Many vertebrates breed in cooperative groups in which more than two members provide care for young. Studies of cooperative breeding behavior within species have long highlighted the importance of environmental factors in mediating the paradox of why some such individuals delay independent breeding to help raise the offspring of others. In contrast, studies involving comparisons among species have not shown a similarly clear evolutionary-scale relationship between the inter-specific incidence of cooperative breeding and any environmental factors. Here, we use a phylogenetically-controlled comparative analysis of a complete, socially diverse group of birds—45 species of African starlings—to show that cooperative breeding is positively associated with living in semi-arid savanna habitats and with temporal variability in rainfall. Savanna habitats are not only highly seasonal, but also temporally variable and unpredictable, and this temporal variability directly influences individual reproductive decisions in starlings and explains inter-specific patterns of sociality. Cooperative breeding is likely to be adaptive in temporally variable environments because it allows both for reproduction in harsh years and for sustained breeding during benign years. This ‘temporal variability' hypothesis is likely to have high explanatory power for the phylogenetic and geographic concentrations of cooperatively breeding species in savanna-like habitats and other temporally variable environments worldwide.