Within evolutionary biology, signalling theory is a body of theoretical work examining communication between individuals. The central question is when organisms with conflicting interests should be expected to communicate "honestly". Mathematical models in which organisms signal their condition to other individuals as part of an evolutionarily stable strategy are the principal form of research in this field .
In sexual selection, traits are selected via the pressure of mate selection, and signalling can be one of those traits. For example, the male gray tree frog, Hyla versicolor, produces a call which is used to attract a female. Once the female chooses a male, this selects for a specific style of male calling, thus propagating a specific signalling ability. The signal, in this context, can be either the call itself, the intensity of a call, its variation style, repetition, etc.
What kind of call should a male use to make sure he's going to get a female's acceptance? Various hypothesis explain why females would select for one call over the other. In the case of compound calls (with more than one signal being modulated), the coevolution of male calling with female preferences can complicate a straightforward answer. The pre-existing sensory bias hypothesis proposes that pre-existing signals can be made more attractive by adding new and novel signals to them. The hidden preference hypothesis explains successful calls as being better able to match some 'hidden preference' in the female.[1]
In biology, signals are traits, including structures and behaviors, that have evolved specifically because they change the behavior of receivers in ways that benefit the signaller.[2] Traits or actions that benefit the receiver exclusively are called cues. When an alert bird deliberately gives a warning call to a stalking predator and the predator gives up the hunt, the sound is a signal. When a foraging bird inadvertently makes a rustling sound in the leaves that attracts predators and increases the risk of predation, the sound is a 'cue'.
Signalling systems are shaped by the extent to which signallers and receivers have mutual interests. An alert bird warning off a stalking predator is communicating something useful to the predator: that it has been detected by the prey; it might as well quit wasting its time stalking this alerted prey, which it is unlikely to catch. When the predator gives up, the signaller can get back to other important tasks. Once the stalking predator is detected, the signalling prey and receiving predator have a mutual interest terminating the hunt.[3][4]
Within species, mutual interests generally increase with kinship.[5] Kinship is central to models of signalling between relatives, for instance when broods of nestling birds beg and compete for food from their parents.[6][7] The distinction between signals and cues is not clear, and probably not useful for immune, endocrine and neural signalling between the cells within an individual, at least to the extent that all of these cells are clonal descendents of a fertilized egg and there are no conflicts of interest between them.
The concept of honesty in animal communication is controversial because it is difficult to determine intent and use that as a criterion to discriminate deception from honesty, as we do in human interactions.[8] Because of this, biologists who use the phrase ”honest signals” use it in a statistical sense. Biological signals, like warning calls or resplendent tail feathers, are considered honest if they are correlated with, or reliably predict, something useful to the receiver. In this usage, honesty is a useful correlation between the signal trait (which economists call ”public information” because it is readily apparent) and the unobservable thing of value to the receiver (which economists refer to as “private information” and biologists often refer to as “quality”). Honest biological signals do not need to be perfectly informative, reducing uncertainty to zero; they only need to be honest “on average” to be potentially useful.[9] Ultimately the value of the signalled information depends on the extent to which it allows the receiver to increase its fitness.[10]
Because there are both mutual and conflicting interests in most animal signalling systems, the fundamental problem in evolutionary signalling games is dishonesty or cheating. Why don’t foraging birds just give warning calls all the time, at random (false alarms), just in case a predator is nearby? If peacocks with bigger tails are preferred by peahens, why don’t all peacocks display big tails? Too much cheating would disrupt the correlation at the foundation of the system, causing it to collapse. Receivers should ignore the signals if they are not useful to them and signallers shouldn’t invest in costly signals if they won’t alter the behavior of receivers in ways that benefit the signaller. What prevents cheating from destabilizing signalling systems? It might be apparent that the costs of displaying signals must be an important part of the answer. However, understanding how costs can stabilize an “honest” correlation between the public signal trait and the private signalled quality has turned out to be a long, interesting process. If many animals in a group send too many dishonest signals, then their entire signalling system will collapse, leading to much poorer fitness of the group as a whole. Every dishonest signal weakens the integrity of the signalling system, and thus weakens the fitness of the group.
