Evolutionary Psychology: Methods and Main Areas of Research


  1. Research methods
  2. Main areas of research
  3. Survival and individual-level psychological adaptations

Research methods

Evolutionary theory is heuristic in the sense that it can generate hypotheses that other theoretical approaches cannot. One of the primary goals of adaptationist research is to determine which organismic traits are likely to be adaptations and which are likely to be byproducts or random variations. As previously stated, adaptations are expected to demonstrate complexity, functionality, and species universality, whereas byproducts or random variation will not. Furthermore, adaptations are expected to manifest as proximate mechanisms that interact with the environment in either an obligate or facultative manner (see above). Evolutionary psychologists are also interested in identifying these proximate mechanisms (also known as "mental mechanisms" or "psychological adaptations"), as well as the types of information they accept as input, how they process that information, and what their outputs are. Evolutionary developmental psychology, or "evo-devo," focuses on how adaptations may be activated during specific developmental times (e.g., losing baby teeth, adolescence, etc.) or how events during an individual's development may alter life-history trajectories.

Several strategies are used by evolutionary psychologists to develop and test hypotheses about whether a psychological trait is likely to be an evolved adaptation. According to Buss (2011), these methods include:
Cross-cultural Consistency. Characteristics that have been demonstrated to be cross-cultural human universals such as smiling, crying, facial expressions are presumed to be evolved psychological adaptations. Several evolutionary psychologists have collected massive datasets from cultures around the world to assess cross-cultural universality.

Function to Form (or "problem to solution"). The fact that males, but not females, risk potential misidentification of genetic offspring (referred to as "paternity insecurity") led evolutionary psychologists to hypothesize that, compared to females, male jealousy would be more focused on sexual, rather than emotional, infidelity.

Form to Function (reverse-engineering – or "solution to problem"). Morning sickness, and associated aversions to certain types of food, during pregnancy seemed to have the characteristics of an evolved adaptation (complexity and universality). Margie Profet hypothesized that the function was to avoid the ingestion of toxins during early pregnancy that could damage fetus (but which are otherwise likely to be harmless to healthy non-pregnant women).

Corresponding Neurological Modules. Evolutionary psychology and cognitive neuropsychology are mutually compatible – evolutionary psychology helps to identify psychological adaptations and their ultimate, evolutionary functions, while neuropsychology helps to identify the proximate manifestations of these adaptations.

Current Evolutionary Adaptiveness. In addition to evolutionary models that suggest evolution occurs across large spans of time, recent research has demonstrated that some evolutionary shifts can be fast and dramatic. Consequently, some evolutionary psychologists have focused on the impact of psychological traits in the current environment. Such research can be used to inform estimates of the prevalence of traits over time. Such work has been informative in studying evolutionary psychopathology.

Experiments, archaeological records, data from hunter-gatherer societies, observational studies, neuroscience data, self-reports and surveys, public records, and human products are all used by evolutionary psychologists for testing. Additional methods and tools based on fictional scenarios, mathematical models, and multi-agent computer simulations have recently been introduced. 

Main areas of research

Survival, mating, parenting, family and kinship, interactions with non-kin, and cultural evolution are broad categories of adaptive problems that arise from evolutionary theory itself.

Survival and individual-level psychological adaptations

Problems of survival are clear targets for the evolution of physical and psychological adaptations. Major problems the ancestors of present-day humans faced included food selection and acquisition; territory selection and physical shelter; and avoiding predators and other environmental threats.


Consciousness meets George Williams' criteria of species universality, complexity, and functionality, and it appears to be a fitness-enhancing trait.

John Eccles argues in his paper "Evolution of Consciousness" that special anatomical and physical adaptations of the mammalian cerebral cortex gave rise to consciousness. Others, on the other hand, have argued that the recursive circuitry that underpins consciousness is much more primitive, having evolved in pre-mammalian species because it improves the capacity for interaction with both social and natural environments by providing an energy-saving "neutral" gear in an otherwise energy-expensive motor output machine. Once in place, this recursive circuitry could have served as the foundation for the subsequent development of many of the functions that consciousness facilitates in higher organisms, as described by Bernard J. Baars. According to Richard Dawkins, humans evolved consciousness in order to become the subjects of thought. According to Daniel Povinelli, large, tree-climbing apes evolved consciousness to account for one's own mass when moving safely among tree branches. Gordon Gallup discovered that chimps and orangutans, but not little monkeys or terrestrial gorillas, demonstrated self-awareness in mirror tests, supporting this hypothesis.

