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    For some time it has been widely accepted that babies learn to do things because certain acts lead to “rewards”; and there is no reason to doubt that this is true. But it used to be widely believed that effective rewards, at least in the early stages, had to be directly related to such basic physiological（生理的）“drives”as thirst or hunger. In other words, a baby would learn if he got food or drink or some sort of physical comfort, not otherwise.
    It is now clear that this is not so. Babies will learn to behave in ways that produce results in the world with no reward except the successful effects. 
    Papousek began his studies by using milk in the normal way to “reward”the babies and so teach them to carry out some simple movements, such as turning the head to one side or the other. Then he noticed that a baby who had had enough to drink would refuse the milk but would still go on making the learned response with clear signs of pleasure. So he began go study he children's responses in situations where no milk was provided. He quickly found that children as young as four months would learn to turn their heads to