Cognitive performance is linked to group size and affects fitness in Australian magpies

The Social Intelligence Hypothesis argues that the demands of social life drive cognitive evolution1–3. This idea receives support from comparative studies linking variation in group size or mating systems with cognitive and neuroanatomical differences across species3–7, but findings are contradictory and contentious8–10. To understand the cognitive consequences of sociality it is also important to investigate social variation within species. Here we show that in wild, cooperatively breeding Australian magpies, individuals living in larger groups show elevated cognitive performance, which in turn is linked to increased reproductive success. Individual performance was highly correlated across four cognitive tasks, hinting towards a “general intelligence factor” underlying cognitive performance. Repeated cognitive testing of juveniles at different ages showed that the group size – cognition correlation emerged in early life, suggesting that living in larger groups promotes cognitive development. Furthermore, we found a positive association between female task performance and three indicators of reproductive success, thus identifying a selective benefit of greater cognitive performance. Together, these results provide critical intraspecific evidence that sociality can shape cognitive development and evolution.