Voluntary action, awareness, or wakefulness are all examples of consciousness. Even voluntary behaviour, however, is influenced by unconscious mechanisms. Many cognitive processes occur in the cognitive unconscious, where they are not visible to conscious awareness. Some behaviours are conscious when first learned, but then become unconscious and appear to be automatic. Learning, particularly implicit learning of a skill, can occur outside of consciousness. For example, many people know how to turn right when riding a bike, but only a few can accurately explain how they do so. Self-deception is viewed as an evolutionary adaptation that can improve one's outcomes in social interactions by evolutionary psychologists.

Sleep may have evolved to conserve energy during times when activity would be less fruitful or more dangerous, such as at night, and especially during the winter season.

Sensation and perception

Many experts, such as Jerry Fodor, believe that perception's primary purpose is to provide knowledge, but evolutionary psychologists believe that perception's primary purpose is to guide action. Depth perception, for example, appears to have evolved not to help us know distances to other objects, but rather to help us move around in space, they claim. According to evolutionary psychologists, animals ranging from fiddler crabs to humans use vision for collision avoidance, implying that vision is primarily used for directing action rather than providing knowledge.

Because it is metabolically expensive to build and maintain sense organs, these organs evolve only when they improve an organism's fitness. More than half of the brain is devoted to sensory information processing, and the brain consumes roughly one-fourth of one's metabolic resources, so the senses must provide exceptional fitness benefits. Perception accurately reflects the world; animals obtain useful, accurate information via their senses.

Perception and sensation scientists have long viewed the human senses as adaptations to their surroundings. Depth perception entails processing over a half-dozen visual cues, each of which is based on a physical world regularity. Vision evolved to respond to a limited range of electromagnetic energy that is abundant and does not pass through objects. Sound waves travel around corners and interact with obstacles, resulting in a complex pattern that contains useful information about the origins and distances of objects. Because of their size, larger animals naturally make lower-pitched sounds. Adaptation, on the other hand, determines the range of an animal's hearing. Even though most smaller animals detect higher-pitched sounds, homing pigeons can hear very low-pitched sounds (infrasound) that travel long distances. Taste and smell respond to chemicals in the environment that are thought to be important for fitness in an evolutionary adapted environment. For example, salt and sugar were apparently both valuable to the human or pre-human inhabitants of the evolutionary adapted environment, so modern humans have an inherent desire for salty and sweet tastes. Touch is a combination of several senses, including pressure, heat, cold, tickle, and pain. Pain, while unpleasant, serves an adaptive function. Range shifting, in which the organism temporarily becomes more or less sensitive to sensation, is an important adaptation for senses. One's eyes, for example, automatically adjust to dim or bright ambient light. Sensory abilities of different organisms frequently coevolve, as is the case with echolocating bats' hearing and that of moths that have evolved to respond to the bats' sounds.

According to evolutionary psychologists, perception demonstrates the principle of modularity, with specialised mechanisms handling specific perception tasks. People who have had damage to a specific part of their brain, for example, suffer from the specific defect of being unable to recognise faces (prosopagnosia). According to evolutionary psychology, this indicates the presence of a so-called face-reading module.

Learning and facultative adaptations

Learning is said to be accomplished through evolved capacities, specifically facultative adaptations, in evolutionary psychology. Facultative adaptations manifest differently depending on environmental input. Sometimes the input comes during the development process and helps shape it. Migrating birds, for example, learn to orient themselves by the stars during a critical period in their development. Evolutionary psychologists believe that humans learn language in the same way that animals do, with critical periods. The input can also come from daily tasks, assisting the organism in adapting to changing environmental conditions. Animals, for example, evolved Pavlovian conditioning to solve problems involving causal relationships. Learning tasks are most easily completed by animals when they are similar to problems they faced in their evolutionary past, such as a rat learning where to find food or water. Learning abilities can differ between sexes in some cases. Due to the effects of male hormones during development, males in many animal species, for example, can solve spatial problems faster and more accurately than females. The same could be said for humans.

