Robin Hanson has wondered why folks seem concerned about inequality based on some stuff, like race, gender, sexuality, ethnicity, and disability, but not other stuff, like height, appearance, intelligence, sleep, conscientiousness, and perhaps most importantly, happiness.

My explanation: Race, gender, sexuality, ethnicity, and disability are fairly discrete ways of classifying people. Most people (though not all) can be categorized fairly neatly in to a single race, birth gender, desired gender, and sexual orientation. By contrast, looks, smarts, and happiness all vary in a continuous fashion. For some/all of these characteristics, there's a bell curve--many people in the middle and fewer people at the extremes.

How would wealth or income inequality relate to this? Wealth and income are clearly not a discrete group, but a large number of people are very concerned about wealth and income inequality.