1. Introduction

Helping behaviours that increase the direct fitness of recipients underpin several major evolutionary transitions [1]. Acts in which helpers provide any resource (e.g. food, time) are interesting because evolutionary theory strongly emphasizes the importance of competition and selfish behaviour. Humans are adept at helping each other. From a quantitative perspective, this trait is not unique in the animal kingdom; arguably, hymenopterans and other eusocial species are even more helpful within their colonies. However, helping by the latter is explained by biological altruism based on kin selection [2,3], while humans also cooperate with unrelated individuals for direct fitness benefits on a scale that is unmatched by any other species. Importantly, the criteria for cooperating are highly flexible: the same individual may cooperate with friends, colleagues, supporters of the same football club, political affiliates, compatriots or even international alliances. Help can be provided in different currencies (e.g. time/money/physical effort) and is also often provided in situations where it is unclear how return benefits may be accrued, from letting a car out at a busy junction to donating to victims of natural disasters in far-away countries.

The frequency and scale of human helping could depend on several factors that appear to be unique to humans: our capacity for spoken and written language, the use of tags to identify groups, societal-level norms and institutions that both prescribe cooperation and punish defection, various media channels that allow for large-scale communication and coordination, and banks to transfer money—a unique non-perishable resource—anywhere. One might therefore wonder to what extent research on human cooperation yields idiosyncratic explanations, rendering comparisons with other species useless. One may also ask whether research on human cooperation is suitable for publication in biological journals like Proceedings of the Royal Society B. Here, we address this question. We first summarize briefly the enormous impact that theoretical concepts and empirical studies of human cooperation have had on research in other species. We then highlight topics of interdisciplinary interest and shared future challenges. It should become clear that we favour an open-minded and inclusive approach, where humans are just another species that can be studied under the general framework of evolutionary theory. While human cooperation might be more peculiar than cooperation in many non-human species, each species would appear unique if every detail was taken into consideration. Therefore, a distinction between disciplines based on study organisms only hinders progress.

2. Theory on human helping as inspiration for biological research

Theoretical approaches to understand helping in humans predate evolutionary concepts of helping. The principal tools used by biologists were developed by economists in the form of game theory—a framework to understand how humans should make decisions in strategic interactions [4]. A ‘game’ is a formal mathematical model of an interaction, defining the payoffs to all players. A key insight is that players’ payoffs are affected by their own decisions and also by those of their partner(s). Thus, the dominant strategy depends on the strategy that is used by the partner(s). Economists assume that payoffs translate into utility and that players maximize utility. Stylized economic games were developed to study optimal decision rules. In their simplest form, these games consist of two players who can each choose between two actions, for example to cooperate or to defect. Games can be one-shot or repeated over a number of rounds. The resulting payoffs of action combinations can be captured by a 2 × 2 matrix. The matrices for well-known games [5], like the prisoner’s dilemma game, the prisoner’s delight game and the snowdrift game (also called hawk–dove game) are summarized in figure 1. These games were subsequently adopted by evolutionary biologists to explore when helping behaviour could be evolutionarily stable [6] in a population. Under this evolutionary approach, strategies are inherited traits that specify behaviours [7]. Rather than utility, evolutionary biologists assume that payoffs translate into fitness, with the accompanying assumption that strategies which, on average, increase fitness will be under positive selection.