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Urban agglomeration economies make cities central to theories of modern economic growth. There is historical evidence for the presence of Smithian growth and agglomeration effects in English towns ...

We thank Tosh et al. (1) for their interest in our research (2) but note that their analyses do not undermine the main findings of our article. Their suggestion that polity population divided by polity area should be one of the social complexity dimensions raises a number of issues. What does this ratio mean at large spatial scales, where populations are concentrated in large urban centers and much of the territory is not heavily populated? How are societies distributed across this variable and why? For example, a small-scale “simple” society could have a very high population density if it has access to a rich resource base. Tosh et al. (1) do not provide sufficient information or context to meaningfully … [↵][1]1To whom correspondence should be addressed. Email: t.currie{at}exeter.ac.uk. [1]: #xref-corresp-1-1

Agricultural productivity, and its variation in space and time, plays a fundamental role in many theories of human social evolution. However, we often lack systematic information about the productivity of past agricultural systems on a large enough scale in order to be able to test these theories properly. The effect of climate on crop yields has received a great deal of attention resulting in a range of empirical and process-based models, yet the focus has primarily been on current or future conditions. In this paper, we argue for a “bottom-up” approach that estimates productivity, or potential productivity based on information about the agricultural practices and technologies used in past societies. Of key theoretical interest is using this information to estimate the carrying capacity of a given region, independently of estimates of population size. We outline how explicit crop yield models can be combined with high quality historical and archaeological information about past societies, in order to infer the temporal and geographic patterns of change in agricultural productivity and potential. We discuss the kinds of information we need to collect about agricultural techniques and practices in the past, and introduce a new databank initiative we have developed for collating the best available historical and archaeological evidence. A key benefit of our approach lies in making explicit the steps in the process of estimating past productivities and carrying capacities, and in being able to assess the effects of different modelling assumptions. This is undoubtedly an ambitious task, yet promises to provide important insights into fundamental aspects of past societies, and will enable us to test more rigorously key hypotheses about human socio-cultural evolution.