments Eyes vs. Fig 3 presents the average quality of hand coverage across treatments.
As mentioned, the RAs were professionally trained to evaluate the extent to which participants followed the WHO guidelines and, in this instance, covered adequately all hand surfaces. Performance was coded as 0 if the participant did not attempt to cover multiple surfaces e. Fig 4 presents the extent to which participants across treatments turned off the tap after washing their hands using a paper towel. This is critical in HH because a lot of bacteria can be found on the water tap.
Participants have therefore been trained that not using a towel reduces considerably the efficacy of HH at combating disease transmission. Performance was coded as 0 if the participant did not use a paper towel at all, as 1 if the participant used a paper towel but improperly e. Neither the fraction of participants using a paper towel properly differs significantly across treatments Eyes: Our paper presents the first empirical test of the impact of surveillance cues on the altruistic behavior of anonymous strangers when reciprocity is precluded and participants are unaware they are being studied.
Despite using cues that have been successfully used previously in the literature, we find no evidence surveillance cues increase the degree of altruistic behavior in our experiment. That is, our findings do not support the hypothesis that altruistic behavior among strangers is maladaptive. One concern with all studies reporting null results is that this is due to the statistical tests being underpowered. This is clearly not the case in our experiment.
Not only do we find no evidence across three distinct measures that the picture of eyes has a significant impact on altruistic behavior in our experiment, but the effect itself is sometimes zero Fig 3 or negative Fig 4. By comparison, the effect of posting a picture of a camera over a wash basin—which as we argued could not be considered as supportive of the evolutionary legacy hypothesis—is also always insignificant and small in size, but at least it is always positive. Therefore, the overall lack of a significant effect cannot be attributed to insufficient statistical power.
Another concern may be that the lack of a positive effect is due to the fact that our experiment investigates the impact of surveillance cues on the quality of hand hygiene intensive margin but not on the decision to wash hands extensive margin. A recent meta-analysis of laboratory studies however contradicts these earlier results, finding no differential effect of cues on the extensive and the intensive margin [ 34 ].
Further, some field studies find the opposite result, i. Therefore, there exist neither clear empirical evidence nor theoretical reasons to expect the automatic activation of the reciprocity-based psychology will operate differently on the decisions on the extensive and intensive margins. However, it is worth emphasizing that similar concerns apply in laboratory environments. In fact, they are arguably greater: not only there are several participants in the lab at the same time— some of whom subjects may know personally—but their decisions are recorded by a computer and possibly observed by the experimenter.
Even if this is not the case, participants—who often partake repeatedly in lab experiments—should anticipate that their final payment will ultimately reveal the extent of their altruistic behavior to the experimenter. It should also be noted that behavior across measures in our experiment falls considerably short of that described in the WHO guidelines.
If participants were concerned about their reputation, one might have expected higher compliance with the guidelines than observed. An altogether different concern with our study could be that hand hygiene prior to treating a patient is in fact not an altruistic act as we claimed. Although similar claims are common in the medical literature [ 31 , 33 ], one might wonder whether hand hygiene is regarded as altruistic, i.
To address this concern, we administered a survey to medical students with the same level of training and background as those who participated in our experiment see section I in S1 Appendix. Respondents were presented with a vignette designed to mimic the situation and incentives in our experiment, and asked whether they would wash their hands prior to treating the patient or not, and the reasons for their decisions.
The survey also included a question to evaluate our claim that the quality of hand-hygiene matters when it comes to reducing infection by asking participants whether they agree that washing hands for longer reduces the risk of infection for the patient. Of the survey respondents who stated they would wash their hands prior to taking the blood pressure, This supports our interpretation of hand hygiene prior to treating a patient as being an altruistic act as it is driven by a concern for the welfare of the patient.
Perhaps unsurprisingly, we find that other concerns also appear to play a role see sections I and J in S1 Appendix , implying that hand hygiene prior to treatment is not driven exclusively by altruistic concerns for everyone. This, however, does not invalidate our analysis which only requires that altruistic motives are an important determinant of behavior in our experiment. At first pass, our findings appear to contradict those in previous field studies finding a strong positive effect of surveillance cues on altruistic behavior in natural environments [ 17 — 25 ].
Such interpretation of our findings however would be wrong. A critical difference between these studies and ours, stemming from the different research aims, is that the cues in these studies were placed in public spaces such as university cafeterias [ 17 , 18 ], public car parks [ 20 ], super markets [ 13 , 21 ] or hospital entrances [ 25 ], over an extended period of time.
This implies that real reputational concerns were at play. For example, participants in all these studies could self-select into several treatments, more than once, suggesting that individuals may be aware of the treatment manipulations and thus suspect they are being monitored.
Similarly, since the manipulations occurred in places frequented by the participants, many of the encounters were likely to be neither anonymous nor one-shot, implying that reciprocity is not precluded by design. For these reasons, these studies suggest a potentially useful, low-cost, policy intervention as was intended by the authors but the evidence cannot inform the debate of whether altruistic behavior between strangers is maladaptive.
