Identification of the social and cognitive processes underlying human cumulative culture

Abstract

The remarkable ecological and demographic success of humanity is largely attributed to our capacity for cumulative culture, with knowledge and technology accumulating over time, yet the social and cognitive capabilities that have enabled cumulative culture remain unclear. In a comparative study of sequential problem solving, we provided groups of capuchin monkeys, chimpanzees, and children with an experimental puzzlebox that could be solved in three stages to retrieve rewards of increasing desirability. The success of the children, but not of the chimpanzees or capuchins, in reaching higher-level solutions was strongly associated with a package of sociocognitive processes-including teaching through verbal instruction, imitation, and prosociality-that were observed only in the children and covaried with performance.

Fig. 1

(A). The cumulative culture puzzlebox, which could be solved at three sequential stages, each building on the preceding stage. (B) Illustration of puzzlebox use. Stage 1 required individuals to push a door in the horizontal plane to reveal a chute through which a low-grade reward was delivered. Stage 2 required individuals to depress a button and slide the door further to reveal a second chute for a medium-grade reward. Stage 3 required the solver to rotate a dial, releasing the door to slide still further to reveal a third chute containing a high-grade reward. All stages could be completed through two parallel options (alternative doors could be slid left or right at stage 1, alternative buttons at the top or bottom could be depressed at stage 2, and alternative colored finger holes enabled rotation of the dial at stage 3), with sets of three chutes on both left and right sides. This two-action, two-option design aided evaluation of alternative social learning mechanisms and allowed two individuals to operate the puzzlebox simultaneously. Replenishment of the chutes by the experimenter allowed the apparatus to be continuously used for long periods. Pilot work established an unambiguous ascendancy in the desirability of reward with stage (food stage 1 = carrot, 2 = apple, 3 = grapes for chimpanzees and capuchins; stickers of increasing size and attractiveness for children).

Fig. 2

(A) Attainment of stages 2 and 3 was exceptionally rare or absent in capuchins and chimpanzees but common in human children. (B) We observed 23 unambiguous instances of teaching, by direct instruction, exclusively in children. (C) In neither chimpanzees nor capuchins was there greater recruitment to the task after, versus before, a food call. Conversely, children who received verbal instruction outperformed those who did not. (D) Children alone performed more matching than nonmatching manipulations, and they produced a greater proportion of matching actions than did either chimpanzees or capuchins. (E) We observed 215 altruistic events (giving an extracted reward to others), exclusively in children. (F) There was no evidence that scrounging hindered performance in any species; children who were victims of scrounging outperformed children who were not. (G) Dominant children and chimpanzees did not monopolize the task, and high-ranking capuchins monopolized the puzzlebox in 2007 but not 2008. (H) Low-rankers did not receive less attention than high-rankers when manipulating the task. (I) In the open condition, where they received rewards at all stages, neither chimpanzees nor children manipulated the puzzlebox less than individuals in the scaffolded condition. In (G) and (H), capuchins (2007 and 2008 pale and dark yellow, respectively) and chimpanzees were split into three (high-, mid-, low-) rank categories, although for clarity and comparability with the child data, we present only analysis of high- versus low-ranked individuals. **P ≤ 0.05; ***P ≤ 0.01; NS, not significant.