To help, cooperate, and share with others are central aspects of human societies, and they range from large-scale cooperation to small acts of charity. How this prosocial behaviour (i.e. “voluntary behavior intended to benefit another”1, p. 646) develops during ontogeny has recently become a hot topic of developmental psychology – so much so that more than 20% of studies on prosocial behaviour in children have been published in the last 6 years2. These studies suggest that prosocial behaviour emerges early in human ontogeny, with some researchers arguing that it forms a biological predisposition3,4. Already by the age of 14 months infants help in simple tasks, such as handing an experimenter an out-of-reach object5. At this age, prosocial behaviour is indiscriminate of the other’s identity and limited to simple helping tasks. By the age of three, children start behaving more selectively by punishing or withdrawing cooperation from uncooperative social partners6. Sharing resources with others develops even later in childhood, as children’s prosocial tendencies increase with experience and they are more strongly influenced by social norms7,8.

One way to experimentally assess children’s propensity to share with others are resource allocation paradigms inspired by behavioural economics: in these “games” a participant (i.e. the donor) is presented with two options that have different payoffs for him- or herself and a partner (i.e. the recipient). In a typical prosocial game (cf. prosocial choice task)9 the donors can choose between one option, where they receive a reward and simultaneously deliver a reward to the recipient (1/1 option), and a second option, where only they themselves receive a reward (1/0 option). In this game there are no costs for the donor, as he or she will always receive one reward item, regardless of their choice. In comparison, in a costly sharing game, the donors are required to choose between the altruistic 1/1 option and a 2/0 option, which would deliver two reward items to them. Consequently, in this game, the donor has to forego maximizing his or her own payoff in order to benefit the recipient.

Brownell and colleagues10 found that in a cost-free prosocial game, 2-year-old children only showed a preference for resource distributions that benefitted an adult recipient, when the recipient verbally expressed a desire for the reward. House and colleagues11 tested 3–8-year-old children with peers (i.e., familiar children from their classroom) as recipients in both the prosocial and the costly sharing game. They found that the children had a tendency to share in the prosocial game, but only when discarding those trials in which the donors laughed while making their choice. In such trials the donors predominantly chose the self-serving option. Also, they chose the prosocial option more often in the test condition, where the donor could actually benefit a recipient, compared to a control condition, where no recipient was present. However, this effect was mainly driven by the 7–8-year-olds. This suggests that only from that age on children make conscious choices about sharing. Recipient requests for the reward occurred in only a few cases in this study and are therefore not likely to have influenced the donors’ prosocial giving11. These results parallel earlier findings conducted in an anonymous setting (i.e. recipient’s identity only represented by a classroom photo)12: while at age 3–4 many children in this study acted selfishly, by 7–8 years the children shared equally with the recipient irrespective of costs for themselves, presumably due to a preference for equal outcomes.

While prosocial sharing had long been regarded a hallmark of human behaviour, recent studies found prosocial tendencies in various non-human primate species (e.g.13,14,15,16, for a review see)17, dogs18 and birds19. Investigating in which extant species prosociality occurs (and in which not) and how non-human prosocial tendencies compare to human ones allows researchers to hypothesize about the evolutionary foundations of human prosociality (e.g. a potential positive effect of cooperative child-rearing on prosocial tendencies in early human societies, cf.20). However, a prerequisite is testing all species with comparable methods. Recently, two studies21,22 tested children in resource allocation paradigms comparable to those used in non-human primates15,23. They used an apparatus that had physical properties that allowed the donor to choose only one option per trial, thereby avoiding elaborate experimenter instructions. In one study21, the 1.5–5-year-old children only behaved prosocially when they had to choose between an option delivering a reward to the recipient (0/1 option) compared to the alternative where there was no reward for either the donor or the recipient (0/0 option). However, in a standard prosocial game (1/1 vs. 1/0) the same children did not take the recipient’s payoff into account. In the other study22, where the donor could decide between delivering a high-quality reward or a low-quality reward to the recipient, 7-year-old children chose to deliver the high-quality reward to the other child only when they themselves also received the high-quality reward. In this respect, their behaviour was comparable with that of adult chimpanzees tested in the same study. In contrast, 5-year-old children did not behave prosocially at all in this study22. Both studies used a side-by-side set-up, where donor and recipient were positioned next to each other and the trays with the rewards were placed in front of them. It is possible that this set-up introduced attentional problems, because the children were so focused on their own payoff that they disregarded the reward distribution on the recipient’s side21.