An example of dishonest signalling comes from Fiddler crabs such as Uca lactea mjoebergi, which have been shown to bluff in regards to their fighting ability. Upon regrowing a lost claw, a crab will occasionally regrow a weaker claw that nevertheless intimidates crabs with smaller but stronger claws.[11][12] The proportion of dishonest signals is low enough that it is not worthwhile for crabs to test the honesty of such signals, as combat can be dangerous and expensive.
The question of whether individuals of the same species might not be attempting to deceive each other was raised by Richard Dawkins and John Krebs in 1978. This thinking was prompted by the application of a gene-centered view of evolution to the use of threat displays. Dawkins & Krebs criticised previous ethologists, such as Nikolaas Tinbergen and Desmond Morris among others, for supporting the view that such displays were used "for the good of the species". Dawkins and Krebs (and Krebs and Dawkins, 1982) argued that such communication ought to be viewed as an evolutionary arms race in which signallers evolve to become better at manipulating receivers, while receivers evolve to become more resistant to manipulation.
The game theoretical model of the war of attrition was applied to the problem, and appeared to suggest that threat displays ought not to convey any reliable information about intentions (Caryl, 1979). This led to a re-examination of the empirical evidence, and much debate .
In 1975, Amotz Zahavi proposed a verbal model for how signal costs could constrain cheating and stabilize an “honest” correlation between observed signals and unobservable qualities, based on an analogy to sports handicapping systems.[13][14] He called this idea the handicap principle. The purpose of a sports handicapping system is to reduce disparities in performance, making the contest more competitive. In horse racing, intrinsically faster horses are given heavier weights to carry under their saddles. In amateur golf, better golfers have fewer strokes subtracted from their raw scores. This creates correlations between the handicap and unhandicapped performance, and if the handicaps work as they are supposed to, between the handicap and handicapped performance. If you knew nothing about two race horses or two amateur golfers except their handicaps, you could infer which horse or golfer has had the better performance in the recent past, and which competitor is most likely to win: the horse with the bigger weight handicap and the golfer with the smaller stroke handicap. By analogy, if peacock tails act as a handicapping system, and a peahen knew nothing about two peacocks but the sizes of their tails, she could “infer” that the peacock with the bigger tail has greater unobservable intrinsic quality, in the sense that it is better able to pay the costs of displaying the tail (here, “infer” is shorthand for the idea that females that prefer bigger tails are at a selective advantage). Display costs can include extrinsic social costs, in the form of testing and punishment by rivals, as well as intrinsic production costs.[15]
The essential idea here is intuitive and probably qualifies as folk wisdom. It was articulated quite nicely by Kurt Vonnegut, in his 1961 short story Harrison Bergeron.[16] In Vonnegut’s futuristic dystopia, the Handicapper General uses a variety of handicapping mechanisms to reduce inequalities in performance. A spectator at a ballet comments: “it was easy to see that she was the strongest and most graceful of all dancers, for her handicap bags were as big as those worn by two hundred pound men.” Zahavi interpreted this analogy to mean that higher quality peacocks with bigger tails are signalling their ability to "waste" more of something important by trading it off for a bigger tail. This resonates with Veblen’s idea that conspicuous consumption and extravagant status symbols can signal wealth.[17]
Zahavi’s conclusions rest on his verbal interpretation of a metaphor, and initially, the handicap principle was not well received by evolutionary biologists.[14] However, in 1984, Nur and Hasson[18] used life history theory to show how differences in signalling costs, in the form of survival-reproduction tradeoffs, could stabilize a signalling system roughly as Zahavi imagined. Later in the decade, several papers using genetic models also began to suggest that the idea just might work, at least some times.[19] The logjam was broken in 1990 by Alan Grafen,[20] who developed a very complicated marginal fitness maximization model of evolutionary signalling games and came to the conclusion that, given certain assumptions, a handicap-like signalling system could be evolutionarily stable, if higher quality signallers paid lower marginal survival costs for their signals.
A specific and widely applicable handicap mechanism was proposed in 1982: parasite-mediated sexual selection.[21] It proposed that due to a never-ending co-evolutionary race between hosts and their parasites, sexually selected signals are indicators of health. This idea led to an explosion of research on the relationship between sexually selected signals, parasites and mate preferences during the 80s and early 90s.
