You know you're supposed to act differently around your boss than you do around your colleagues, and if you've ever watched a group of dogs interact, you'll see that they have a social order, too.
There's a "top dog" that's in charge, and the others know their place.
Monkeys and other animals have rules about who does what, too. It's called the social hierarchy. And it has a huge influence on us -- on how we act, whom we spend time with, even where we go and what we buy.
Now, researchers have found evidence that our brains may actually be hard-wired for hierarchy. And in fact, we may be wired to value the "top dog" over the people who rank below us.
To understand how our brains process social ranking, Caroline Zink, a neuroscientist at the National Institute of Mental Health, and her colleagues set out to capture images of the brain while having volunteers play a simple computer game.
The researchers looked for differences in brain activity after volunteers were told they were "superior" or "inferior" to others playing at the same time.
"I actually did the analysis twice because I couldn't believe how strong these differences were," Zink said. "It was striking."
The study, published today in the medical journal Neuron, involved placing volunteers in a special MRI scanner. As each person played the computer game, the researchers were able to see what parts of the brain were most in use. They told the volunteers that someone else would be playing the game at the same time and that the other person was either more- or less-skilled.
The researchers made it clear this was not a competition: Players earned a dollar for each correct response, no matter what the other person did. Although the volunteers were shown a photo of the other player, they were really just playing against a computer.
While some parts of the brain always "lit up" no matter who the volunteers thought was playing, an additional pattern emerged when the players thought their opponent ranked higher than they did.
One area that became more active in volunteers facing a higher-ranked player was the ventral striatum, a part of the brain that responds to money and rewards. Essentially, it recognizes things that are of value to us.
The brain activity suggests that players not only placed more value on the superior player, but even cared more about that person.
Another active area in the same scenario was the occipital parietal cortex, which processes vision and attention.
"We pay more attention to the superior person than the inferior person," Zink explained. "We see that in primates, too. They spend more time looking at the dominant monkey than at the inferior or submissive one."
Volunteers' comments also showed they were paying more attention to the higher-ranking player, as they would comment on that person's appearance, even remembering their hair color.
Conversely, the volunteers didn't say much about the lower-ranking players.
Because social rank is so important, you might think our brains would spend a lot of effort figuring out where people stand. And in fact, although the brain doesn't exactly create social status, certain regions do seemed to be wired to interpret it.
A part of the brain called the parahippocampal gyrus is involved in this social sorting. It helps us do something called "contextual processing" -- basically, figuring out where something or someone fits into our world. Another brain region, the dorsolateral prefrontal cortex, also helps us interpret rank.
In another version of the experiment, the researchers made it possible for players to move up or down in the rankings. When people thought they could gain higher status, new areas of the brain came into play.
Some of these areas are involved in preparing for movement. Even though the volunteers weren't really going to get up and do anything, the brain seemed to be getting ready for that climb up the social ladder.
One way to think about it, said Zink, is that "they prepare more, show more motivation, based on these activations. It's sort of, 'I'm going to beat you.'"
In an attempt to find out whether the study results were due more to competitive nature than an evaluation of social rankings, researchers set up a third experiment.
This time people knew the other players weren't real. They still were given rankings, but they knew they were playing against a computer. It turned out that some parts of the brain tuned in only when volunteers were comparing themselves to other humans.
Dr. Andreas Meyer-Lindenberg, one of the study's authors, pointed out that humans aren't the only animals who pay attention to social status.
Social rankings probably "existed long before consciousness or humankind, and I think they are embedded in brain and the architecture that shapes it," Meyer-Lindenberg said in an e-mail. That doesn't mean brain circuits cause those social rankings, but that we're built to recognize and act on them.
"Social hierarchies change," said Dr. David Amodio, an assistant professor of psychology at New York University who did not take part in the research. "This work shows how different mental systems come online to support the different challenges and goals associated with superior or inferior members of the group."
Zink hopes the study will help lay the groundwork for future research.
"We've all seen someone act inappropriately," she said, referring to how others might misbehave at work, with friends or at school. "Hierarchies are so important to every decision we make and all behaviors. And this is the first time we've been able to look at how the brain processes this information."
Some illnesses, such as autism, schizophrenia and depression, include misinterpreting or failing to recognize where we "fit" in our social world.
Zink said she hopes knowing how the brain processes social ranking will help shed light on these and other conditions.