Emotion and motivation

Motivations direct and energise behaviour, whereas emotions, whether positive or negative, provide the affective component to motivation. Paul Ekman and colleagues began a line of research in the early 1970s that suggests that many emotions are universal. He discovered evidence that humans have at least five fundamental emotions: fear, sadness, happiness, anger, and disgust. Social emotions clearly evolved to motivate adaptive social behaviours in an evolutionary adapted environment. For example, spite appears to work against the individual, but it can help an individual establish a reputation as someone to be feared. Shame and pride can motivate behaviours that help one maintain one's status in a community, and self-esteem is one's assessment of one's own status. Motivation has a neurobiological basis in the brain's reward system. It has recently been proposed that reward systems may evolve in such a way that there may be an inherent or unavoidable trade-off in the motivational system for short-term versus long-term activities.


Internal representations of the world and internal information processing are referred to as cognition. From the standpoint of evolutionary psychology, cognition is not "general purpose," but rather employs heuristics, or strategies, that increase the likelihood of solving problems that our ancestors faced on a regular basis. For example, modern humans are far more likely to solve logic problems involving detecting cheating (a common problem given humans' social nature) than the same logic problem expressed purely abstractly. [Because our ancestors did not encounter truly random events, we may be cognitively predisposed to incorrectly identify patterns in random sequences. One example is the "Gamblers' Fallacy." Gamblers may mistakenly believe they have hit a "lucky streak" when each outcome is random and unrelated to previous trials. Most people believe that if a fair coin is flipped 9 times and Heads appears each time, there is a greater than 50% chance of getting Tails on the tenth flip. Humans find it far easier to make diagnoses or predictions when presented with frequency data than when presented with probabilities or percentages, presumably because our ancestors lived in relatively small tribes (usually with fewer than 150 people) where frequency information was more readily available.


The primary goal of evolutionary psychology is to discover commonalities between people, or basic human psychological nature. The fact that people have fundamental differences in personality traits is initially perplexing from an evolutionary standpoint. (Note: Behavioral genetics is concerned with statistically separating differences between people into genetic and environmental sources of variation. However, understanding the concept of heritability can be difficult – heritability refers only to differences between people, never to the degree to which an individual's traits are due to environmental or genetic factors, because traits are always a complex interweaving of both.)

Personality traits are thought to be the result of normal variation around an optimum, frequency-dependent selection (behavioural polymorphisms), or facultative adaptations, according to evolutionary psychologists. Some personality traits, like height variability, may simply reflect inter-individual variability around a general optimum. Alternatively, personality traits may represent different genetically predisposed "behavioural morphs" – alternate behavioural strategies that vary according to the frequency of competing behavioural strategies in the population. For example, if the majority of the population is generally trusting and gullible, being a "cheater" (or, in extreme cases, a sociopath) may be advantageous. Finally, personality traits, like many other psychological adaptations, may be facultative – sensitive to typical variations in the social environment, particularly during early development. Later-born children, for example, are more likely than firstborns to be rebellious, less conscientious, and more open to new experiences, which may benefit them given their specific niche in family structure. It is important to recognise that shared environmental influences do influence personality and are not always less important than genetic factors. However, shared environmental influences frequently decrease to near zero after adolescence but do not disappear completely.


The universal human ability to learn to talk between the ages of 1 and 4, essentially without training, suggests that language acquisition is a distinctly human psychological adaptation, according to Steven Pinker, who builds on Noam Chomsky's work (see, in particular, Pinker's The Language Instinct). According to Pinker and Bloom (1990), language as a mental faculty shares many similarities with complex organs of the body, implying that, like these organs, language evolved as an adaptation, as this is the only known mechanism by which such complex organs can develop.

Pinker agrees with Chomsky that the fact that children can learn any human language without explicit instruction suggests that language, including most grammar, is innate and only needs to be activated through interaction. Chomsky himself believes that language did not evolve as an adaptation, but rather as a byproduct of another adaptation, a so-called spandrel. However, Pinker and Bloom argue that the organic nature of language strongly suggests that it evolved through adaptation.

According to evolutionary psychologists, the FOXP2 gene may have played a role in the evolution of human language. Myrna Gopnik, a psycholinguist from the United Kingdom, discovered a dominant gene that causes language impairment in the KE family in the 1980s. This gene was discovered to be a mutation of the FOXP2 gene. Humans have a distinct allele of this gene, which has otherwise been highly conserved throughout most of mammalian evolution. This one-of-a-kind allele appears to have appeared between 100 and 200 thousand years ago, and it is now nearly universal in humans. However, the once-popular notion that FOXP2 is a "grammar gene" or that it triggered the evolution of language in Homo sapiens has been widely debunked.