Since the early 90s attention has shifted slightly towards that idea that carotenoids have dual but mutually incompatible roles in immune function and signalling.[22] Given that animals cannot synthesize carotenoids de novo, they must be obtained from food. The hypothesis states that animals with carotenoid-depended sexual signals are actually demonstrating their ability to “waste” carotenoids on sexual signals at the expense of their immune system. This hypothesis has been the topic of an extensive body of work (e.g.,.[23]) These red, orange and yellow carotenoid-dependent ornaments are hypothesized to be a general form of an immunocompetence handicap,[24] and a specific mechanism by which the handicap principle might work.
Efforts to test the handicap principle empirically have not been decisive, partly because of inconsistent interpretations of Zahavi’s metaphor and Grafen’s marginal fitness model, and partly because of conflicting empirical results: in some studies individuals with bigger signals seem to be paying higher costs, in other studies they seem to be paying lower costs.[25][26] Recent theoretical analyses have uncovered a possible explanation for the inconsistent empirical results. A series of papers by Getty[27][28][29][30] shows that Grafen’s proof of the handicap principle is based on the critical simplifying assumption that signallers trade off costs for benefits in an additive fashion, the way humans invest money to increase income in the same currency. Grafen’s proof is formally similar to a classic monograph on economic market signalling by Nobel laureate Michael Spence.[31] This assumption that costs and benefits trade off in an additive fashion might be valid for some biological signalling systems, but is not valid for the survival cost – reproduction benefit tradeoff that is assumed to mediate the evolution of sexually selected signals. Fitness depends on the production of offspring and this is a multiplicative function of reproductive success given an individual is still alive times the probability of still being alive, given investment in signals.[18]
Survival-reproduction tradeoffs do not correspond to sports handicaps in any simple, useful way. Zahavi’s intuition was correct in the very general sense that “differences in costs” can stabilize the evolution of an “honest” signalling system, but in sexually selected signalling, “differences in costs” are properly decreasing proportional (or log) marginal costs.[30] The mathematics can be interpreted to mean that higher quality signallers are more efficient at converting signal costs into reproductive benefits. This re-analysis undermines the idea that higher quality signallers are demonstrating their ability to waste more because the pattern of absolute signal costs across signallers of different quality remains undetermined. Depending on the specific form of the tradeoffs in any particular system, higher quality signallers might pay absolutely more or less for big signals than lower quality signallers pay for small signals. This might explain why the empirical data on the relationship between signals and costs are so inconsistent.
The effort to discover how costs can constrain an “honest” correlation between public signals and private qualities within signallers is built on strategic models of signalling games, with many simplifying assumptions. These models are most often applied to sexually selected signalling in diploid animals, but they rarely incorporate an important feature of diploid sexual reproduction that was pointed out by Ronald Fisher in the early 20th century: if there are “preference genes” correlated with choosiness in females as well as “signal genes” correlated with display traits in males, choosier females should tend to mate with showier males. Over generations, showier sons should also carry genes associated with choosier daughters and choosier daughters should also carry genes associated with showier sons. This correlation could introduce an evolutionary dynamic known as a Fisherian runaway. Russell Lande explored this with quantitative genetic models and his work inspired a very active line of research in the quantitative genetic framework.[19] These analyses revealed that Fisherian diploid dynamics are very sensitive to signalling and search costs. Recent models have begun to bridge the gap between the costly-signalling and Fisherian-runaway traditions by developing modelling frameworks that incorporate both simultaneously.[32][33] These models recognize that if fitness depends on both survival and reproduction, having sexy sons and choosy daughters (in the stereotypical model) can be adaptive, increasing fitness just as much as having healthy sons and daughters.
Human behaviors may also serve as examples of costly signals. Evidence for costly signalling has been found in many areas of human interaction, including risk taking, hunting, and religion. In general, these signals provide information about a person’s phenotypic quality or cooperative tendencies.
