Pam Jarvis, of Leeds Metropolitan university, spent 18 months observing pupils at play. Most of the mixed-gender games involved chasing. The process generally began with a small group of girls approaching one or two boys and issuing an invitation to play, usually touch-and-run. The boys would then pretend to be frightening monsters, chasing the girls.
"This is how interaction between adults starts," said Dr Jarvis. "The woman gives out a signal that she's free and would be interested, if the man is willing. But the boy does the chasing."
The games tended to be directed by the females, who often told specific boys which girls they should be chasing. Female conversation during breaks in play would revolve around how disgusting boys were, each girl impressing on her friends how much she had been chased, while sympathising with her playmates about the boys' over-eager behaviour.
Dr Jarvis says: "The human social world is about collaboration and competition. Girls collaborate to get boys to chase them, and then compete to be the most chased. And boys compete to be the one who chases best."
Occasionally, girls would complain about over-enthusiastic boys. But the boys appeared to relish admonishments, which marked them out as rule-breakers. "If you're really bad, no one likes you," says Dr Jarvis.
"But if you have a touch of danger about you, it's really attractive."
The games, she says, are a foray into the grown-up activity of flirting.
"Humans are highly evolved primates," she says. "We have these evolved tendencies and we have the ability to step out of role. But they're like a pair of worn shoes: you tend to feel more comfortable in them."