Eckart Voland

Human life histories and demographic outcomes are impacted by kin behaviour in diverse ways, and human evolutionary theory is essential to understanding how environmental context and kin relationship moderate this behaviour in cooperative versus competitive directions. However, kin presence is simultaneously correlated with behavioural and non-behavioural factors such as risk of infection or familial wealth. As such, it can be hard to disentangle evolutionary effects from other factors correlated within a family. In this chapter we discuss how historical family reconstitution databases have assisted in the investigation of kin effects and their advantages in disentangling these behavioural kin effects from non-behavioural accompanying factors. A variety of family reconstitution studies exist across varying geographic and cultural contexts, and their application to kinship research has resulted in varying findings. This highlights how kin impacts differ depending on circumstance. We describe several family reconstitution studies and discuss the importance of evolutionary theory in understanding kin effects across different populations.

To address claims of human exceptionalism, we determine where humans fit within the greater mammalian distribution of reproductive inequality. We show that humans exhibit lower reproductive skew (i.e., inequality in the number of surviving offspring) among males and smaller sex differences in reproductive skew than most other mammals, while nevertheless falling within the mammalian range. Additionally, female reproductive skew is higher in polygynous human populations than in polygynous nonhumans mammals on average. This patterning of skew can be attributed in part to the prevalence of monogamy in humans compared to the predominance of polygyny in nonhuman mammals, to the limited degree of polygyny in the human societies that practice it, and to the importance of unequally held rival resources to women’s fitness. The muted reproductive inequality observed in humans appears to be linked to several unusual characteristics of our species—including high levels of cooperation among males, high dependence on unequally held rival resources, complementarities between maternal and paternal investment, as well as social and legal institutions that enforce monogamous norms.

Historically, mothers producing twins gave birth, on average, more often than non-twinners. This observation has been interpreted as twinners having higher intrinsic fertility – a tendency to conceive easily irrespective of age and other factors – which has shaped both hypotheses about why twinning persists and varies across populations, and the design of medical studies on female fertility. Here we show in >20k pre-industrial European mothers that this interpretation results from an ecological fallacy: twinners had more births not due to higher intrinsic fertility, but because mothers that gave birth more accumulated more opportunities to produce twins. Controlling for variation in the exposure to the risk of twinning reveals that mothers with higher twinning propensity – a physiological predisposition to producing twins – had fewer births, and when twin mortality was high, fewer offspring reaching adulthood. Twinning rates may thus be driven by variation in its mortality costs, rather than variation in intrinsic fertility.

We describe the natal dispersal patterns of the Krummhörn population as the outcome of intrafamilial competition. Depending on the affiliation with a specific social group and the sex of the individual, this competition is driven by different factors and obeys a different functional logic: The dispersal patterns of the daughters of landless workers allow a mate competition scenario to be detected, whereas the dispersal behavior of the children of farmers (especially of the sons) is driven by a resource competition scenario. Am. J. Hum. Biol., 2008.

Zusammenfassung: Wir argumentieren, dass die Psychologie des Schwiegermutter/Schwiegertochter-Konflikts auf einen genetischen Interessenskonflikt zurückgeht. Anhand von Statistiken zur Totgeburtlichkeit zeigen wir, dass dieser Konflikt unter den sozio-ökologischen Bedingungen der ländlichen Krummhörner Bevölkerung des 18. und 19.Jahrhunderts (Ostfriesland) reproduktive Kosten verursacht hat. Drei Szenarien könnten zur Evolution des Schwiegerkonflikts geführt haben: in dem “Monopolisierungs-Szenario” geht es der Schwiegermutter um eine best mögliche Erhöhung der Vaterschaftswahrscheinlichkeit ihres Sohnes. In dem Szenario zur “Steigerung des Paarungserfolgs” brechen Schwiegermütter starke emotionale Bande zwischen ihren Söhnen und deren Partnerinnen auf, um den Söhnen weitere, außereheliche Paarungsmöglichkeiten zu erleichtern. In dem “Ausbeutungs-Szenario” sind Schwiegermütter bestrebt, die ökonomischen Erträge aus der Arbeit von Schwiegertöchtern in die eigene Familienökonomie zu überführen. Die Krummhörner Schwiegerkonflikte entsprechen wahrscheinlich am besten der Logik des “Ausbeutungs-Szenarios”. Wir schließen mit der Hypothese, dass die soziale und reproduktive Manipulation der Schwiegertöchter durch die Schwiegermütter ein weiterer evolutionärer Weg zur Entstehung der postgenerativen Lebensspanne gewesen sein könnte.