Taken together, the field evidence suggests that surveillance cues may be effective in promoting altruistic behavior in circumstances in which there are real opportunities to build a good reputation. In these instances, the cues may serve as a signal of what the expected behavior is and that behavior is monitored. In line with this is the finding that the surveillance-cue effect appears to be strongest when peer effects are modest [ 13 , 18 ], possibly due to the increased difficulty of monitoring behavior in large groups.
Additional studies can help explore the underlying mechanism through which surveillance cues operate. Our findings indicate that surveillance-cues effects should not readily be interpreted as evidence that altruistic behavior between strangers is maladaptive. Informed consent was obtained by participants in a way which would not make participants aware that their behavior in the field experiment was studied see section A in S1 Appendix.
Neither the existence nor the purpose of the field experiment that took place during the POSCE was revealed to participants at the end of the experiment.
All aspects of our study, including the methods of consent and disclosure were carried out in accordance with relevant guidelines and regulations at United Arab Emirates University, and the IRB-approved protocols.
We thank the Editor, two anonymous reviewers, Loukas Balafoutas and Ernesto Reuben for useful comments on the paper, and Iain Blair for assisting us with the experiment. Browse Subject Areas?
Click through the PLOS taxonomy to find articles in your field. Abstract The degree of altruistic behavior among strangers is an evolutionary puzzle.
Funding: The authors received no specific funding for this work. Introduction The degree of altruistic behavior among strangers in modern societies is a major evolutionary puzzle [ 1 , 2 ]. The experiment The experiment was conducted in a large university, which is well-regarded locally for its medical program: the United Arab Emirates University see section A in S1 Appendix.
Download: PPT. Fig 1. Picture of the wash basin in the private examination room featuring the watching eyes. Results Our measure of altruistic behavior is the quality of hand hygiene prior to treating the patient. Fig 2. Average time spent washing hands across treatments with percent confidence intervals.
Fig 3. Average quality of hand coverage across treatments with percent confidence intervals. Fig 4. Average compliance with turning-off-tap-with-paper-towel rule across treatments with percent confidence intervals. Discussion Our paper presents the first empirical test of the impact of surveillance cues on the altruistic behavior of anonymous strangers when reciprocity is precluded and participants are unaware they are being studied.
Supporting information. S1 Appendix. Details about experimental procedures, power calculations, robustness checks and surveys. S1 Data. Acknowledgments We thank the Editor, two anonymous reviewers, Loukas Balafoutas and Ernesto Reuben for useful comments on the paper, and Iain Blair for assisting us with the experiment.
References 1. Fehr E. Nowak M. Five Rules for the Evolution of Cooperation. Science , — Cosmides L. Evolutionary psychology: New perspectives on cognition and motivation. Annual Review of Psychology , 64, — Tooby J.
The past explains the present: Emotional adaptations and the structure of ancestral environments. View Article Google Scholar 5. Haxby J. The distributed human neural system for face perception. Trends in Cognitive Science , 4, — View Article Google Scholar 6. Izuma K. The social neuroscience of reputation. Neuroscience Research , 72 4 , — Winston J. Automatic and intentional brain responses during evaluation of trustworthiness of faces.
Nature Neuroscience , 5, — Burnham T. Engineering human cooperation. Human Nature , 18 2 , 88— The biological and evolutionary logic of human cooperation. View Article Google Scholar Haley K. Subtle cues affect generosity in an anonymous economic game. Evolution and Human Behavior , 26, — Nettle D. 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. Despite the attention paid to reciprocal altruism by theoreticians, clear-cut empirical examples in non-human animals are relatively few Hammerstein , Sachs et al. This is probably because the pre-conditions for reciprocal altruism to evolve- multiple encounters and individual recognition—are not especially common.
It is quite common for a vampire bat to fail to feed on a given night. This is potentially fatal, for bats die if they go without food for more than a couple of days. On any given night, bats donate blood by regurgitation to other members of their group who have failed to feed, thus saving them from starvation. Since vampire bats live in small groups and associate with each other over long periods of time, the preconditions for reciprocal altruism are likely to be met.
Wilkinson and his colleagues' studies showed that bats tended to share food with their close associates, and were more likely to share with others that had recently shared with them. These findings appear to accord with reciprocal altruism theory. Trivers describes an apparent case of reciprocal altruism between non con-specifics. The interaction is mutually beneficial—the large fish gets cleaned and the cleaner gets fed.
However, Trivers notes that the large fish sometimes appear to behave altruistically towards the cleaners. If a large fish is attacked by a predator while it has a cleaner in its mouth, then it waits for the cleaner to leave before fleeing the predator, rather than swallowing the cleaner and fleeing immediately. Trivers explains the larger fish's behaviour in terms of reciprocal altruism. Since the large fish often returns to the same cleaner many times over, it pays to look after the cleaner's welfare, i.
So the larger fish allows the cleaner to escape, because there is an expectation of return benefit—getting cleaned again in the future. As in the case of the vampire bats, it is because the large fish and the cleaner interact more than once that the behaviour can evolve.
