If you've never seen the opposition run a play on you at a bar, you haven't been paying attention. "Is the mating game sometimes a team sport?" two researchers ask in a paper. The answer is yes.
The age-old rulebook dictates that men need help accessing females, while women need assistance avoiding riffraff, and that's exactly the kind of aid Josh Ackerman of MIT and Douglas Kenrick of ASU witnessed in their study. When a woman is flirting with a desirable guy, her girlfriends will tend to leave her alone, but when she's interacting with an undesirable, they'll step in. Conversely, guys will leave a buddy alone if he's stuck with a dud and provide support if he's onto something good.
Three quarters of participants also reported that they'd used a pal as a decoy mate, typically (for men) to demonstrate desirability to other women or (for women) to ward off other guys.
The top reasons people offered for cooperation in courtship were self-satisfaction, help with future access, and friendship maintenance. As competitive as the dating world is, humans advance—and defend—in packs.