Several competing theories about the evolutionary origins of language coexist at the moment, with none achieving widespread acceptance. [85] Language acquisition researchers in primates and humans, such as Michael Tomasello and Talmy Givón, argue that the innatist framework has overstated the role of imitation in learning and that the existence of an innate grammar module is not required to explain human language acquisition. According to Tomasello, studies of how children and primates actually acquire communicative skills suggest that humans learn complex behaviour through experience, and that language is acquired by the same cognitive mechanisms that are used to acquire all other types of socially transmitted behaviour.

Pinker agrees with Chomsky that the fact that children can learn any human language without explicit instruction suggests that language, including most grammar, is innate and only needs to be activated through interaction. Chomsky himself believes that language did not evolve as an adaptation, but rather as a byproduct of another adaptation, a so-called spandrel. However, Pinker and Bloom argue that the organic nature of language strongly suggests that it evolved through adaptation. Terrence Deacon makes a similar argument in The Symbolic Species, arguing that the various features of language have co-evolved with the evolution of the mind and that the ability to use symbolic communication is integrated into all other cognitive processes.

If the theory that language evolved as a single adaptation is accepted, the question of which of its many functions served as the foundation for adaptation becomes relevant. Several evolutionary hypotheses have been proposed, including the notion that language evolved for social grooming, displaying mating potential, or forming social contracts. According to evolutionary psychologists, these theories are all speculative, and much more evidence is needed to understand how language may have been selectively adapted.


Given that sexual reproduction is how genes are passed down to future generations, sexual selection plays a significant role in human evolution. Thus, evolutionary psychologists are interested in human mating in order to investigate evolved mechanisms for attracting and securing mates. This interest has spawned several lines of research, including studies of mate selection, mate poaching, mate retention, mating preferences, and sex conflict.

Robert Trivers published an influential paper on sex differences in 1972, which is now known as parental investment theory. The fundamental, defining difference between males (small gametes – sperm) and females (large gametes – ova) is the size difference of gametes (anisogamy). Anisogamy, according to Trivers, usually results in different levels of parental investment between the sexes, with females initially investing more. Trivers proposed that this disparity in parental investment results in the sexual selection of different reproductive strategies between sexes and sexual conflict. For example, he proposed that the sex that invests less in offspring will generally compete for access to the sex that invests more in offspring in order to increase their inclusive fitness (also see Bateman's principle . Trivers proposed that sexual dimorphisms in mate choice, intra- and inter-sexual reproductive competition, and courtship displays evolved as a result of differential parental investment. Females make a much larger parental investment than males in mammals, including humans (i.e. gestation followed by childbirth and lactation). A subset of life history theory is parental investment theory.

According to Buss and Schmitt's (1993) Sexual Strategies Theory, humans have evolved sexually dimorphic adaptations related to "sexual accessibility, fertility assessment, commitment seeking and avoidance, immediate and enduring resource procurement, paternity certainty, mate value assessment, and parental investment" as a result of differential parental investment. According to their Strategic Interference Theory, sex conflict occurs when one sex's preferred reproductive strategies interfere with those of the other sex, resulting in the activation of emotional responses such as anger or jealousy.

Women are generally more picky when it comes to mates, especially when it comes to long-term mating. Short-term mating can, however, provide benefits to women in some cases, such as fertility insurance, trading up to better genes, reducing the risk of inbreeding, and insurance protection for her offspring.

Sex differences in the domains of sexual jealousy have been discovered as a result of male paternity insecurity. Females are more likely to be hurt by emotional infidelity, while males are more likely to be hurt by sexual infidelity. This pattern is predicted because the costs of mating for each sex are different. Women, on average, should prefer a mate who can provide resources (e.g., financial, commitment); thus, a woman risks losing such resources if she marries an emotionally inept partner. Men, on the other hand, can never be certain of their children's genetic paternity because they do not bear the offspring themselves ("paternity insecurity"). This implies that for men, sexual infidelity is more aversive than emotional infidelity because investing in another man's offspring does not result in the propagation of their own genes.