Nevertheless, it is also possible that children’s prosocial behaviour is not indiscriminate and depends on the characteristics of the donor, the recipient, or both24. One of the potential features to influence prosocial tendencies is the quality of the relationship between donor and recipient. Adults believe that prosocial behaviour is an important element of friendship25 and preschool children already expect more prosocial behaviour between friends than between non-friends26,27. Moreover, elementary school children are more sociable and cooperative with friends than with peers with whom they have less affectionate ties28, and similar patterns have been found among non-human animals29,30,31. Moore32 found that friendship influenced 4.5–6-year-old children’s choices in a resource allocation experiment. The children had to name a friend with whom they liked to play and a non-friend with whom they did not prefer to play at all. They shared more often with the friend than with the non-friend, with allocations to an unfamiliar child taking an intermediate level. However, these children were not tested in real interactions with their peers, and instead, drawings were used to represent the identity of the recipient. In direct peer interactions, some studies did not find a difference in prosocial donating between friends and acquaintances33, or even a reduced preference to benefit friends, if the interaction was framed in a competitive context34,35. Therefore, it is important to investigate resource allocations in the presence of a familiar peer. Interestingly, using such a choice paradigm with direct interactions, long-tailed macaques do not necessarily choose to benefit their friends, and their choices seem more influenced by status effects (i.e., dominance hierarchy)13,14.

Among children, social status can be defined as a summary measure of the degree to which a group member is liked or disliked by peers as a whole36. This popularity can be assessed by the number of peer nominations as best friend or by the number of interaction partners during free play37,38. Moreover, the higher an individual’s social status in the group, the more the other children pay attention to this individual37,38,39. A further key aspect of social status is social dominance, which has been described as the ability to acquire valuable resources, regardless of whether they are obtained by coercive behaviour or prosocial, cooperative interactions40. It is not clear if prosocial behaviour is used more by individuals with a high or with a low social status in the group. High-status individuals could use prosocial behaviour in order to consolidate their social rank (cf.13), and in turn, prosocial behaviour could lead to high status41. On the other hand, low-status individuals could use prosocial behaviour in order to avoid sanctions from dominant individuals40 (cf.14). In line with the second argument, a recent study showed that children with a low social status – determined by a previous resource competition experiment – donated more rewards to an anonymous recipient than children with high social status42.

One final aspect that has been underrepresented when investigating prosocial behaviour is a potential physiological determinant. In adult humans it has been shown that prosocial behaviour in economic games is influenced by administration of testosterone, with some studies showing testosterone to decrease prosocial behaviour (e.g.43), while others demonstrate increased prosocial tendencies after testosterone administration (e.g.44). Additionally, the effects of prenatal androgen exposure seem to independently modulate prosocial behaviour later in life45. Prenatal androgen exposure can be approximated non-invasively, by measuring the second-to-fourth digit (2D:4D) ratio46. While the early foundations of the 2D:4D approach to prenatal testosterone exposure relied heavily on correlational inference (e.g., sexually dimorphic 2D:4D ratios, cord blood measures), Zheng and Cohn47 demonstrated the developmental and molecular pathways of the association between prenatal testosterone exposure and the 2D:4D ratio in a mouse model. Moreover, several studies have found that a direct manipulation of prenatal androgen levels, or a manipulation of the binding potential of prenatal androgens with the receptors of the embryo(s) leads to differences in adult 2D:4D ratios in the predicted directions in both mice and rats48,49,50. However, it has remained controversial whether 2D:4D is mainly determined by prenatal testosterone, or by a balance of prenatal testosterone relative to prenatal estrogen46, and effects as well as effect sizes in relation to other investigated traits are currently still debated.

While the ratio increases with age at least until early adulthood, the rank order is relatively stable across the life span51,52. Importantly, the 2D:4D ratio of the right hand has been argued to show a stronger association with prenatal androgens than that of the left hand53. Individuals with high prenatal androgen exposure have a lower 2D:4D ratio, while individuals with low exposure have a relatively higher 2D:4D ratio. Although women tend to have a larger 2D:4D ratios than men, between-sex overlap and within-sex variability are usually large54. The variation in 2D:4D ratios has been closely related to variation in gender-typed appearance and behaviours within each sex (for a review see55,56). Males with higher 2D:4D ratios have a less masculine behavioural phenotype57 and less physical strength compared to those with lower 2D:4D58. Females with lower 2D:4D, in turn, score higher on male-dominated dimensions, such as spatial abilities59 and a systemizing personality60, than same-sex individuals with a higher 2D:4D ratio. Therefore, one can argue for a general masculinisation effect of prenatal androgen exposure on multiple phenotypic levels, from brain organization to appearance and behaviours.

