Experiment conducted in a school building in Emmaus, a rural location in a remote valley in the DrakensbergMountains.


Participants were asked to impose a cost upon themselves in the form of pain from a physical exercise in return for a proportionate material reward given to an individual or organization, whose identity was disclosed to the participant in advance of carrying out the exercise. The exercise was a standard isometric ski-training exercise (sitting as though on a chair with the back against a wall, with the calves and thighs at right angles to each other: Cabanac, 1986). Participants were asked to maintain this position for as long as possible. The position becomes increasingly painful with time; after approximately 100 sec, it increases massively.


The length of time a participant maintained the position on a given trial was transformed into a material benefit for a recipient who varied in biological relatedness to the participant (the coefficient of relationship, r, the independent variable). Each participant repeated the experiment with the recipient varied systematically between four principal categories: the participant (r=1.0), a sibling (r=0.5), an aunt, uncle, niece or nephew (r=0.25), a cousin (r=0.125). Before the experiment, each participant drew up a list of individuals for the three categories of genetic relatedness from with recipients were randomly selected by the experimenter. Participants also maintained the position for a category of r=0 (in London, a children's charity, in South Africa a local school). In two previous experiments conducted in the UK, participants also performed the exercise for a unrelated friend of the same sex (r=0 but with possibility of reciprocation).


For each experiment, the relatedness category for the recipient of the payout was randomized across trials. Before each trial, participants were told that the proceeds would be sent directly to a nominated individual, chosen at random by the experimenter from among those listed by the participant. Recipients were limited to individuals who were of reproductive age (16-45 years) and not cohabiting with the participant.


At the end of each trial, the duration for which the position was held was determined, and the fee appropriate to this calculated. Where the participant was the beneficiary, this was paid directly to them; otherwise, a cheque for the appropriate fee was sent or delivered to the nominated beneficiary along with a note explaining the origins of the donation.


In the U.K., participants received a payment at a rate of £1.50 per 20 sec for which the position was held. The payments for the Zulu participants consisted of food hampers (containing 6 items considered desirable by local informants: oil, sugar, soya protein, tinned fish, instant soup and matches) of the same monetary value £1.50, equivalent to approx. ZAR20.00. Participants who maintained the exercise for 20 sec received one hamper, while for 40 sec they received 2 hampers, etc.


If participants follow Hamilton’s Rule, investment (time for which the position was held) should increase with the recipient’s relatedness to the participant.


There was no cultural effect, as this kinship altruism occurred among both urban English & rural South African Zulus. As the degree of relatedness decreases, so does the degree of altruism.

The results confirm that the effect of kinship on altruism is trans-cultural. Hamilton’s Rule of kin selection is not limited to the particular cultural environment now characteristic of the post-industrialised West. The results thus support the relevance of Hamilton’s Rule as a functional explanation for human behaviour. (This however, does not tell us anything about the motivations that underpin the behaviour, nor the cues that are used to infer relatedness).


Kinship thus represents a baseline against which humans make judgements when deciding how to behave towards others. Judgements that may be later coloured by issues of reciprocity, obligation, prosociality and other ethical considerations.         


These findings provide the first unequivocalexperimental evidence that kinship plays a role in moderating altruistic behaviour. It is the first empirical study of altruism that imposes a genuine cost on the altruist & looks at altruistic behaviour rather than declared inclinations (as in e.g. questionnaire studies: as anthropologists and psychologists have repeatedly shown, humans commonly say one thing but do something entirely different).


One feature of the results however does suggest a cross-cultural difference, although overlaid on a general pattern. In contrast to the London sample, there was in the two South African societies no discrimination between investment in kin with r=0.5 (siblings) and 0.25 (aunt, uncles, nieces and nephews). This difference in patterns of investment may reflect that siblings and close kin in Zulu societies cohabit during the early stages of life & that this kind of association may function as a proximate cue of relatedness among kin, in a way similar to the way in which early childhood association activates adaptive kin-recognition responses among nonkin (see the Westermarck Effect).


There was a slight bias for women to rely less on kinship, possibly because women have evolutionarily spent most of their reproductive lives in groups with few genetic relatives and may thus be more attuned to maintaining close relationships with unrelated people and kin.