Large game hunting has been studied extensively as a signal of men’s willingness to take physical risks, as well as showcase strength and coordination (Bliege Bird et al. 2001; Gurven and Hill 2009; Hawkes 1990; Weissner 2002). Costly Signaling Theory (CST) is a useful tool for understanding food sharing among hunter gatherers because it can be applied to situations in which delayed reciprocity is not a viable explanation (Bliege Bird and Bird 1997; Gurven et al. 2000; Hawkes 1993). Instances that are particularly inconsistent with the delayed reciprocity hypothesis are those in which a hunter shares his kill indiscriminately with all members of a large group (Wiessner 1996). In these situations, the individuals sharing meat have no control over whether or not their generosity will be reciprocated, and free riding becomes an attractive strategy for those receiving meat. Free riders are people who reap the benefits of group-living without contributing to its maintenance (Barrett et al. 2002). Fortunately, the CST can bring resolution to some of the gaps that the delayed reciprocity hypothesis cannot fill (Sosis 2000; Smith and Bliege Bird 2000). Hawkes (1991, 1993) has suggested that men target large game and publicly share meat in order to draw social attention or to “show-off”. This favorable attention can improve a hunter’s reputation by providing information about his phenotypic quality. High quality signallers are more successful in acquiring mates and allies. Thus, CST is helpful in unravelling an array of human evolutionary puzzles because it can explain wasteful and altruistic behavior. (Getty 1998; Grafen 1990; Johnstone 1995, 1997; Smith and Bliege Bird 2000; Zahavi 1975, 1977).
In order to be effective, costly signals must fulfill specific criteria (Bliege Bird et al. 2001; Hawkes and Bliege Bird 2002; Zahavi 1975). The first criterion requires signallers to incur different levels of cost and benefit for signalling behavior. Second, levels of cost and benefit must reflect the signallers’ phenotypic quality. Third, the information provided by a signal should be directed at an audience and easy to access. A receiver can be anyone who stands to benefit from information the signaller is sending, such as potential mates, allies, or competitors. Honesty is guaranteed when only individuals of high quality can pay high costs to generate this signal. Hence, the high costs of honest signalling make it impossible for low-quality individuals to fake a signal and fool a receiver (Bliege Bird et al. 2001; Hawkes and Bliege Bird 2002; Zahavi 1975).
Bliege Bird et al. (2001) observed turtle hunting and spear fishing patterns in a Meriam community in the Torres Strait of Australia. Here, only some Meriam men were able to accumulate high caloric gains for the amount of time spent turtle hunting or spear fishing (kcal/h) compared to the other men. Since a daily catch of fish is carried home by hand and turtles are frequently served at large feasts, members of the community know when men are successful at fishing and hunting. When members of the community were asked to identify the best hunter, they named the men who most reliably brought turtle meat and fish back to the community. Thus, turtle hunting qualifies as a costly signal because Meriam men have differential hunting and fishing success and members of their community use this public information to judge the quality of each hunter. Furthermore, turtle hunting and spear fishing requires a great deal of time and energy and results in significantly lower gains in kcal/h than foraging for shellfish. Shellfish collecting success was dependent only on the amount of time dedicated to searching, so it is not an optimal activity to serve as a signal for skill or strength. This suggests that energetic gains are not the primary reason men take part in turtle hunting and spear fishing. In a follow-up study, Smith et al. (2002) found that successful Meriam hunters do experience greater social benefits and reproductive success than men who are less skilled hunters.
Marlowe (2010) reports that the Hadza also participate in food sharing. Costly signalling could be an important factor in explaining this behavior since those who share food gain reputational benefits. The suggestion that hunters mainly share to provision their families or to gain reciprocal benefits cannot account for all of the meat sharing that occurs. For example, in this community, teenage boys often give away their meat even though they do not have wives or children yet. Instead, sharing food that is difficult to acquire provides information about the donor’s phenotypic quality (Hawkes 2001). These qualities include good eyesight, coordination, strength, knowledge, endurance, or bravery. It is common for Hadza hunters to pair with more fertile, hard-working wives than non-hunters (Hawkes et al. 2002). A woman benefits from mating with a man who possesses such qualities because there is a high probability that her children will inherit qualities that increase fitness and survivorship. She may also benefit from her husband’s high social status. Thus, hunting is a reliable signal of phenotypic quality because it is costly and can be broadcasted to the rest of the community (Smith et al. 2000).
