Income and the vote in Mexico

I just came across a really interesting analysis by Jeronimo Cortina and Andrew Gelman of income and voting choice in Mexico. In a paper (which I skimmed, but will read later), the key graph of which is posted at the blog, Statistical Modelling, Causal Inference, and Social Science, Cortina and Gelman analyze the relationship between income and the vote in each state and the DF.

The key finding, from the 2000 presidential election, is that:

The slopes are higher–that is, income is a stronger predictor of the vote–in poor states.

This is similar to a relationship found in the USA by Gelman, et al.:

One difference between the two countries is that in the U.S., the conservative party does better in the poor states, but in Mexico, the conservative party does better in the rich states. But at the level of individual voting, the patterns in the two countries seems similar.

In carrying out their analysis, the authors use a left-right scale that places the PRD at the left, PRI in what they call the “blurry center,” and PAN at the right. Nothing at all wrong with that ideological description, but is this the expected relationship of income level to the vote? I would think not: the poorest Mexicans probably have the strongest tendency to vote PRI, with the PRD strongest in the lower-middle classes (at least in 2006). In fact, the paper contains a set of graphs of the percent vote per state, arrayed by state income. The PRD shows hardly any relationship, while there is a negative relationship for the PRI (and the unsurprising rise for the PAN). I should emphasize that the paper (and the graph at the blog) analyzes individual data in each state (in a multilevel model); whether the ordering of the parties under the assumption of poor–>PRD/ middle–>PRI/ wealthy–>PAN matters to their results is not clear to me (at least until such time as I have actually read the entire paper).

Cortina and Gelman note that they hope to replicate their study when (if!) 2006 exit-poll data are made public. Believe me, they are not the only ones waiting to get their hands on those data!

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