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Aim: Species geographic ranges are the 'fundamental units' of macroecology. Range size is a major correlate of extinction risk in many groups, and is also critical in studies of biotic responses to climate change. Despite this, there is a lack of studies exploring the role of environmental, historical and anthropogenic processes in determining large-scale patterns in range size. We perform the first global analysis of putative drivers of range size variation in any group, choosing amphibians as our study taxon. Our aims are to disentangle the many hypothesized causes of range size variation and evaluate support for 'Rapoport's rule', the observation that range size correlates with latitude. Location: Global. Methods: We develop a global map of gridded median range size using the International Union for Conservation of Nature (IUCN) distribution maps. From this we perform spatial and non-spatial regressions to explore relationships between range size and nine hypothesized variables in six biogeographic realms. We use information-theoretic model selection to compare multiple competing variables, simultaneously evaluating the relative support for each one. Results: Current climate – environmental water and energy, and temperature seasonality – is consistently highly ranked in spatial and non-spatial analyses. Human impacts and other environmental measures (topographic and landscape complexity, effective area, climate extremes) show mixed support, and glacial history is consistently unimportant. Our findings add further evidence to the view that Rapoport's rule is a regional, not global, phenomenon. Main conclusions: The primary importance of temperature seasonality may explain why Rapoport's rule is largely restricted to northern latitudes, as this is where se asonality is most pronounced. More generally, the dominance of contemporary climate in our analyses (even when accounting for space) has stark implications for the future status of amphibians. Changes in climate will almost certainly interact with the anthropogenic processes already threatening a third of amphibians globally, with the effects being most keenly felt by species with a restricted range.