Biting other people is common in children ages 3 and younger. It becomes a problem if it happens frequently, occurs after age 3, injures others, or occurs with other aggressive behaviors.
A baby who is teething may bite in response to the sensation in the mouth or to relieve the pressure on the gums. Children may also bite as a way to cope with strong emotions—such as powerlessness, fear, or frustration—because they lack the social and language skills to express these feelings appropriately.
Usually, a firm "no" and stern expression will stop a child from biting. Children who bite frequently, especially if age 3 or older, should be evaluated by a doctor.