Matrilateral biases in the investment of aunts and uncles

In a study of the kin investment of aunts and uncles we show that the laterality effect expected as a result of paternity uncertainty is statistically reliable but somewhat smaller than the sex effect. Matrilateral aunts invest significantly more than patrilateral aunts, and the same is true for uncles. Regardless of laterality, however, aunts invest significantly more than uncles. Multivariate controls show that the matrilateral bias is fully independent of any age or distance confounds that might result from sex differences in age at marriage or dispersal. We discuss our results in relation to recent findings on the kin investment of grandparents (Euler and Weitzel 1996). In addition, we propose a simple method for estimating the level of paternity uncertainty from kin investment data; application of this method to our data on aunts and uncles suggests that between 13% and 20% of children are not the offspring of their putative father. Our parallel analyses of Euler and Weitzel’s (1996) data on grandparental investment suggest a similar estimate, that paternity uncertainty lies between 9% and 17%.     

Altruism, cheating, and anticheater adaptations in cellular slime molds

Cellular slime molds (CSMs) possess a remarkable life cycle that encompasses an extreme act of altruism. CSM cells live as individual amoebae until starved, then aggregate and ultimately transform themselves into a multicellular fruiting body. This fruiting body consists of stalk cells (altruists that eventually die) and spores (the beneficiaries of this sacrifice). Altruistic systems such as this are vulnerable to cheaters, which are individuals unrelated to the altruists that obtain the benefits provided by them without reciprocating. Here, we investigate two forces that can maintain CSM altruism despite cheating: kin selection and anticheater adaptations. First, we present new kinship-based models based on CSM developmental biology to evaluate the efficacy of kin selection. These models show that stalk-making genotypes can still be maintained when aggregations are initiated by multiple "founder" spores, provided that spores of stalkless fruiting bodies have low rates of dispersal and dispersal success is a concave function of stalk height. Second, we review proposals that several features of CSM development, such as the chemical suppression of the redifferentiation of prestalk cells into prespores, act as anticheater adaptations.     

Testing kin altruism in long-tailed macaques (Macaca fascicularis) in a food-sharing experiment

Social grooming and coalition formation have been main foci in studies concerning altruism in monkeys. Results have been inconclusive because the altruistic nature of these behaviors remains unclear. I investigated altruism in a more transparent context via an apparatus in which captive long-tailed macaque females had the choice to exploit a food source themselves or to yield the food to a test partner. I hypothesized that if potential donors behaved altruistically toward kin, they would yield the food sources longer to kin than to nonkin. Of 11 tested potential donors, 8 did not discriminate between daughters or sisters and nonkin. Thus, the experiments do not support the kin altruism hypothesis. Three females let their youngest offspring have more food than matched juvenile nonrelatives. Behavioral observations strongly suggested,however, that it was primarily spiteful behavior toward juvenile nonrelatives that caused the differences between kin and nonkin in these three cases.     

Migration as a determinant of marriage pattern: preliminary report on consanguinity among Afghans

Two sample populations, one refugee and one resident, were studied. The frequencies of consanguineous marriages came out to be 49.8%and 55.4%, respectively, for the refugees and the residents. Caste endogamy was dominant both in the residents and the refugees. The mean coefficient of inbreeding was calculated to be 0.0303 for the refugee population and 0.0332 for the resident population samples. First cousin marriage was the dominant type of marriage in both samples; father's brother's daughter (FBD) marriage was more frequent among the refugees while mother's brother's daughter(MBD) marriage was more frequent among the residents. Education has no decreasing effect on the incidence of consanguineous marriages. A significant difference in the pattern of marriages in the refugees is observed after the Saur Revolution of 1979.     

Altruism among kin vs. nonkin: effects of cost of help and reciprocal exchange

Evolutionary principles suggest that there will be differences in the nature of altruism directed toward kin vs. nonkin. The present study sought to explore these differences. Participants were 295 undergraduate students who each completed a questionnaire about help exchanged with siblings, cousins, acquaintances or friends. For siblings, cousins and acquaintances, greater relatedness was associated with higher levels of helping. Friends were an exception, however, receiving as much or more help than kin. Consistent with an evolutionary analysis, as the cost of helping increased, kin received a larger share of the help given, whereas nonkin received a smaller share. For low-cost help, people helped friends more than siblings; for medium-cost help, they helped siblings and friends equally; and for high-cost help, they expressed a greater willingness to help siblings than friends. As expected, the level of reciprocal exchange was higher among acquaintances than among friends; however, there was also an unexpectedly high level of reciprocal exchange among kin.     

