According to Hamilton's rule, matrilineal-biased investment restrains men in matrilineal societies from maximising their inclusive fitness (the ‘matrilineal puzzle'). A recent hypothesis argues that when women breed communally and share household resources, a man should help his sisters' household, rather than his wife's household, as investment to the later but not the former would be diluted by other unrelated members (Wu et al., 2013). According to this hypothesis, a man is less likely to help on his wife's farm when there are more women reproducing in the wife's household, because on average he would be less related to his wife's household. We used a farm-work observational dataset, that we collected in the matrilineal Mosuo in southwest China, to test this hypothesis. As predicted, high levels of communal breeding by women in his wife's households do predict less effort spent by men on their wife's farm, and communal breeding in men's natal households do not affect whether men help on their natal farms. Thus, communal breeding by women dilutes the inclusive fitness benefits men receive from investment to their wife and children, and may drive the evolution of matrilineal-biased investment by men. These results can help solve the ‘matrilineal puzzle'.

Microfinance is an economic development tool that provides loans to low-income borrowers to stimulate economic growth and reduce financial hardship. Lenders typically require joint liability, where multiple borrowers share the responsibility of repaying a group loan. We propose that this lending practice creates a cooperation dilemma similar to that faced by humans and other organisms in nature across many domains. This could offer a real-world test case for evolutionary theories of cooperation from the biological sciences. In turn, such theories could provide new insights into loan repayment behaviour. We first hypothesise how group loan repayment efficacy should be affected by mechanisms of assortment from the evolutionary literature on cooperation, i.e. common ancestry (kin selection), prior interaction (reciprocity), partner choice, similarity of tags, social learning, and ecology and demography. We then assess selected hypotheses by reviewing 41 studies from 32 countries on micro-borrowers’ loan repayment, evaluating which characteristics of borrowers are associated with credit repayment behaviour. Surprisingly, we find that kinship is mostly negatively associated with repayment efficacy, but prior interaction and partner choice are both more positively associated. Our work highlights the scope of evolutionary theory to provide systematic insight into how humans respond to novel economic institutions and interventions.

Premunition is the state in a disease where an existing infection protects the host from reinfection with the same species. The cause of premunition is not clearly understood. In this study, we hypothesized that kin-selection might be a contributing factor in premunition. To test this theory, sheep were infected either once with a linguiform or smooth vulval morphotype of Haemonchos contortus, twice with the same morphotype or twice with different morphotypes. All infections resulted in a similar number of adult parasites. However, there were differences in the morphotypes recovered providing potential evidence of kin selection. Negative interference competition might also contribute to the reduction of the incoming population. Allelopathic or physical interactions between the parasites may be the mechanism behind the observed phenomena.

Parasite virulence, or the damage a parasite does to its host, is measured in terms of both host costs (reductions in host growth, reproduction and survival) and parasite benefits (increased transmission and parasite numbers) in the literature. Much work has shown that ecological and genetic factors can be strong selective forces in virulence evolution. This review uses kin selection theory to explore how variations in host ecological parameters impact the genetic relatedness of parasite populations and thus virulence. We provide a broad overview of virulence and population genetics studies and then draw connections to existing knowledge about natural parasite populations. The impact of host movement (transporting parasites) and host resistance (filtering parasites) on the genetic structure and virulence of parasite populations is explored, and empirical studies of these factors using Plasmodium and trematode systems are proposed.

Virulence is part of the proximate base to the understanding of host–parasite interactions, and therefore it is crucial to interpret parasite-induced damage as an outcome of co-evolution between parasite and host. We experimentally investigated the impact of 2 congeneric ticks with contrasting transmission modes and host specificity, on the health of songbird nestlings (Parus major). The nidicolous Ixodes arboricola lives in tree holes where it obtains blood from adult birds and their nestlings, hence is prone to vertical transmission. All developmental stages of the field-dwelling Ixodes ricinus feed on different host taxa, hence are prone to horizontal transmission. Within each nest we infested 3 nestlings with varying loads of I. ricinus and 3 with Ixodes arboricola. Ixodes arboricola had no negative effects on nestling health (haematocrit, inflammation, body size, body condition). Body size was positively associated with the number of feeding I. arboricola adults. In contrast, I. ricinus infestations by nymphs and adult ticks led to haematocrit reductions. Furthermore, I. arboricola was a more prudent feeder, with longer feeding durations for smaller bloodmeals. Although the negative effects on nestling health were limited, our results are in line with current theories, stating that virulence increases with horizontal transmission, reduced dependence on individual hosts and reduced relatedness among co-exploiting ticks.

A new theory of attraction and liking based on kin selection suggests that people detect genetic similarity in others in order to give preferential treatment to those who are most similar to themselves. There are many sources of empirical and theoretical support for this view, including (1) the inclusive fitness theory of altruism, (2) kin recognition studies of animals raised apart, (3) assortative mating studies, (4) favoritism in families, (5) selective similarity among friends, and (6) ethnocentrism. Specific tests of the theory show that (1) sexually interacting couples who produce a child are genetically more similar to each other in blood antigens than they are either to sexually interacting couples who fail to produce a child or to randomly paired couples from the same sample; (2) similarity between marriage partners is most marked in the more genetically influenced of sets of anthropometric, cognitive, and personality characteristics; (3) after the death of a child, parental grief intensity is correlated with the child's similarity to the parent; (4) long-term male friendship pairs are more similar to each other in blood antigens than they are to random dyads from the same sample; and (5) similarity among best friends is most marked in the more genetically influenced of sets of attitudinal, personality, and anthropometric characteristics. The mechanisms underlying these findings may constitute a biological substrate of ethnocentrism, enabling group selection to occur.

In most cultures, extramarital sex is highly restricted for women. In most of those cultures, men transfer wealth to their own sons (patrilineal inheritance). In some cultures extramarital sex is not highly restricted for women, and in most of those cultures, men transfer wealth to their sisters' sons (matrilineal inheritance). Inheritance to sisters' sons ensures a man's biological relatedness to his heirs, and matrilineal inheritance has been posited as a male accommodation to cuckoldry—a paternity strategy—at least since the 15th century. However, longitudinal analysis of the cumulative effect of female extramarital sex indicates that matrilineal inheritance is most advantageous for women and would be more accurately considered a grandmaternity strategy. That is, if the probability that men's putative children are their biological children (ρ = probability of paternity) is less than 1, the probabilistic degree of relatedness between a female and her matrilineal heirs is higher than her corresponding relatedness to her patrilineal heirs. The same holds true for men only if ρ is very low (< 0.46). The upshot is that for moderate levels of female extramarital sex, matrilineal inheritance, relative to patrilineal inheritance, is highly advantageous for women and disadvantageous for men. Consideration of female variance in reproductive success beyond the first generation, and of a man's network of obligation to the inclusive fitness of his relatives, suggests that although the establishment of matrilineal inheritance may require extremely high levels of female extramarital sex, once established, it is likely to be maintained at levels of ρ that reasonably characterize many societies in the ethnographic record. New analysis of previously published data shows a strong association between matrilineal inheritance and moderate to low probability of paternity, and an even stronger relationship between patrilineal inheritance and high probability of paternity.