It usually involves placing the thumb into the mouth and rhythmically repeating sucking contact for prolong duration. The presence of the habit is considered normal till the age of 3-4 years. Persistence of the habit beyond this age can lead to various malocclusions.

Causes:-

Freudian theory:

This theory was proposed by sigmond freud in the early part of this century. He suggested that a child passes through various distinct phases of psychological development of which the oral and the anal phases are seen in the first three year of life. In the oral phase, the mouth is believed to be an oro-erotic zone. The child has the tendency to place his fingers or any other object into the oral cavity. Prevention of such an act is believed to result in emotional insecurity and poses the risk of the child diversifying into other habits.

Oral drive theory of sears and Wise:

Sears and Wise in 1950 proposed that prolonged sucking can lead to thumb sucking.

Benjamin’s theory:

Benjamin has suggested that thumb sucking arises from the rooting or placing reflex seen in all mammalian infants. Rooting reflex is the movement of the infant’s head and tongue towards an object touching his cheek. The object is usually the mother’s breast but may also be a finger or a pacifier. This rooting reflex disappears in normal infants around 7-8 months of age.

Psychological aspects: 

Children deprived of parental love; care and affection are believed to resort to this due to a feeling of insecurity.

Learned pattern: According to some authors, thumb sucking is merely a learned pattern with no underlying cause or psychological bearing.  

Effects of thumb sucking:

1. Labial tipping of maxillary anterior teeth resulting in proclination of maxillary anterior.
2. The overjet increases due to proclination of the maxillary anterior.
3. some children rest their hand on the mandibular anterior during the sucking acts. In such children lingual tipping of the mandibular incisor can be expected which further increases the overjet.
4. Anterior open bite can occur as a result of restriction of incisor eruption and supraeruption of the buccal teeth.
5. The cheek muscles contract during thumb sucking resulting a narrow maxillary arch which predisposes to posterior cross bites.
6. The child may develop tongue thrust habit as a result of the open bite.
7. The upper lip is generally hypotonic while the lower part of the face exhibits hyperactive mentalis activity.

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