Should policymakers care about inequality? Inequality has been linked to a range of negative societal outcomes, affecting life expectancy and mental health, social cohesion, violence and social mobility, with psychosocial factors (including stress) acting as the mechanism. However, the evidence is far from conclusive. In health, many studies find no association with health outcomes. Thus, whether policymakers should focus on reducing poverty or inequality remains an open question. This study contributes to the debate on whether economic inequality is statistically associated with health outcomes independent of poverty, and whether this effect is substantively relevant. We exploit the fact that the social gradient in cancer mortality differs by cancer type. “Extrinsic” cancers, those linked to health behaviours (smoking, etc) such as lung cancer, show a significantly larger social gradient than “intrinsic” cancers, those caused by bad luck and genetic predisposition, such as breast cancer. Intrinsic cancers, therefore, serve as placebo test for the effect of socioeconomic factors on cancer mortality operating via health behaviours. We show that economic inequality is correlated with cancer mortality for the most extrinsic cancers - lung, liver, and oesophagus – but not for intrinsic cancers; and that the predicted effect of economic inequality is lower than that of average household income for all cancers (except liver). Thus, we conclude that economic inequality affects cancer mortality via its effect on health behaviours. As such, policymakers should focus on extrinsic cancers, with the greatest impact achieved by focusing on poverty reduction and the link to adverse health behaviours.