Among the men of Ifaluk atoll, costly signalling theory can also explain why men torch fish (Sosis 2000). Torch fishing is a ritualized method of fishing on Ifaluk whereby men use torches made from dried coconut fronds to catch large dog-toothed tuna. Preparation for torch fishing requires significant time investments and involves a great deal of organization. Due to the time and energetic costs of preparation, torch fishing results in net caloric losses for fishers. Accordingly, Sosis (2000) argues that torch fishing is a handicap that serves to signal men’s productivity. Torch fishing is the most advertised fishing occupation on Ifaluk. Women and other individuals from the community usually spend time observing the canoes as they sail beyond the reef. In addition, local rituals help broadcast information about which fishers are successful and enhance fishers’ reputations during the torch fishing season. Several ritual behavioral and dietary constraints clearly distinguish torch fishers from other men. First, males are only permitted to torch fish if they participated on the first day of the fishing season. The community is well informed as to who participates on this day, and can easily identify the torch fishers. Second, torch fishers receive all of their meals at the canoe house and are prohibited to eat certain foods. Community members serve as signal receivers and frequently discuss the qualities of each individual who participates in the costly display. On Ifaluk, women claim that they are looking for hard-working mates (Sosis et al. 1998). With the distinct sexual division of labor on Ifaluk, industriousness is a highly valued characteristic in males (Sosis 1997). Torch fishing provides women with reliable information on the work ethic of prospective mates. This observable display fits the criteria of an honest costly signal as suggested by Smith and Bliege Bird (2000).
In many human cases, a strong reputation built through costly signalling enhances a man’s social status over the statuses of men who signal less successfully (Kelly 1995; Dowling 1968; Wiessner 1996). Among northern Kalahari foraging groups, traditional hunters usually capture a maximum of two or three antelopes per year (Lee 1979). Thomas (1959) recorded this anecdote about a particularly successful hunter:
“It was said of him that he never returned from a hunt without having killed at least a wildebeest, if not something larger. Hence the people connected with him ate a great deal of meat and his popularity grew.”
Although this hunter was sharing meat, he was not doing so in the framework of reciprocity (Thomas 1959). The general model of costly signalling is not reciprocal; rather, individuals who share acquire more mates and allies (Bliege Bird et al. 2001; Zahavi 1975). The CST model can be applied to situations in Kalahari foraging groups where giving often goes to recipients who have little to offer in return. A young hunter is motivated to impress community members with daughters so that he can obtain his first wife. Older hunters may be motivated to attract the attention of women interested in an extramarital relationship, or in becoming a sister wife (Lee 1993; Shostak 1981). In these northern Kalahari groups, the killing of a large animal indicates that a man has mastered the art of hunting and is able to support a family (Marshall 1976). Generally, many women seek a man who is a good hunter, has an agreeable character, is generous, and will provide advantageous social ties (Lee 1979; Marshall 1976; Shostak 1981). Since hunting ability is a prerequisite for marriage, men who are good hunters enter the marriage market earliest. Ultimately, costly signalling theory has the capacity to explain many puzzling facets of human foraging strategies – it has the potential to rationalize wasteful foraging displays because higher quality individuals gain higher benefits for producing high cost displays (Hawkes et al. 2002).
In addition to hunting, CST can be applied to other situations involving physical strain and risk of physical injury or death (Bleige Bird et al. 2001; Nell 2002; Farthing 2005). Research on physical risk taking is important because information regarding why people, especially young males, take part in high risk activities can help in the development of prevention programs (Farthing 2005; Nell 2002). Reckless driving is a particularly lethal problem among adolescent and young adult men in western societies (Nell 2001). Zahavi (1975) suggests that a male who takes a physical risk is sending the message that he has enough strength and skill to survive extremely dangerous activities. Nell (2001) uses Zahavi’s (1975) hypothesis to explain the high level of reckless driving among adolescent males in the United States. He suggests that males use surviving high-risk driving situations (high speeds, intoxication, etc.) as a signal of their overall phenotypic quality. This signal is directed towards peers and potential mates.