Differential facial resemblance of young children to their parents: who do children look like more?

In humans, paternal investment is highly variable and is modulated by paternity uncertainty. Facial phenotypic similarity between a father and a child is one possible paternity indicator. However, whether such paternal-biased traits are expressed in children is unclear, as previous empirical results are contradictory. Therefore, we quantified the facial resemblance between a child and each of his or her parents, from birth to 6 years old. Resemblance was assessed from pictures of the face by nonrelated judges. We found that, at all ages, children resemble both their parents more than would be expected by chance, although there is a differential resemblance toward one or the other parent depending on the age and sex of the child. For newborns, boys and girls resemble their mothers more, this differential resemblance persisting through time for girls. For boys, an inversion occurs and they resemble their fathers more between 2 and 3 years of age. The resemblance ascribed by the parents shows that, at birth, mothers ascribe a resemblance to the father, as previously found, although assessment by external judges revealed the opposite. These results suggest that facial appearance is a cue for kin recognition between a father and a child. Patterns of differential resemblance are discussed within the context of evolutionary theories on parental investment.     

The evolution of trans-generational altruism: kin selection meets niche construction

A cornerstone result of sociobiology states that limited dispersal can induce kin competition to offset the kin selected benefits of altruism. Several mechanisms have been proposed to circumvent this dilemma but all assume that actors and recipients of altruism interact during the same time period. Here, this assumption is relaxed and a model is developed where individuals express an altruistic act, which results in posthumously helping relatives living in the future. The analysis of this model suggests that kin selected benefits can then feedback on the evolution of the trait in a way that promotes altruistic helping at high rates under limited dispersal. The decoupling of kin competition and kin selected benefits results from the fact that by helping relatives living in the future, an actor is helping individuals that are not in direct competition with itself. A direct consequence is that behaviours which actors gain by reducing the common good of present and future generations can be opposed by kin selection. The present model integrates niche-constructing traits with kin selection theory and delineates demographic and ecological conditions under which altruism can be selected for; and conditions where the 'tragedy of the commons' can be reduced.     

The presence of a paternal grandmother lengthens interbirth interval following the birth of a granddaughter in Krummhörn (18th and 19th centuries)

Because only daughters inherit the paternal X-chromosome, an asymmetry in adaptive investment decisions has been suggested for certain patrilineal kin. Namely, paternal grandmothers (PGMs) may favor a granddaughter over a grandson, because (within the limits of paternity uncertainty) the former definitely carries one of their X-chromosomes, while the latter definitely does not. Here, we test the hypothesis that the PGMs' sex-specific favoritism influences reproductive scheduling. Using family-reconstitution data, we analyzed interbirth intervals (IBIs) in the historical population from the Krummhörn (Ostfriesland, Germany). In order to account for potentially timevarying effects on IBIs we applied (and combined) both the additive hazards regression of Aalen and the Cox proportional hazards model. We found that the presence of the PGM but not that of the maternal grandmother (MGM), correlates with the IBI following the birth of a grandchild as a function of the grandchild's sex. Specifically, in the presence of a PGM, the IBIs following the birth of a granddaughter are longer than in her absence. However, contrary to predictions from theoretical life history framework, model estimates for a PGM's effect on a mother's IBI did not significantly vary over time This study supports the hypothesis that PGM behavior differs according to her grandchild's sex. Further research should now explore the biological mechanism underlying this phenomenon.     

Kin selection,kin avoidance and correlated strategies

Summary Kin selection of correlated strategies is examined for both weak and strong altruism under simple haploid inheritance. While kin assortment enhances the range of evolutionary stability for (strongly altruistic) correlated strategies (defined herein), kin avoidance is possible under a weakly altruistic correlated strategy. When social competition induces role assignments of variable fitness, group mates may prefer association with non-relatives. Even when group life is mandatory, an individual may accept the risk of abandonment (and reproductive death) rather then associate with kin: a competitive superior may behave altruistically by permitting competitively inferior kin to emigrate. Thus, kin selection and social competition are not necessarily mutually supportive processes within groups. I conclude by interpreting dominance as a strongly altruistic correlated strategy in two social hymenopteran contexts.     