Another intriguing area of study is the study of women's mate preferences throughout the ovulatory cycle. Theoretically, ancestral women would have evolved mechanisms to select mates with specific characteristics based on their hormonal status. The ovulatory shift hypothesis proposes that, during the ovulatory phase of a woman's cycle (days 10–15 of a woman's cycle), a woman who mated with a male of high genetic quality would have been more likely to produce and bear a healthy offspring than a woman who mated with a male of low genetic quality. Because a potential male mate would only be offering genes to a potential offspring, these putative preferences are expected to be especially pronounced in short-term mating domains. This hypothesis allows researchers to investigate whether women choose mates with high genetic quality characteristics during the high fertility phase of their ovulatory cycles. Indeed, research has shown that women's preferences change throughout the ovulatory cycle. Haselton and Miller (2006), in particular, demonstrated that highly fertile women prefer creative but poor men as short-term mates. Creativity could serve as a proxy for good genes. 109] According to Gangestad et al. (2004), highly fertile women prefer men who exhibit social presence and intrasexual competition; these traits may act as cues that help women predict which men have or can acquire resources.


Reproduction is always expensive for women, and it can be for men as well. Individuals are limited in the amount of time and resources they can devote to producing and raising their children, and such expenditure may also be detrimental to their future condition, survival, and reproductive output. Parental investment refers to any parental expenditure (time, energy, etc.) that benefits one child at the expense of the parents' ability to invest in other aspects of fitness (Clutton-Brock 1991: 9; Trivers 1972). Fitness components (Beatty 1992) include the well-being of existing offspring, the future reproduction of parents, and inclusive fitness through assistance to kin (Hamilton, 1964). A subset of life history theory is parental investment theory.

According to Robert Trivers' theory of parental investment, the sex that invests the most in lactation, nurturing, and protecting offspring will be more discriminating in mating, while the sex that invests the least in offspring will compete for access to the higher investing sex (see Bateman's principle). [99] Parental effort differences between sexes are important in determining the strength of sexual selection.

The benefits of parental investment in their offspring are substantial, and they are associated with effects on condition, growth, survival, and, ultimately, reproductive success. However, these advantages may come at the expense of the parent's ability to reproduce in the future, such as an increased risk of injury when defending offspring against predators, a loss of mating opportunities while rearing offspring, and an increase in the time between reproductions. Overall, parents are chosen to maximise the difference between benefits and costs, and parental care will most likely evolve when the benefits outweigh the costs.

The Cinderella effect refers to the alleged high rate of stepchildren being physically, emotionally, or sexually abused, neglected, murdered, or otherwise mistreated at the hands of their stepparents. It gets its name from the fairy tale character Cinderella, who is cruelly mistreated by her stepmother and stepsisters in the story. "Evolutionary thinking led to the discovery of the most important risk factor for child homicide – the presence of a stepparent," Daly and Wilson (1996) wrote. Parental efforts and investments are valuable resources, and selection favours parental psyches that effectively allocate effort to promote fitness. The adaptive problems that challenge parental decision-making include accurately identifying one's offspring and allocating one's resources among them while being sensitive to their needs and abilities to convert parental investment into fitness increments.... Stepchildren were rarely, if ever, as valuable to one's expected fitness as one's own offspring, and those parental psyches that were easily parasitized by any appealing youngster must have always been at a selective disadvantage" (Daly & Wilson, 1996, pp. 64–65). They do, however, point out that not all stepparents "want" to abuse their partner's children, and that genetic parenthood does not provide any protection against abuse. Stepparent care is primarily viewed as a "mating effort" toward the genetic parent.

Family and kin

The sum of an organism's classical fitness (how many of its own offspring it produces and supports) and the number of equivalents of its own offspring it can add to the population by supporting others is referred to as inclusive fitness. Hamilton refers to the first component as "classical fitness" (1964).

From the gene's perspective, evolutionary success is ultimately determined by leaving the greatest number of copies of itself in the population. Until 1964, it was widely assumed that genes did this solely by causing the individual to produce the greatest number of viable offspring. W. D. Hamilton, on the other hand, demonstrated mathematically in 1964 that, because close relatives of an organism share some identical genes, a gene can also increase its evolutionary success by promoting the reproduction and survival of these related or otherwise similar individuals. Hamilton came to the conclusion that this causes natural selection to favour organisms that behave in ways that maximise their overall fitness. Natural selection also favours behaviour that maximises personal fitness.