In 5–7-year-old children, prosocial tendencies scored via a teacher questionnaire were positively correlated with their 2D:4D ratios, suggesting that children with high 2D:4D ratios (indicating lower prenatal androgen exposure) were perceived as more prosocial61. Millet and Dewitte62 found that adults with lower 2D:4D ratios (indicating higher prenatal androgen exposure) were more likely to give a fair share and less likely to give either more or less than the fair share in a public goods game, where multiple participants could contribute to a shared financial pay-off. In a further study, the authors investigated how much money the participants would donate to an anonymous recipient63. Here, participants with lower 2D:4D ratios actually donated more money. However, after exposure to aggression cues, this relationship was inverted and subjects with high 2D:4D ratios acted more prosocially63. Notwithstanding the results above, to date there are no experimental studies investigating the connection between prenatal androgen exposure and allocations in a resource allocation experiment in children.

The aim of the current study was to investigate 6–9-year-old children’s prosocial tendencies in a resource allocation paradigm that, comparable to animal studies, was constrained by the apparatus’ physical properties and not by rules established by the experimenter. To avoid attentional biases (cf.21), we used a set-up where the two children were facing each other instead of a side-by-side set-up (Fig. 1). We used a prosocial game to assess the children’s propensity to choose the 1/1 option (i.e., one reward item on their side and one reward item on the recipient’s side) versus the 1/0 option (i.e., one reward item on their side, but no reward item on the recipient’s side) in the prosocial test condition when a recipient was present compared to a non-social control condition when they were tested alone. The experiment started with a warm-up phase, where the donor children could learn how to operate the apparatus and understand the consequences of their choices (cf.21,22). Each donor child was then paired with one familiar, same-sex recipient from their classroom and received 10 consecutive trials in both the test and the non-social control (sequence of conditions counterbalanced across children). Further, we tested whether friendship, social status and prenatal androgen exposure (approximated by their 2D:4D ratio) influenced children’s prosocial behaviour in this task. Friendship was assessed by questionnaires given to the children and the classroom teachers. Half of the children were paired with a recipient that they indicated as a friend, the other half was paired with a recipient that they had not indicated as a friend. Social status was assessed live during free play observations, where we scored each child’s average number of interaction partners and number of peers paying attention to the child. Additionally, classroom teachers were asked to score the children’s perceived social dominance. Digit ratios were measured from hand scans made prior to the behavioural experiment. We tested 48 children from 4 different classrooms. Three children failed the warm-up in the resource allocation experiment and were excluded from data analysis. Of the 45 remaining children, seven children were missing data from one other part of the study (see methods section for details). In the results section, we report analyses on the maximum sample size per variable (see also Table 1). We also analysed the reduced sample from only those children with a complete data set and the results are fully equivalent to the maximum sample (see Supplementary Results file).

Figure 1 (a) Drawing of the experimental set-up of the prosocial test condition. The apparatus is placed on the tables that serve as a barrier between the two sides. The donor child is on the right side. By putting a coin in one of the small coin receptacles on his side, the donor can open the connected larger reward box. The illustration shows the apparatus after the donor chose the right reward box, whose lid is open. Illustration by Nadja Kavcik-Graumann. (b) Schematic bird’s eye view of the experimental apparatus. The hexagons represent the reward items (i.e., stickers). The 1/1 option is on the left side and the 1/0 option is on the right side. Full size image

Table 1 Sample size and descriptive statistics (mean, standard deviation, median, minimum, maximum) on all variables, split by sex and for the total sample. Full size table

We predicted that 6–9-year-old children would choose the prosocial 1/1 option more when a peer was present as a recipient than when tested alone. Further, we predicted that the children would share more when they were paired with a friend than when they were paired with a non-friend32. Regarding the connection between social status and prosocial tendencies, we hypothesized that children with a lower social status would behave more prosocially42. If the effects of prenatal testosterone exposure (assessed via 2D:4D) modulate the children’s choices, two competing hypotheses can be phrased in the light of the existing literature. The first line of argument is based on the overall masculinising effect with increasing prenatal testosterone exposure together with sex-typical behavioural tendencies55,56. Feminine evolutionary strategies are characterized by increased sharing, an earlier onset of theory of mind, more empathy and more donations64. Thus, it is expected that a lower prenatal testosterone exposure (higher 2D:4D) is associated with relatively more 1/1 choices. For the masculine strategy, it could be speculated that striving for exclusive ownership of a limited, valued good is beneficial in male-male competition and in terms of female choice. Therefore, lower 2D:4D ratios would be related with more 1/0 decisions. Both the outlined feminine and masculine strategies are reflected in a positive correlation between 2D:4D and the amount of 1/1 choices. A second, competing, line of argument is derived from the findings that higher prenatal testosterone exposure (lower 2D:4D) has been related to a stronger preference for fair contributions in adults62. In this regard, the 1/1 choice (as compared to the 1/0 alternative) represents the fair choice. If fairness rather than prosocial tendencies drive the decision process, one would expect a higher amount of 1/1 choices in children with lower digit ratios, i.e. a negative correlation between 2D:4D and the amount of 1/1 choices.