Half-Siblings in Human Evolutionary History
KeywordsInclusive Fitness Serial Monogamy Human Evolutionary History Inclusive Fitness Theory Human Behavioral Ecology
Half-siblings are either sired by the same father (paternal half-siblings) or born by the same mother (maternal half-siblings), but not both.
In altricial species, such as primates, maternal half-siblings and full-siblings live usually in the proximity of each other and are raised up by the same mother. In contrast, the relation between paternal half-siblings is much less tight. The outcome of this difference in terms of social interactions, such as cooperation, aggression, and agonistic behavior, has been studied extensively in many mammalian species. Most research studying human behavioral ecology and the role of kin network, however, focused on full-siblings only. The current knowledge about the mating behavior of Homo sapiens and extinct Homo species suggests that this form of relatedness had always been present during the evolutionary history of the genus. Therefore, studying the interactions between half-siblings is expected to contribute significantly to our understanding of how resources are allocated between, and how support is given to, human individuals with different degrees of genetic relatedness.
Relation of Half-Siblings
Though the degree of genetic relatedness is the same, a distinction has to be made between maternal and paternal half-siblings. The relation of siblings within the two types is qualitatively different: on average, maternal half-siblings exhibit more interactions with each other, compared to paternal half-siblings (e.g., Pollet 2007; Tanskanen and Danielsbacka 2014). The main reason for this is that after a couple splits up, the children usually stay with the mother, so children from a new relationship grow up with the mother’s other children. The shared family environment is an influential determinant of the half-siblings’ relation (Pollet 2007).
Half-siblings – just as full-siblings – could either compete for resources (social, emotional, material, etc.) or provide resources to each other or both. Both types of relation can be explained within the frameworks of evolutionary theories, such as: (1) kin selection, (2) parent–offspring conflict, and (3) parental investment theory.
Siblings as Rivals
From an evolutionary point of view, the rivalization between siblings can be explained by kin selection (Hamilton 1964) and parent–offspring conflict (Trivers 1974). Trivers assumed that competition among full-siblings is less intense than among half-siblings. The results of empirical research, however, are contradictory (Pollet 2007; Tanskanen and Danielsbacka 2014; Salmon and Hehman 2015; Tanskanen et al. 2016).
In their study, Tanskanen et al. (2016) compared older (62–67 years) and younger (21–50 years) siblings. They proposed that the fact that they lean on the resources of the very same providers puts more strain on the relation of full-siblings. In general, the older generation showed less conflicts. Members of the younger generation had indeed more conflicts with full-siblings than with half-siblings. They also had more conflicts with maternal half-siblings than paternal ones, just like the older generation. However, the older full-siblings in the sample reported an increased likelihood of having conflicts with full-siblings over paternal half-siblings only, but not over maternal ones. The data were also controlled for emotional closeness, contact frequencies, and other demographic and socioeconomic family variables. This suggests that these within-family relations might be more successfully explained within the frames of the parental investment, rather than the inclusive fitness theory.
Siblings as Resources
Studies inspired by the kin selection theory yielded confirmatory data in the case of resource allocation. It was suggested that individuals will invest more in full-siblings than in half-siblings or step siblings (Emlen 1997). This view received support from Jankowiak and Diderich (2000), who found that half-siblings displayed significantly lower solidarity in a polygamous Mormon community in the USA. Solidarity was expressed in monetary gifts, requests to babysit, feelings of closeness, favoritism, and attendance at birthday and wedding celebrations. This happened despite a strong cultural ideology for equal treatment of half-siblings.
Half-Siblings and Sexual Selection
In The Mating Mind Miller (2001) suggested that the mating pattern of the human ancestors in the Pleistocene might be best described as serial monogamy. This is similar to what is observed in extant hunter-gatherer communities. Exclusively monogamous pairs are formed for a couple of years and may split up either because of the death of one party or because of deserting. Then, new relations could be formed. On average, 3 months with regular copulations is necessary for conception. Therefore, in a relation which lasted for several years, one baby was likely to be born in every 2–5 years. Just like in most of the recent human societies, the major part of the costs of rearing a child was inflicted on the ancient hominid females. After the couples split up, this was even more the case.
Because of the serial monogamy, it is likely that the mating behavior of hominid females with one or more dependent offspring was combined with their parental behavior. Miller suggested that males hardly had any chance to court childless females. Hence, the main question was not whether a female has any children; her attractiveness and other reproductively important qualities were more of a concern. Sexual competition between females was, therefore, competition between mothers. It was proposed that mothers in the Pleistocene paid attention to how their children judged the prospective sexual partner. So the mothers’ choice was influenced by the children’s choice. Therefore, the courting of hominid males was directed partly on the offspring of the potential female sexual partner. An inevitable but somewhat surprising consequence of the children’s preference was that through influencing their mother’s choice, they influenced sexual selection and the genetic constitution of their younger half-siblings.
The kin network might have been much more complex during human evolutionary history than it is often suggested by the traditional view of families in Western societies. “Patchwork” families of nowadays may stand closer to what was common in the evolutionary past. As the number of such families is increasing, and the relations between full- and half-siblings are getting more complicated, and eventually stressful, the reconsideration of the view of a “normal” human family is warranted both from a scientific and a therapeutic perspective.
- Miller, G. (2001). The mating mind (pp. 171–179). New York: A Division of Random House.Google Scholar