Kids are capable of thinking strategically, including using competitiveness and lies to their own benefit, by the time they're 7. So report researchers out of the University of Minnesota who studied 69 kids ages 3 to 9 playing two games. Kids typically figure out how to infer what others are thinking and what motivates them—and act strategically based on this information—somewhere between 6 and 7 years old, reports ABC Science. What's more, by that age they've sorted out that they don't have to trust others, and they don't have to be truthful themselves. The researchers call this the "strategic theory of mind."
In the first game, kids play either sender or receiver; the sender knows which of two boxes contains candy, and the receiver must guess after the sender points to one of the boxes. The sender keeps the candy if the receiver guesses the wrong box, and the receiver keeps it if he or she picks the correct box. In the second game, a kid and an experimenter are tasked with choosing one to five stickers simultaneously; the player who selects the fewest keeps that number of stickers, the other doesn't get any, and neither gets any if they pick the same number. By 6.5 years of age, kids' thinking while playing such games is sophisticated enough to resemble an adult's, reports Medical Xpress. (Another recent study found that we understand probability as toddlers.)