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"Children’s progress “may have been slowed by overly academic preschool experiences that introduced formalized learning experiences too early for most children’s developmental status,” Dr. Marcon wrote. Nevertheless, many educators want to curtail play during school. “Play is often perceived as immature behavior that doesn’t achieve anything,” says David Whitebread, a psychologist at Cambridge University who has studied the topic for decades. “But it’s essential to their development. They need to learn to persevere, to control attention, to control emotions. Kids learn these things through playing.”