Freud's psychoanalytic theories
The first stage is the oral stage. An infant is in this stage from birth to eighteen months of age. The main focus in the oral stage is pleasure-seeking through the infant’s mouth. During this stage, the need for tasting and sucking becomes prominent in producing pleasure. Oral stimulation is crucial during this stage; if the infant’s needs are not met during this time frame, he or she will be fixated in the oral stage. Fixation in this stage can lead to adult habits such as thumb-sucking, smoking, over-eating, and nail-biting. Personality traits can also develop during adulthood that are linked to oral fixation; these traits can include optimism and independence or pessimism and hostility.