The evolutionary theories described above, in particular kin selection, go a long way towards reconciling the existence of altruism in nature with Darwinian principles.
The grounds for this view are easy to see. Ordinarily we think of altruistic actions as disinterested, done with the interests of the recipient, rather than our own interests, in mind. But kin selection theory explains altruistic behaviour as a clever strategy devised by selfish genes as a way of increasing their representation in the gene-pool, at the expense of other genes. This is a tempting line of argument.
Indeed Trivers and, arguably, Dawkins were themselves tempted by it. But it should not convince. The key point to remember is that biological altruism cannot be equated with altruism in the everyday vernacular sense. Biological altruism is defined in terms of fitness consequences, not motivating intentions.
Ants and termites, for example, presumably do not have conscious intentions, hence their behaviour cannot be done with the intention of promoting their own self-interest, nor the interests of others. Thus the assertion that the evolutionary theories reviewed above show that the altruism in nature is only apparent makes little sense. As we have seen, the gene's-eye perspective is heuristically useful for understanding the evolution of altruistic behaviours, especially those that evolve by kin selection.
Any evolutionary explanation of how a phenotypic trait evolves must ultimately show that the trait leads to an increase in frequency of the genes that code for it presuming the trait is transmitted genetically. Do theories of the evolution of biological altruism apply to humans? This is part of the broader question of whether ideas about the evolution of animal behaviour can be extrapolated to humans, a question that fuelled the sociobiology controversy of the s and is still actively debated today cf.
Boyd and Richerson , Bowles and Gintis , Sterelny All biologists accept that Homo sapiens is an evolved species, and thus that general evolutionary principles apply to it. However, human behaviour is obviously influenced by culture to a far greater extent than that of other animals, and is often the product of conscious beliefs and desires though this does not necessarily mean that genetics has no influence.
Nonetheless, at least some human behaviour does seem to fit the predictions of the evolutionary theories reviewed above.
In general, humans behave more altruistically in the biological sense towards their close kin than towards non-relatives, e. It is also true that we tend to help those who have helped us out in the past, just as reciprocal altruism theory would predict.
On the other hand, humans are unique in that we co-operate extensively with our non-kin; and more generally, numerous human behaviours seem anomalous from the point of view of biological fitness. Think for example of adoption. Parents who adopt children instead of having their own reduce their biological fitness, obviously, so adoption is an altruistic behaviour.
But it does not benefit kin—for parents are generally unrelated to the infants they adopt—and nor do the parents stand to gain much in the form of reciprocal benefits. So although evolutionary considerations can help us understand some human behaviours, they must be applied judiciously.
What is the relationship between these two concepts? They appear to be independent in both directions, as Elliott Sober has argued; see also Vromen and Clavien and Chapuisat An action performed with the conscious intention of helping another human being may not affect their biological fitness at all, so would not count as altruistic in the biological sense.
Conversely, an action undertaken for purely self-interested reasons, i. Sober argues that, even if we accept an evolutionary approach to human behaviour, there is no particular reason to think that evolution would have made humans into egoists rather than psychological altruists see also Schulz On the contrary, it is quite possible that natural selection would have favoured humans who genuinely do care about helping others, i.
Suppose there is an evolutionary advantage associated with taking good care of one's children—a quite plausible idea. Then, parents who really do care about their childrens' welfare, i. Contrary to what is often thought, an evolutionary approach to human behaviour does not imply that humans are likely to be motivated by self-interest alone.
Altruism and the Levels of Selection 2. Kin Selection and Inclusive Fitness 2. Conceptual Issues 3. Reciprocal Altruism 5. Altruism and the Levels of Selection The problem of altruism is intimately connected with questions about the level at which natural selection acts. Kin Selection and Inclusive Fitness The basic idea of kin selection is simple. Player 2 Altruist Selfish Player 1 Altruist 11,11 0,20 Selfish 20,0 5,5 Payoffs for Player 1, Player 2 in units of reproductive fitness The question we are interested in is: which type will be favoured by selection?
Conceptual Issues Altruism is a well understood topic in evolutionary biology; the theoretical ideas explained above have been extensively analysed, empirically confirmed, and are widely accepted. The payoff matrix is thus: Player 2 Weak Altruist Non Player 1 Weak Altruist 30,30 10,20 Non 20,10 0,0 Payoffs for Player 1, Player 2 in units of reproductive fitness 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.
Reciprocal Altruism 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. Bibliography Abbot, P. Avital, E. Axelrod, R. Birch, J. Bowles, S. Boyd, R. Levinson and N. Enfield eds. Bourke, A. Carter, G. Clavien, C. Cronin, H. Darwin, C.
Dawkins, R. Dugatkin, L. Fletcher, J. Frank, S. Gardner, A. Grafen, A. Krebs and N. Davies eds. Hamilton, W. Fox ed. Hammerstein, P. Hammerstein ed. Kerr, B. Lehmann, L. Leigh, E. Marshall, J. Maynard Smith, J. Michod, R. Nowak, M.
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