The Effects of Extra-Somatic Weapons on the Evolution of Human Cooperation towards Non-Kin

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.

人类对非亲属的合作与利他行为，以及‘强互惠（strong reciprocity）’——即合作/利他者无法获得当下或未来回报的行为——均是重大的进化谜题。本文检验了“体外武器（extra-somatic weapons）的发展可能影响人类合作行为的演化”这一假说，从而为这两大谜题提供全新解释。武器的广泛使用，会使人亚科类群内部的冲突更具致命性，同时也能平衡个体间的权力差距。在这样的文化生态位中，不合作者参与此类致命冲突的频率可能远高于合作者，进而提升与合作行为相关的基因的相对适合度。我们采用两种版本的进化版重复囚徒困境（Iterated Prisoner's Dilemma, IPD）模型——一种模拟武器使用场景，另一种不模拟——并测试了25种IPD策略的表现，以评估武器使用对其产生的影响。研究结果显示，在模拟武器使用的场景下，合作型策略的表现显著更优，而非合作型策略则显著更差。尤为重要的是，与‘强互惠’等价的‘始终合作’IPD策略，其表现提升幅度远高于其他所有合作型策略。我们由此得出结论：体外武器的发展为人类利他与合作行为的演化，尤其是强互惠行为的演化，提供了新的阐释视角。人类特有的利他与合作行为或许曾在过往环境中成为适应性性状，但在现代世界中已不复存在，这一观点为解释这一重大进化谜题的理论体系增添了全新的维度。