This paper synthesises the state of knowledge on the economic effects of density. We consider 15 outcome categories and 347 estimates of density elasticities from 180 studies. More than 100 of these estimates have not been previously published and have been provided by authors on request or inferred from published results in auxiliary analyses. We contribute own estimates of density elasticities of 16 distinct outcome variables that belong to categories where the evidence base is thin, inconsistent or non-existent. Along with a critical discussion of the quality and the quantity of the evidence base we present a set of recommended elasticities. Applying them to a scenario that roughly corresponds to an average high-income city, we find that in per-capita present value terms (at a 5% discount rate), a 1%-increase in density implies an increase in wage

Most developed countries now pursue policies that implicitly or explicitly aim at promoting compact urban form. This report analyses more than 300 academic papers that study the effects of compact urban form, and finds that 69% of the papers reviewed find positive effects associated with compact urban form. Over 70% of studies find positive effects of economic density (the number of people living or working in an area). A smaller majority of studies attribute positive effects to mixed land use (58%) and the density of the built environment (56%). These averages hide significant variation across specific dimensions of urban development. In order to understand the effects of compact urban form, the report estimates the monetary per capita value of the change in 15 outcomes in response to a 10% change in economic density. The major benefits of economic density arise from improved productivity and better access to jobs and services. Further benefits are generated through the preservation of urban green space, greater energy efficiency, pollution reduction and safer urban environments. The major costs of higher economic density are related to congestion, health and well-being. Increasing compactness can also contribute to higher land values and housing costs, which are borne disproportionately by renters and first-time buyers. Increasing economic density therefore requires accompanying policy interventions to maximise the benefits and minimise the costs associated with compactness. In particular, policymakers need to facilitate large-scale investment in housing supply and public transport networks to ensure efficient and equitable access to housing, services and jobs in compact cities.

This article argues for a citizen social science methodology in which residents from the sites of inquiry play a central role in key activities of the research process and beyond: research design and data collection, presentation and publication of findings, and design and implementation of urban interventions that address challenges to quality of life. This is a way of democratising the research process through sustained engagement with communities and an emphasis on co-designing pathways to impact. The article draws on the authors' experience of running a citizen social science project in Beirut, Lebanon, where citizen scientists, university academics, and NGOs have worked collaboratively to understand what prosperity means for local residents, develop context-specific measures of prosperity, and design and implement small-scale interventions for local challenges.

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