The First Inequality

As far as hot button issues go, income inequality is certainly having a moment. A worldwide, popular movement was spurred on by it, several Nobel-prized economists have damned it in no uncertain terms, Barack Obama made it a pillar of his presidency (though, on this, he may have begun to waver), and The New York Times has devoted a regular series to it.

This is all with good reason. Although the data continue to roll in, we have growing confidence that income inequality erodes trust, diminishes health, undermines democracy, and damages the economy, among many other things. Which raises an important question: Why would inequality have such corrosive effects in the first place?

The answer might seem straightforward at first. Isn’t it inherently obvious that we should want to have as much as everyone else? Well, no. Theoretically, we could instead be happier for others to have more than we do, rather than find this cause for upset. Moreover, what would we prefer to do if we had more than everyone else — keep what we had and maintain the inequality or give up our surplus so that everyone can be equally well off? We may be inclined to believe we’re the sorts to give until we can give no more, but the evidence suggests otherwise. So, if our concerns about inequality run in certain directions, we need an explanation for them. Below, I’ll supply such an account and, over this and the next few blog posts, offer some important illustrations of it.


The Evolution of Inequality

Evolutionary theory tidily explains the problem of inequality in the form of reproductive variance. If some individuals produce more offspring than others, then there will be some variance (or dispersion, or spread) in reproduction. Take the figure below, for example, representing the reproduction of two different groups or populations over subsequent generations. Although individuals of both groups produce the same average of three offspring per generation, those from the blue group are more likely than those from the red group to produce anywhere between zero and two offspring and anywhere between four and six offspring; they’re also less likely to produce exactly three offspring. In other words, reproduction is more spread out among members of the blue group than among members of the red group.


Reproductive variance amplifies the effects of competition among individuals, because a small advantage or disadvantage will have a larger effect on reproduction in variable groups. That is, it will push reproduction to the extreme ends of the distribution. All puns aside, individuals in the blue group get more “bang” for their competitive “buck” than do individuals in the red group.

So, genes that produce costly traits — ones, say, that make their bearers take greater risks (like fighting off rivals) or invest in expensive signals (like colorful plumage) — will be more likely to permeate through the population when reproduction is more rather than less variable. This is because their benefits increase with reproductive variance. Eventually, everyone will be more pugnacious or colorful than in previous generations.

Reproductive variance was the first inequality, and it holds court over all others. Indeed, we should have evolved to care about any other form of inequality, like differences in the ability to obtain food or shelter, only insofar as they have left their marks on the relative number of offspring that we produced over time. This doesn’t mean, of course, that organisms (humans included) should fuss over reproductive inequality alone, or even at all. Rather, they are expected to act as if they care about those inequalities that would have predicted success in reproductive competition in the past — things like inequality of access to sexual partners or resources.

The traces of inequality are everywhere. We find them in the reliable differences between the sexes in terms of size, armaments, and ornaments in species where the reproductive variance of one sex (usually males) is larger than that of the other sex (usually females). We find them in the shape of our social organization. And, according to compelling work that has been accumulating over the years, we find them in our willingness to kill one another.


To The Bitter End

Martin Daly and the late Margo Wilson have built a strong case that fatal violence is subject to the same stresses of inequality as most everything else. In their first study of the problem, they analyzed the association between income inequality and homicide rates in the 77 neighborhoods of Chicago. They found that inequality was a strong, positive predictor of homicide, and this association was independent of the only other significant predictor in their study (male life expectancy at birth).

In a second study, Daly, Wilson, and Shawn Vasdev tackled a larger issue: whether income inequality better predicts patterns of homicide than income alone. This is an important problem, because base income and income inequality may themselves be correlated; as income inequality rises, base incomes might shrink. If so, then what appears to be an effect of inequality might instead be an effect of poverty. This time, the researchers studied the association between income inequality, average (i.e. median) household income, and homicide rates across the 50 US states and the 10 Canadian provinces. What they found was striking.

Canada has a homicide rate that is about one quarter the size of the US rate and, unlike the US, income inequality in Canada is positively associated with household incomes — that is, the more unequal, the wealthier the province. When the researchers analyzed the data from the two countries together, they found again that income inequality, but not average income, strongly predicted homicide rates, as shown in the figure below. In fact, they found that the association between inequality and homicide in Canada (black circles) was almost identical to the association between inequality and homicide in the US (white circles)! This significantly weakens the popular argument that what separates Americans from Canadians in their proclivities for violence is simply a different cultural trajectory — the cultures of fear and honor being two obvious examples.

Finally, in a third study, Daly and Wilson demonstrated this again by contrasting the effects of inequality on homicide in the US northern and southern states, known for large differences in their homicide rates. As before, they found that income inequality significantly predicted homicide rates whereas average household income did not. Moreover, the distinction between the southern and northern states vanished once inequality was taken into account.

Homicide is just one glaring example of how deep the roots of inequality run. Stick around for more.


Cover image by Timothy Krause.