Isn't that inequality? It sounds like if it's equal (both get grapes or both get cucumbers) everything is fine.

The study is implying that people/monkeys get agitated only when people/monkeys of their own perceived level/rank/group do better.

For example, you and your brother aren't be agitated because your dad has all the money. That inequality doesn't bother you. But if your dad gave you $10 and your brother $100, I'm sure you'd get upset.

Or if your company's payroll was leaked, you aren't upset that the founder of the company is making so much than you. But if you found out your fellow co-workers made more than you, you'd be pissed.

So the study is implying the inequality issue is more about people seeing other people their own level getting ahead rather than seeing the uber wealthy getting richer. I believe there are other studies in psychology with back this up. But whether you agree with the studies implications is up to you.

Another interesting note is that the people who are most upset at zuckerburg and the new tech billionaires are the elite wealthy whom they surpassed in wealth.

Well, it's still inequality. It's just not ine-quantity.

Each gets something different -- whether we perceive them as equal market value is not probably relevant to the monkey.