Adaptive Problem of Detecting Kinship
Evolution of cues used by humans to detect kin in order to avoid incest and promote altruistic behavior toward genetic relatives.
As a highly social species, humans interact and cooperate with many individuals; however cooperation among relatives tends to be greater than among unrelated individuals (Hamilton 1964). Throughout history, humans of all known cultures have always tended to feel and behave differently toward family members than to individuals to whom they were not genetically related. This behavior, which is not exclusive to humans, is distinguished by two important traits: on the one hand, the display of a highly altruistic behavior in which their own welfare could be sacrificed to help family members and, on the other hand, an innate aversion to any sexual contact with genetic relatives. From the evolutionary point of view, natural selection has favored this behavior because it conferred a selective advantage to the individuals displaying it. These desirable features increased in frequency to the point that nowadays they have become a universal feature of the species.
Species’ typical traits have been favored by evolution because they solved an adaptive problem for the organism that displayed it. Therefore, it is correct to infer that the behaviors displayed by humans toward family members have been selected because they solved an adaptive problem, at least in the environment of evolutionary adaptiveness. However, it is important to note that thousands of species display social behaviors that are influenced by genetic relatedness. Altruistic behavior or nepotism correlates with the amount of shared genes that are identical by descent. Therefore, organisms display this behavior in response to cues of genetic relatedness. In humans, genetic relatedness correlates with the willingness to help in a hypothetical situation and the amount of imbalance tolerated in a reciprocal relationship (Burnstein et al. 1994).
Social groups and closeness to relatives lead to two entirely different adaptive problems, avoidance of inbreeding and willingness to support kin. Breeding with relatives increases the probability of conceiving individuals with two copies of recessive deleterious genes. These genes tend to produce either an in utero early death or the birth of offspring with congenital abnormalities. Therefore, genes that coded for behaviors that led to incestuous behavior have eventually caused their own disappearance by being present in always fewer handicapped bodies. On the other hand, genes that promoted behavior that avoided sex with genetic relatives have been favored by natural selection. This have led to their increased presence in offspring that would have been healthier and reproductively more successful that the ones produced by incestuous sex. As a result, the successful genes have spread in the population to a point of ubiquity in human populations.
Development of Altruism
The second adaptive problem associated to genetic relatedness involves the ability to identify kin in order to support them. Altruistic behavior toward kin is a consequence of genes aiming to spread themselves; thus natural selection has favored the permanence of motivational systems that, in an all else equal situation, increase the willingness to help close genetic relatives as opposed to unrelated ones. A genetic makeup that encourages helping relatives so that they reproduce more effectively would increase the probability of spreading new copies of those genes in the population. This means that genetic designs that promote altruistic behaviors will always be favored as long as the cost to the individual’s own reproduction is offset by the benefit to the reproduction of its kin member. The degree of relatedness between relatives has a great impact on the expression of this behavior since the more closely related two individuals are the higher the probability that they share the same genetic makeup.
Kin Detection Systems
In order to effectively solve the above adaptive problems, organisms would have developed the ability to identify who is and who is not a genetic relative and estimate the closeness of this relatedness. Detecting genetic similarity is therefore an important adaptive problem, and not an easy one to solve. Organisms cannot “see and compare” DNAs. It appears that the problem has been somehow solved by the development of a kin detection system. By analyzing and outweighing several unrelated cues, this system seems to generate an internal index of relatedness. Based on the results, the system then feeds two different motivational systems, one that promotes sexual attraction/aversion and the other family-directed altruism. It is important to note that familiar relationships do not seem to be identified in the same way, for instance, the system uses different cues to determine relatedness between parents and offspring when compared to siblings.
One of the main cues used to establish relatedness to offspring is similarity. People tend to rate strangers that look similar to them as more trustworthy and less sexually attractive. Mammalian mothers have almost 100 % confidence in their maternity. Therefore they regard any infant who is present after birth as their own child even if they do not resemble her. Fathers on the other hand must use other cues such as phenotypic similarity. A study by Apicella and Marlowe (2004) found that fathers favor children who look more like them (Apicella and Marlowe 2004). This not only refers to physical similarity (ancestral males would have had very little exposure to their own image) but nonvisual cues of self, such as personality, behavioral traits, and smell. In addition, since his mother and sibling share a proportion of his genes, a male could also use their faces as templates.