Cues to paternity: Do partner fidelity and offspring resemblance predict daughter-directed sexual aversions?

Despite the profound influence of relatedness on mating and cooperative behavior in humans, the cues men use to assess paternity and guide offspring-directed behavior have yet to be fully resolved. According to leading theories of kin detection, kinship cues should influence both sexual and altruistic motivations because of fitness consequences associated with inbreeding and welfare tradeoff decisions, respectively. Prior work with paternity assessment, however, has generally evaluated candidate cues solely by demonstrating associations with altruism. Here we (i) replicate past work that found effects of phenotypic resemblance and perceived partner fidelity on offspring investment; and (ii) evaluate whether both phenotypic resemblance and perceived partner fidelity meet the more stringent criteria suggested by theory—that is, whether they also predict inbreeding aversions. We report on two studies, one from a population-based sample of Finnish fathers (N?=?390), the other from a Mechanical Turk sample (N?=?700), and furnish evidence in strong support of perceived partner fidelity as a cue to paternity. Support for resemblance as a cue to paternity was decidedly weaker. We discuss a non-kin-based role that resemblance might play in altruistic decision-making, consider whether men might use additional kinship cues to meet the computational challenges associated with paternity assessment, and provide suggestions for future research.     

The neurobiology of punishment

Animals, in particular humans, frequently punish other individuals who behave negatively or uncooperatively towards them. In animals, this usually serves to protect the personal interests of the individual concerned, and its kin. However, humans also punish altruistically, in which the act of punishing is personally costly. The propensity to do so has been proposed to reflect the cultural acquisition of norms of behaviour, which incorporates the desire to uphold equity and fairness, and promotes cooperation. Here, we review the proximate neurobiological basis of punishment, considering the motivational processes that underlie punishing actions.     

Hamilton's rule and kin competition: the Kipsigis case

Evolutionary studies of human behavior have emphasized the importance of kin selection in explaining social institutions and fitness outcomes. Our relatives can nevertheless be competitors as well as sources of altruism. This is particularly likely when there is local competition over resources, where conflict can lead to strife among nondispersing relatives, reducing or even negating the effects of relatedness on promoting altruism. Here, I present demographic data on a land-limited human population, utilizing large within-population variation in land ownership to determine the interactions between local resource competition and the benefits of kin in enhancing child survival, a key component of fitness in this population. As predicted, wealth affects the extent of kin altruism, in that paternal relatives (specifically father's brothers) appear to buffer young children from mortality much more effectively in rich than in poor households. This interaction effect is interpreted as evidence that the extent of nepotism among humans depends critically on resource availability. Further unanticipated evidence that maternal kin play a role in buffering children from mortality in situations where paternal kin control few resources speaks to the important role that specific local circumstance plays in shaping kin contributions to child welfare.     

Resolving the evolution of sterile worker castes: a window on the advantages and disadvantages of monogamy

Many social Hymenoptera species have morphologically sterile worker castes. It is proposed that the evolutionary routes to this obligate sterility must pass through a ‘monogamy window’, because inclusive fitness favours individuals retaining their reproductive totipotency unless they can rear full siblings. Simulated evolution of sterility, however, finds that ‘point of view’ is critically important. Monogamy is facilitating if sterility is expressed altruistically (i.e. workers defer reproduction to queens), but if sterility results from manipulation by mothers or siblings, monogamy may have no effect or lessen the likelihood of sterility. Overall, the model and data from facultatively eusocial bees suggest that eusociality and sterility are more likely to originate through manipulation than by altruism, casting doubt on a mandatory role for monogamy. Simple kin selection paradigms, such as Hamilton''s rule, can also fail to account for significant evolutionary dynamics created by factors, such as population structure, group-level effects or non-random mating patterns. The easy remedy is to always validate apparently insightful predictions from Hamiltonian equations with life-history appropriate genetic models.     

Exact versus heuristic models of kin selection

This paper examines the conditions under which the classical inclusive fitness formulation of Hamilton (1964) provides an adequate approximation to the dynamics of gene frequency change and to conditions for genetic equilibrium, in the “additive” model of altruism between sibs of Uyenoyama and Feldman (1981). It is concluded that the classical formulation is adequate, provided that either the effect of the gene on the probability of behaving altruistically is low or the costs and benefits of altruism are small, unless the benefit/cost ratio k is very close to 2, the value that must be exceeded for altruism to be favoured. In addition, the gene for altruism must be underdominant, recessive or partially recessive in its effect on the probability of behaving altruistically, for the inclusive fitness predictions to break down significantly.     

