Local competition promotes hurtful behaviour towards unknown others

Humans frequently cooperate to achieve benefits unattainable through selfish behaviour. Punishment of free-riders is key for sustaining cooperation, but it is also costly, raising questions about the origins of such behaviour. Some evolutionary models suggest that individuals may be willing to hurt unknown others in competitive environments. However, empirical evidence about the effect of the scale of competition on strategies involving hurtful behaviour in human groups, such as spite, retaliation for hurting and punishment of free-riders, is missing. Using a laboratory experiment, we manipulate the scale of competition to investigate its influence on hurtful and helpful behaviours, between unknown humans in an indirect reciprocity game. We observe distinct behavioural patterns between local and global competition. When competition is local and thus confined to take place within isolated groups, we find frequent hurting as an expression of spite and retaliation. In contrast, when competition extends globally across several groups, hurting is used primarily for punishment of free-riders, whereas helping behaviour is rewarded, which together promotes cooperation. Thus, while isolated competition fosters inefficient, anti-social behaviour, global competition encourages pro-sociality and cooperation.