In social groups, dominant mammals generally have the easiest access to resources that garner survival and reproductive success. Psychologist Sheri Johnson of the University of California, Berkeley describes that humans and other mammals respond accordingly to their place in these social hierarchies.

"Many of us are used to thinking about hierarchy in the animal world. Wolf packs have a hierarchy of who’s the chief wolf pack leader and on down from there. But in human context, we also have a hierarchy, and where you are in that hierarchy may even shift across different contexts. You might be the high-ranking person in one area of your life, like you might be captain of your sports team, but then you go to work, and maybe you’re not captain there."

Johnson says that feeling subordinated in these hierarchies is linked to poor mental health. Recent studies have found that people living in developed countries with the highest income inequality are three times as likely to develop depression.

"There are tons of studies out there showing that poverty is very bad for mental health."