Why babies look like their daddies: paternity uncertainty and the evolution of self-deception in evaluating family resemblance

It has been suggested that in a socially monogamous system where fathers invest in their mate's offspring but paternity is far from certain, it will be adaptive on the part of infants to conceal their father's identity; but the opposite claim has also been made that this is against the genetic interests of the fathers, and a high frequency of adulterine births will select instead for paternal resemblance. In this article, I present a simple theoretical model that suggests that neonatal anonymity benefits fathers, mothers, and children. Once anonymity becomes established, however, all babies start paying the cost of paternity uncertainty, that is, the reduction in paternal care due to fathers not knowing whether they have truly sired their mate's offspring. By diminishing the fitness of babies, such a cost bounces back as lowered fitness for parents as well. We should then expect the evolution of maternal strategies directed to decrease paternity uncertainty, in the form of instinctive and unsolicited comments on babies' resemblance to their putative fathers. In contradiction to the widespread belief that it would be in fathers' interest to be skeptical of these allegations, the model suggests that, under conditions of infant anonymity, fathers will actually promote their own fitness by believing their spouses. Received in revised form: 5 September 2001 Electronic Publication     

Social dynamics of mammals: Reproductive success, kinship and individual fitness

Apparent altruism, in which an individual seemingly decreases its evolutionary fitness by assisting others, can confer benefits if the individual assists kin. Thus, an animal can increase its total or inclusive fitness by producing offspring (direct fitness) and/or helping kin to reproduce (indirect fitness). Although kin selection has been suggested as the mechanism underlying the formation of mammalian societies, many species act as if they attempt to maximize the direct fitness component of their inclusive fitness.     

Kinship appeals and conservation social marketing

Increasing environmental problems and the need to obtain public support to help address them make effective appeals in conservation fundraising campaigns indispensable. However, social marketing messages based on data, characteristics of focal species, self-interest, and moral responsibility tend to work best on targeted, and so limited, audiences. As conservation organizations reach out to broader audiences, they will require strategies that appeal to more potential donors. This paper argues that use of kinship symbolism to describe non-human species should make conservation marketing campaigns more effective. Evolutionary theories of altruism predict the power of kinship-recognition cues in encouraging and reinforcing sacrifice in non-kin, unreciprocated contexts, and these cues can be manipulated in marketing campaigns to protect threatened species and resources. People often behave altruistically toward “fictive” kin, and the labeling of non-humans as kin in many traditional, small-scale societies appears to be associated with environmental resource management. Characterizing non-human species, and even non-living resources, as kin to humans in marketing campaigns may promote a willingness to contribute to conservation-related causes.     

Matriliny as daughter-biased investment

From an evolutionary perspective, matriliny presents a puzzle because men in matrilineal societies transmit wealth to their sisters' sons, to whom they are only half as related as to their own sons. It has been argued that such systems would only maximise fitness under unrealistically high levels of paternity uncertainty. In this paper, we propose that matriliny can arise from daughter-biased investment by parents and/or grandparents. We show that daughter-biased investment is adaptive if the marginal benefit of wealth to sons (compared to daughters) does not outweigh the risk of nonpaternity in sons' offspring. We argue that such conditions will be rare where resource-holding polygyny is prevalent but could otherwise be widespread under even moderate levels of paternity uncertainty. The daughter-biased investment model explains two well-known characteristics of matrilineal descent: (a) matriliny's association with high levels of paternity uncertainty and (b) matriliny's ecological correlates, including its association with horticulture, its rarity in pastoralist and agro-pastoralist societies, and the tendency for matriliny to be replaced by son-biased inheritance during economic development. We present data on wealth, sex, and reproductive success (RS) in two African societies, the matrilineal Chewa in Malawi and patrilineal Gabbra in Kenya, which support the daughter-biased investment theory.     

Evolution of surname distribution under gender-equality measures

We consider a model for the evolution of surname distribution under a gender-equality measure currently being discussed by the Spanish Parliament (whereby children would adopt their mother's and father's surnames in alphabetical order). We quantify how this would bias the alphabetical distribution of surnames, and analyze its effect on the present distribution of surnames in Spain.