Theories of why income inequality correlates with violence suggest that inequality erodes social capital and trust, or inhibits investment into public services and infrastructure. Past research sensed the importance of these causal paths but few have examined them using tests of statistical mediation. We explored links between income inequality and rates of homicide in 33 countries and then tested whether this association is mediated by an indicator of social capital (interpersonal trust) or by public spending on health and education. Survey data on trust were collected from 48 641 adults and matched to country data on per capita income, income inequality, public expenditures on health and education and rate of homicides. Between countries, income inequality correlated with trust (r = -0.64) and homicide (r = 0.80) but not with public expenditures. Trust also correlated with homicides (r = -0.58) and partly mediated the association between income inequality and homicide, whilst public expenditures did not. Multilevel analysis showed that income inequality related to less trust after differences in per capita income and sample characteristics were taken into account. Results were consistent with psychosocial explanations of links between income inequality and homicide; however, the causal relationship between inequality, trust and homicide remains unclear given the cross-sectional design of this study. Societies with large income differences and low levels of trust may lack the social capacity to create safe communities.