In a study on general attitudes regarding risk, Farthing (2005) tested male and female college student’s perceptions of risk takers and applied CST to his results. The study results suggest that some types of risk are viewed more favorably than others. Physical risk for another’s benefit (heroic risk) received more positive ratings than risks involving drugs and physical risks that would not benefit anyone. In addition, males and females valued different degrees of heroic risk for mates and same-sex friends. Males valued a high level of heroic risk taking in same-sex friends and a lower level of heroic risk taking in female mates. Females valued a high level of heroic risk taking in male mates and a lower level of heroic risk taking in same sex friends. Farthing (2005) suggests that females are attracted to males who may be inclined to physically defend them and their children, so they prefer a high level of heroic risk taking. He also postulates that males prefer a high level of heroic risk taking in male friends because heroic risk taking indicates that another male could be a good ally with whom to form coalitions.
In western societies, voluntary blood donation is another common, yet less extreme, form of risk taking. Lyle et al. (2009) were the first to apply evolutionary theory to the phenomenon of human blood donations in which donors receive no monetary compensation. Costs associated with these donations are pain, anxiety, risk of infection, and time lost (Schreiber et al. 2006). In the study, they proposed that if blood donation is an opportunity to send costly signals, then donors will have lower perceived costs of donating, will actually experience lower costs of donating, and will be perceived by others as generous and physically healthy (Zahavi 1975; Lyle et al. 2009). Through a series of survey questions Lyle et al. (2009) assessed the general attitudes of donors and non-donors among a population of college students. The survey data indicated that, since blood donors are screened for a range of diseases and health concerns, knowledge of a person’s blood donor status can confer reliable information about the donor’s general health. In addition, a willingness to donate blood reliably indicated reduced stress and anxiety regarding pain and needles. Lyle et al. (2009) propose that signal receivers may assume that a signaller’s reduced anxiety about experiencing pain is telling of how the signaller will respond in other stressful situations. Thus, costly signalling theory may be useful in explaining why humans choose to donate blood without pay. Both donors and non-donors expressed that they had specific perceptions of the health, generosity, and ability of blood donors to operate in stressful situations (Lyle et al. 2009).
The existence of costly religious rituals such as male circumcision, food and water deprivation, and even consumption of toxic substances (see for example Snake handling) has presented a paradox for evolutionary scientists. In this sense, devout religious belief appears to be a maladaptive trait (Tuzin 1982). Many evolutionary psychologists have proposed that religion arose as a means of increasing and maintaining intragroup cooperation (Steadman and Palmer 2008). Cooperation is the basis of the social organization of humans and leads to altruistic behavior (Bulbulia 2004). Costly signalling helps promote altruistic behavior because expensive signals provide information about an individual’s quality (Zahavi 1975). Irons (2001) proposes that all religions involve costly and elaborate rituals, which are performed publicly, in order to demonstrate loyalty and commitment to other members of the religious group. In this way, group members increase their allegiance to the group by signalling that they are invested in promoting group interests as well as being able to easily identify other members. However, as group size increases among humans, the threat of free riders grows as well (Barrett et al. 2002). CST accounts for this by proposing that these religious rituals are costly enough to deter free riders from benefiting from the collective production of religious goods (Iannaccone 1992). (See Evolutionary psychology of religion)
William Irons (1996) was one of the original proponents of using CST to explain costly religious behavior. He argued that these hard-to-fake religious displays served as a way to enhance both trust and solidarity in a community, producing emotional and economical benefits. He demonstrated that display signals among the Yomut Turkmen of northern Iran helped to secure trade agreements (Irons 1996). These “ostentatious” displays signalled to strangers and group members alike their commitment to Islam.
Richard Sosis, a University of Connecticut anthropologist, has since become one of the leading supporters of this idea (Bulbulia 2004). He has done many comparative studies of religious and secular communities. In one case, he was able to demonstrate that people in religious communities are four times more likely to live longer than their secular counterparts (Sosis 2000). He also demonstrated that these longer lifespans were positively correlated with the number of costly requirements demanded from these religious community members (Sosis and Bressler 2003). However, there has been controversy regarding the correlation between religion and increased longevity due to the presence of confounding variables (Hood et al. 2009).
Lawrence Iannaccone has also studied the effects of costly signals on religious communities. He found from a general self-reported survey that as the strictness of a church increased, the attendance and contributions to that church increased proportionally (1992). In effect, people were more willing to participate in a church that has more stringent demands on its members.
