Last week, in an interview with the New York Times Magazine "Play," current Yankee director Hank Steinbrenner had this to say about the Boston Red Sox’s recent (they’ve won the World Series twice in four years) rise to national prominence:

“Red Sox Nation? What a bunch of [expletive] that is. That was a creation of the Red Sox and ESPN, which is filled with Red Sox fans. Go anywhere in America and you won’t see Red Sox hats and jackets, you’ll see Yankee hats and jackets. This is a Yankee country. We’re going to out the Yankees back on top and restore the universe to order.”

I’ve spent much of this past week trying to figure out what “expletive” was so horrible that the NY Times felt the need to remove it and have decided to go with “bull-pucky,” but that’s besides the point.

The point is that there’s a ton of interesting evolutionary psychology wound around Steinbrenner’s statement and the rivalry between modern sports teams.

“Humans come with an innate mechanism for dividing themselves into us versus them groups,” says Gad Saad, author of The Evolutionary Bases of Consumption and an evolutionary behavioral scientist at the John Molson School of Business in Montreal. “We fight and vie for our home team, just like we used to fight and vie for our clan.”

David Goetze, an Associate Professor of Political Science at Utah State University and Executive Director of Politics and Life Sciences (a group which studies the intersection of evolution and social behavior) explains further, “just like hunter-gather bands, sports teams are essentially small, tight groups competing for resources."

At the core of this is the idea of inclusive fitness: that humans enhance the spread of their genes by being nice to their close relatives. In recent years, evolutionary psychologists have extended the inclusive fitness argument slightly, claiming that our hunter-gatherer roots (where we lived in groups of 50-200) require bonding with an extended surrogate family.

Why this same type of bonding mechanism extends to the players from fans was first explained by gossip researcher Robin Dunbar. Dunbar was looking into the evolution of social networks and realized that our brains come equipped with roughly 150 slots for “friends.” These slots correspond to the average size of hunter-gather bands. Unfortunately, in the modern world, most of us don’t have 150 friends so we fill up those slots with the most familiar faces available: our favorite TV characters and our sports heroes.

“It makes sense,” says Saad. “We turn on the TV and ‘invite’ these people into our living rooms every night—as far as our brain is concerned they're just extended family.”

Scientists have also found that these sporting attachments actually influence hormonal levels. In the 1994 World Cup final, researchers found that levels jumped 27 percent in ‘winning’ Brazilian fans and dropped on average by the same amount in most of the ‘losing’ Italian fans.

Which also explains why researchers studying a phenomena known as ‘basking in reflected glory’ found that the day after their team wins, students are significantly more likely to wear their college’s colors than if they lose.

“We want to be associated with winners,” says Saad, “just like primates want to be in close proximity to the alpha male.”