Ed Yong in The Atlantic:
For decades, psychologists have argued that the answers depend on how old you are, and whether you're the one with the bigger or smaller share. Adults seem to reject inequality of any form, and will pay a personal cost to avoid it even if they stand to get a bigger slice of the pie. Children are more nuanced.
In 2011, Katherine McAuliffe and Peter Blake showed that 8-year-olds, like adults, will reject any unequal offer. But younger children, aged 4 to 7, only bristle at situations when they are disadvantaged. In other words, they'd take the four pieces of candy, thank you very much, and screw the other kid.
“They start out with this very self-focused idea that they recognize unfairness when it’s unfair to me,” says Blake. “It takes more years for different psychological processes to kick in before they can flip that, and say: What's unfair to you is also unfair in general.”
These and other experiments have shown that our aversion to advantageous inequity (when we get more than others) is distinct from our aversion to disadvantageous inequity (when others get more than us). These two reactions involve different parts of the brain. They appear at different ages. They appear in different species: Chimpanzees and capuchins don't like disadvantageous inequity, but they'll tolerate the advantageous kind just as much as 4-year-old humans.
Now, McAuliffe and Blake have found that this distinction also depends on where we come from.
More here.