The above map is from a paper by Kimuli Kasara and Pavithra Suryanarayan (pdf), “When do the Rich Vote Less than the Poor and Why? Explaining Turnout Inequality across the World.” The abstract:
The conventional wisdom that the poor are less likely than the rich to vote is based upon research on voting behavior in advanced industrialized countries. However, in many places the relationship between turnout and socioeconomic status is reversed. We argue that the potential tax exposure of the rich explains the positive relationship between income and voting in some places and not others. Where the potential tax exposure of the rich is high, they have more of an incentive to participate in politics. Therefore, where states have the capacity to tax the rich, politicians use fiscal policy to gain support. Using survey data from a wide range of developed and developing countries, we demonstrate that the rich turn out to vote at higher rates than the poor where the state has the bureaucratic capacity to tax the rich and where the political preferences of the rich and poor diverge.
Joshua Tucker points out “the role of the US – and, interestingly, Poland and Zambia – as outliers in terms of having over-representation of the well-off citizens among voters.”