To gauge relatedness to siblings, the system has evolved to detect mainly two cues: maternal perinatal association (MPA) and duration of coresidence during the period of parental investment. MPA is established when an individual sees their mother caring and nursing a newborn; this means with a high probability that the newborn is his sibling and will be tagged as such. As a consequence, the motivational systems will be promoting high levels of altruistic behavior and high levels of sexual aversion. It is obvious that MPA only works for older siblings. Thus, in order to detect genetic relatedness to older siblings, the brain uses a different cue which takes into account the coresidence period with other children when growing up (approximately between ages 0 and 18). In hunter-gatherer societies, the duration of childhood coresidence is highly correlated with the degree of kinship (Lieberman et al. 2007). There are several studies that have shown that early coresidence causes sexual disinterest between siblings and even unrelated individuals raised together from childhood (Westermarck effect) (Wolf 1995). Interestingly time of coresidence is proportional to the degree of aversion to incest; thus, the longer a youth coresides with an older sibling, the more disgust elicited by the thought of incestuous sex. This extends to moral judgments of third parties where the length of coresidence predicts how morally wrong youngsters find third-party incest.
The MPA cue is overly predominant over coresidence; in other words they are not additive. MPA is enough to generate high levels of altruism and sexual disinterest which do not seem to be modified by time of coresidence. In order to obtain similar levels of altruism and sexual aversion in younger siblings, 14–16 years of coresidence are needed. Thus, coresidence only has an effect if the MPA cue is absent. Since both cues are not additive, they must somehow be associated; it has been suggested this is done by a “kinship estimator.” The kinship estimator combines the cues in a non-compensatory way, following a decision tree pattern. In addition the kinship index seems to be generated in an unconscious way and therefore not affected by conscious beliefs. This is evidenced by cases of step or adoptive siblings. In this case coresidence generates altruistic behavior and sexual aversion despite the fact that the individuals know they are not genetically related. This shows that the criteria used by the kinship estimator prevail over conscious beliefs. Although the two cues mentioned above seem to be the strongest ones, in their absence other cues such as physical resemblance and olfactory signals indicating similarity of the major histocompatibility complex are also taken into account by the system. This is especially relevant in case of distinguishing for instance, maternal half-siblings from full-siblings (DeBruine et al. 2008).
It is important to note that the mechanisms described above are prevalent across species. Nowadays modern humans possess other more sophisticated means of assessing kinship which have been added, not replaced, to existing ones (Geary 2005). These processes seem to be controlled by the neocortex and tend to be complemented and overridden by aforementioned more primitive ones (Park et al. 2008).
As discussed above, animals are not born with knowledge of which individuals are kin. Kin-recognition mechanisms are dependent on learning, enabling individuals to acquire the association between specific members of the group and kin-relevant responses. As a consequence, kin recognition is susceptible to mistakes. A minority of individuals are still inclined to engage in incest. This could arise from the lack of exposure to cues of kinship during upbringing. For instance, opposite-sex siblings that had been separated for considerable time during childhood are more likely to engage in incestuous activity (Bevc and Silverman 2000). In this case these behaviors can occur with intact brain mechanisms. Another explanation for this behavior could be a malfunction of the kin detection system in which due to damage, the motivational system that causes sexual attraction is not turned off by cues of kinship.
Although the presence of a kinship estimator seems to agree with an evolutionary premises, some issues are worthy of consideration. The evolutionary approach bases its assumptions on contemporary hunter and gatherer societies. All these societies express on one way or another incest taboos, but there is evidence that not all ancestral groups conformed to the structural organization of the groups alive today. Modern hunter and gatherer lifestyle is quite restricted to few areas that share similar landscapes and limited immigration/emigration. Our ancestors, on the other hand, occupied a wealth of different habitats ranging from the African savannah to arctic regions. As a consequence of this, it is very likely that social structures were quite diverse among those groups.
It is reasonable to deduct that the kinship model of incest avoidance is more adaptive in societies that do not involve significant immigration/emigration. Here kin detection is important in order to favor altruistic behavior and an increase in the number of kin.
In many animal species that live in social groups, this excess of kin members is usually solved by emigrating before mating season. Usually as soon as males reach reproductive age, they move away from the familiar unit reducing the risk of inbreeding. In these cases there is no evolutionary pressure to develop an incest avoidance mechanism. In the case of humans, it is not well known if this was the case during ancestral times. Although there is some evidence that at least in some early H. sapiens groups, males moved away, it is not known whether this happened in the majority of groups. It has been argued that without this information the adaptive benefit of evolving a kinship detector is not well supported (Richardson 2015).
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- Wolf, A. P. (1995). Sexual attraction and childhood association: A Chinese brief for Edward Westermarck. Stanford: Stanford University Press.Google Scholar