Thumb sucking has been proven to be a natural reflex and soothing habit that infants gain in the first year of their life.
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It is the safe zone for babies, that’s why they turn to the thumb when scared, bored, tired, or upset to comfort and entertain themselves.
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Dopamine and Endorphins produced while a babies are sucking their finger is what gives them pleasure and comfort.
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Before the age of 2 years old, thumb sucking is a normal habit. And it becomes an issue after 4 years old.