Richard Wilkinson, Kate Pickett

… differences in average income or living standards between whole populations or countries don’t matter at all, but income differences within those same populations matter very much indeed … what matters in rich countries may not be your actual income level and living standard, but how you compare with other people in the same society. Perhaps average standards don’t matter and what does is simply whether you are doing better or worse than other people …
… men have an incentive to achieve as high a status as possible because their sexual competitiveness depends on it … men use violence when their status is threatened, and more so when there is little status to defend. … The association between inequality and violence is strong and consistent. The evolutionary importance of shame and humiliation provides a plausible explanation of why more unequal societies suffer more violence.