The studies of Sosis and Irons, as well as other researchers, have provided many examples in support of their hypothesis. Bradley Ruffle and Richard Sosis (2003) tested the relationship between religious rituals and cooperative behavior in an Israeli kibbutz. A kibbutz is a modern-day commune where each member of the kibbutz earns an equal share of the income generated by it. This presents an opportunity for free riding since the wealth is equally divided, regardless of profession. In this experiment, members of a kibbutz, both secular and religious, were asked to play a “common-pool resource game” mirroring the problems faced daily by this community. In these kibbutzim, cooperation and self-restraint are necessary to prevent draining of the shared resources. By including both secular and religious kibbutz members, Sosis and Ruffle were able to see if religious rituals affect cooperative behavior even when compared to secular members living under the same social constraints.
In Judaism, men’s religious practices include obligations to pray collectively three times a day, follow kosher dietary laws, and dress modestly. Interestingly, women are not required to perform the costly ritual of thrice daily communal prayers. Moreover, while males’ rituals are performed publicly, women’s religious requirements are generally performed privately. One using the CST would predict that this dichotomy would result in a greater prevalence of cooperative behavior among men than women.
The results of this experiment show that: First, males are more cooperative toward other male members of their commune than towards females. Second, males that attend synagogue were found to be more cooperative than males who do not attend synagogue. There seemed to be no similar correlation in religious females. Third, secular males and females showed the same degree of cooperation. Finally, religious male members were found to be the most cooperative subpopulation over both secular members and religious female members. The disparity between males and females demonstrated the importance of public costly signalling in predicting cooperation (Ruffle and Sosis 2003).
Examples of cooperative behaviors have been observed in several other religions. Orbell et al. (1992) conducted a prisoner’s dilemma game on university students in a largely Mormon area of Utah and a town in Oregon, which has one of the lowest church attendance rates in the United States. In the religious subgroup, frequency of church attendance was positively correlated with the prevalence of cooperative behavior. However, in the secular subgroups from both Oregon and Utah, there was no correlation observed between these two attributes.
There is also evidence for the maintenance of religious costly signals via sexual selection. In the Bay Islands of Honduras, Irons (2001) investigated how men viewed women based on whether or not the women attended church regularly. Men claimed that they considered women who attended church on a regular basis to be more desirable as long-term partners because they viewed them as more likely to be “faithful wives”. In a community where fidelity is of extreme importance, women who attended church were observed to be of better or higher quality.
There has also been research on the economic gains of religious devotion. Ensminger (1997) argued that the spread of Islam throughout Africa was a result of the trading advantages reaped by conversion to Islam. He claims that the religious rites of Islam functioned as a social contract between people in Africa. These signals helped them build trust with a trading partner despite the long-distance over which the trading had to take place. In effect, the costly signals of Islam deterred free riders from infringing on trade agreements and allowed agreements to be more efficient and profitable.
Despite the experimental support for this hypothesis, it is very controversial and there are many critiques against it. One of the most common critiques is that devoutness is, in fact, very easy to fake. Attending a religious service, which is generally all that is required of some religions, can be done by almost anyone. Therefore, if attendance gives free riders an advantage then there will be nothing stopping them from exploiting it (Rees 2011). While this critique is impossible to completely refute, it fails to account for the fact that the hypothesis predicts that people are more likely to join a religious group and contribute to it when the rituals of that group are costly (Iannacone 1992).
Another critique specifically asks: why religion? There seems to be no evolutionary advantage to evolving religion over any other signal of commitment such as nationality etc., a flaw that Irons himself admits (Bulbulia 2004). However, it is possible that the reinforcement of religious rites as well as the intrinsic reward and punishment system found in religion makes it an ideal candidate for increasing intragroup cooperation.
Finally, there has also been a lack of sufficient evidence for an increase in fitness as a result of religious cooperation. If this is the case, then it becomes hard to establish what natural selection was acting on (Bulbulia 2004). However, there has been evidence of benefits from religion itself, such as increased longevity, improved health, assistance during crises, and greater psychological well being (Sosis 2002). Thus, the costly signalling hypothesis provides an ultimate explanation for religion, while the immediate benefits listed are proximate explanations. It is therefore impossible to directly tie cooperation to increased fitness since they are different levels of analysis.