In his State of the Union Address last night, President Obama made a big deal about the huge income inequality among Americans, with much of the wealth in the hands of a few while manylive in poverty. Although we’re a relatively wealthy nation in terms of gross domestic product per capital, we’re also one of the most unequal in the world. This inequality has been increasing in the U.S. for several decades.

It’s not often realized that, regardless of per-capita wealth, income inequality is correlated with a number of indices of social dysfunctionality.

You can see this relationship graphically (in both senses) at Sociological Images, in a post called “Income inequality is bad for society. Really bad.“. Here are a few correlations among nations between income inequality and indices of social dysfunction (these are apparently taken from Richard Wilkinson and Kate Pickett‘s book, The Spirit Level: Why More Equal Societies Almost Always Do Better). Note the extreme position of the United States.

Infant mortality (I’m not sure what statistic they use to measure income inequality; it may well be the “Gini Index”):

Homicide rate:

Rate of incarceration:

Child well being (UNICEF measure):

Now of course this doesn’t imply that income inequality is the cause of all the dysfunctionality, for there could be other variable that affect dysfunctionality and these other measures as well. Insecurity could be such a factor. Nevertheless, it’s a good working hypothesis that this kind of inequality breeds not only social unrest, which leads to crime, drug use, incarceration, and the like, but also bespeaks a lack of caring for the welfare of the poor, leading to things like high infant mortality. The Equality Trust goes into these figures in detail, explaining the alternative hypotheses.

And then there’s another index of what most of us would see as dysfunctionality: the degree of religious belief. The work of Solt et al., which I’ve discussed before, shows that income inequality is also highly correlated among nations with religiosity: the more unequal a nation, the more religious its inhabitants. Here’s a figure showing the positive correlation between income inequality (measured using the Gini index) and various measures of religiosity, taken from Solt et al.. Each dot is one country (see the link above for further information). The work of Tomas Rees leads to the same conclusion.

Here the pathway of causation is clearer: as I wrote before:

[As Solt et al. note], “Increases in inequality in one year predict substantial gains in religiosity in the next,” while “past values of religiosity do not predict future values of inequality.” In other words, the correlation between religiosity and inequality is driven by the former responding to the latter, and not the other way around. Unequal incomes lead to societies becoming more religious.

Finally, we know that religiosity is highly correlated among nations with acceptance of evolution: the less religious nations have higher acceptance of that theory. Here’s a graph I made of that correlation among 34 countries, with data taken from Miller and Scott (2006), the Eurobarometer surveys, and a few other places. Note again the bad position of the US: next to the bottom in accepting evolution (the most resistant nation is Turkey).

As for the causes of this, I think it’s more likely that the inherent religiosity of a nation affects its inhabitant’s acceptance of evolution rather than reverse theory: acceptance of evolution makes a country more atheistic. I favor the former idea because people imbibe religion earlier in their lives than they learn evolution (if they learn it at all). ]

While this suggests that the way to make evolution more acceptable is to weaken the grasp of religion on the world, doing that may require larger structural changes in society, for the work cited above suggests that religiosity is itself a byproduct of social dysfunction, which itself may result from a grossly unequal distribution of incomes (see also the work of Greg Paul that I’ve discussed several times).

But of course the acceptance of evolution is a matter far less pressing than issues like homicide, drug use, and child mortality. If we can reduce much of that dysfunctionality—and religiosity—by creating a more just and equal society, I predict that the acceptance of evolution will increase as well. One could regard of acceptance of evolution itself as one sign of a healthy society. We can start by eliminating the tax loopholes that enable the very wealthy to pay far fewer taxes than they should (viz. Mitt Romney).

h/t: Matthew Cobb via Ed Yong