Abstract
Human cooperation and altruism towards non-kin is a major evolutionary puzzle, as is ‘strong reciprocity’ where no present or future rewards accrue to the co-operator/altruist. Here, we test the hypothesis that the development of extra-somatic weapons could have influenced the evolution of human cooperative behaviour, thus providing a new explanation for these two puzzles. Widespread weapons use could have made disputes within hominin groups far more lethal and also equalized power between individuals. In such a cultural niche non-cooperators might well have become involved in such lethal disputes at a higher frequency than cooperators, thereby increasing the relative fitness of genes associated with cooperative behaviour. We employ two versions of the evolutionary Iterated Prisoner's Dilemma (IPD) model – one where weapons use is simulated and one where it is not. We then measured the performance of 25 IPD strategies to evaluate the effects of weapons use on them. We found that cooperative strategies performed significantly better, and non-cooperative strategies significantly worse, under simulated weapons use. Importantly, the performance of an ‘Always Cooperate’ IPD strategy, equivalent to that of ‘strong reciprocity’, improved significantly more than that of all other cooperative strategies. We conclude that the development of extra-somatic weapons throws new light on the evolution of human altruistic and cooperative behaviour, and particularly ‘strong reciprocity’. The notion that distinctively human altruism and cooperation could have been an adaptive trait in a past environment that is no longer evident in the modern world provides a novel addition to theory that seeks to account for this major evolutionary puzzle.
Highlights
The puzzle of cooperation and strong reciprocity Human cooperation and altruism towards non-kin poses two fundamental questions for biology and the behavioural sciences
‘Strong reciprocity’, has been observed in one-off, anonymous encounters under experimental conditions [12] where the reciprocal and reputation effects required by reciprocal altruism or indirect reciprocity theory could not influence the behaviour of participants [4]
The higher incidence of disputes encountered between non-cooperative players and the adverse effect this had on individual fitness appears to explain the patterns found, despite the usual payoffs from the Iterated Prisoner’s Dilemma (IPD) model
Summary
The puzzle of cooperation and strong reciprocity Human cooperation and altruism towards non-kin poses two fundamental questions for biology and the behavioural sciences. ‘strong reciprocity’ is defined as a propensity to cooperate unconditionally even if this is costly and provides neither present nor future rewards to the co-operator/altruist [3], [4] Why, does this behaviour persist as it incurs a cost and/or offers no present or future rewards [3], [4]?. ‘Strong reciprocity’, has been observed in one-off, anonymous encounters under experimental conditions [12] where the reciprocal and reputation effects required by reciprocal altruism or indirect reciprocity theory could not influence the behaviour of participants [4]. Instead, it has been explained by ‘new’ group selection linked to extinction-threatening events [3],. Others explain it as a response to conditions of uncertainty in reciprocal relationships [18] or as a maladaptive ‘misfiring’ of evolutionary mechanisms in modern, experimental settings [19]
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