Hamilton's rule describes mathematically whether or not a gene for altruistic behavior will spread in a population:
rb > c
  • c is the reproductive cost to the altruist, 
  • is the reproductive benefit to the recipient of the altruistic behavior, and 
  • is the probability, above the population average, of the individuals sharing an altruistic gene – commonly viewed as "degree of relatedness"
The concept explains how natural selection can sustain altruism. Because relatives are likely to share genes with the altruist due to common descent, if there is a "altruism gene" (or complex of genes) that influences an organism's behaviour to be helpful and protective of relatives and their offspring, this behaviour increases the proportion of the altruism gene in the population. Altruists may also be able to recognise altruistic behaviour in unrelated people and be inclined to support them. This must be distinguished from the green-beard effect, as Dawkins points out in The Selfish Gene (Chapter 6) and The Extended Phenotype.

Although it is generally true that humans are more altruistic toward their kin than toward non-kin, the proximate mechanisms that mediate this cooperation have been debated (see kin recognition), with some arguing that kin status is determined primarily through social and cultural factors (such as coresidence, maternal association of sibs, etc.), while others have argued that kin recognition can also be mediated by biological factors such as facial resemblance (MHC).

Whatever the proximate mechanisms of kin recognition are, there is substantial evidence that humans act more altruistically toward close genetic kin than genetic non-kin.

Interactions with non-kin / reciprocity

Although interactions with non-kin are generally less altruistic than interactions with kin, cooperation with non-kin can be maintained through mutually beneficial reciprocity, as proposed by Robert Trivers. If the same two players in an evolutionary game have repeated encounters in which they can choose to "cooperate" or "defect," then a strategy of mutual cooperation may be preferred, even if it pays each player to defect when the other cooperates in the short term. Only if the probability, w, of another encounter between the same two individuals exceeds the cost-to-benefit ratio of the altruistic act can direct reciprocity lead to the evolution of cooperation:
w > c/b
Reciprocity can also be indirect if previous interactions are shared. Reputation enables the evolution of cooperation through indirect reciprocity. Natural selection favours strategies in which the decision to help is based on the recipient's reputation: studies show that people who are more helpful are more likely to receive help. The calculations for indirect reciprocity are complex, and only a small portion of this universe has been discovered, but once again, a simple rule has emerged.   Indirect reciprocity can only promote cooperation if the probability, q, of knowing someone's reputation is greater than the cost-to-benefit ratio of the altruistic act:
q > c/b
One significant flaw in this explanation is that individuals may evolve the ability to obscure their reputation, lowering the probability, q, that it will be known.

According to Trivers, friendship and other social emotions evolved in order to manage reciprocity. He claims that liking and disliking evolved to help modern humans' ancestors form coalitions with others who reciprocated and exclude those who did not. Moral indignation may have evolved to protect one's altruism from being exploited by cheaters, and gratitude may have motivated modern humans' ancestors to reciprocate appropriately after benefiting from the altruism of others. Similarly, modern humans feel guilty when they do not reciprocate. These social motivations are consistent with what evolutionary psychologists expect to see in adaptations that evolved to maximise the benefits of reciprocity while minimising the drawbacks.

According to evolutionary psychologists, humans have psychological adaptations that evolved specifically to help us identify nonreciprocators, also known as "cheaters." Robert Frank and colleagues discovered in 1993 that participants in a prisoner's dilemma scenario could often predict whether their partners would "cheat" based on a half-hour of unstructured social interaction. Linda Mealey and her colleagues, for example, discovered in a 1996 experiment that people were better at remembering people's faces when those faces were associated with stories about those people cheating (such as embezzling money from a church).

Strong reciprocity (or "tribal reciprocity")

Humans may have evolved a set of psychological adaptations that predispose them to be more cooperative with members of their tribal in-group and more nasty with members of their tribal out-group. These adaptations could have resulted from tribal warfare. Humans may also be predisposed to "altruistic punishment" – punishing in-group members who break in-group rules, even when this altruistic behaviour cannot be justified in terms of helping those to whom you are related (kin selection), cooperating with those with whom you will interact again (direct reciprocity), or cooperating to improve your reputation with others (indirect reciprocity).



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