thumb-sucking Between 75% and 95% of all infants suck their thumb, making thumb-sucking the most prevalent kind of nonnutritive oral activity in infants and young children. Thumb-sucking continues in approximately 45% of American pre-school children, but in only 30% of Swedish children of the same age. In a significant percentage of American 7–11-year-olds, thumb-sucking persists. Among Inuit, American Indian, and African children thumb-sucking is rare.

Thumb-sucking may begin before birth. The thumb has been observed in the mouths of fetuses as young as 18 weeks of gestational age, and true sucking movements and protrusion of the lips may occur by 24 weeks. New-borns often have blisters on their hands and arms, indicating the probable occurrence of sucking before birth. After birth, infants have a strong rooting reflex: they turn their head toward an object touching the cheek. This reflex allows them to find the nipple. Around the age of 3 months, an infant may accidentally discover its thumb or another digit and suck on it. Thumb-sucking is usually established before the first birthday.

A number of theories have been advanced concerning the origins and import of thumb-sucking. Freud believed that thumb-sucking is an instinctual activity. Following S. Lindner's 1879 argument that thumb-sucking is sexual in nature, Freud found that it fulfilled all the criteria to be considered the model of infantile sexuality. It relies on a physical function — the need for nourishment and the pleasure derived from it; it is autoerotic, or directed toward self-gratification; and it is controlled by an erogenous zone, the lips, which first experience the pleasure of sucking. Freud believed that thumb-sucking (or sucking another part of the body) arises when the gratification of the erogenous zone becomes separated from the desire to take nourishment. During thumb-sucking an infant desires to grasp something, such as its own or another's ear, and pull it rhythmically. Since this activity is very pleasurable and may become associated also with rubbing sensitive parts of the body, such as the breast or genitals, Freud believed that thumb-sucking led to masturbation.

The behaviourist John B. Watson rejected instinct as an explanation for human behaviour. In The Psychological Care of the Infant and Child (1928), he promoted child-rearing practices based on his belief that children are made, not born, and that almost all behaviours result from conditioning. An implication of this view was that thumb-sucking, a bad habit, resulted from a conditioned response associated with eating, and must be cured within the first few days after birth. Charles Anderson Aldrich and Mary Aldrich, the authors of Babies are Human Beings (1938), by contrast, viewed thumb-sucking as a prenatal sport designed to exercise the facial muscles, and assured their readers that a child would stop it as soon as he developed other interests.

Other explanations for the origin and significance of thumb-sucking abound. Some consider it a natural stage of development that usually ceases by the end of the third year, though it may become a habit. Still others characterize it as a means by which infants comfort themselves. In contrast to these explanations, one study finds that thumb-sucking seems to improve respiratory function in babies with an immobile tongue (ankyloglossia) and a deviated epiglottis and larynx. Another group of theories links the development of thumb-sucking with inadequate sucking during breast-feeding or with bottle feeding, which takes less time. Some children suck their thumb only while falling asleep or sleeping. In older children thumb-sucking occurs in stressful situations, such as fatigue, boredom, hunger, insecurity, parental deprivation, or frustration.

Thumb-sucking may entail certain risks to physical health. It pushes the upper incisors out and the lower incisors in. Such malocclusions resolve spontaneously if thumb-sucking stops before the permanent teeth erupt. Other undesirable effects can include problems with jaw movements, narrowing of the cheek bones due to the contractions of the cheek muscles, ulcerations beneath the tongue, and root resorption. More commonly, the thumb may develop calluses or an irritant eczema, and the digit itself may become deformed.

Parental and expert responses to thumb-sucking underwent significant changes during the twentieth century. Before the end of the nineteenth century, little was said or written about it. In the final decades of the century, however, it became a cause for concern. Freud noted disapprovingly that, in the nursery, thumb-sucking was treated like any other ‘sexual naughtiness’. It was recognized that thumb-sucking could also lead to a misshapen mouth. Luther Holt recommended mittens or a splint to the elbow to prevent it. In 1922 Charis Barlow warned that thumb-sucking introduced dirt and germs into the mouth, caused adenoid inflammation, weakened the digestion, and spoiled the mouth and thumb shape. To prevent thumb-sucking, she recommended pinning the sleeves shut or wrapping the baby in a shawl to sleep, while Watson advised sewing mitts onto pajama sleeves. In the first edition of the Common Sense Book of Baby and Child Care (1945), Dr Benjamin Spock recommended increasing the time taken in breast- or bottle-feeding in young infants to discourage thumb-sucking, but cautioned parents against using restraints such as aluminum mittens for older children, as such devices frustrate the child.

Pediatricians now recommend that no action against thumb-sucking be initiated during the first two or three years. In the older child, an environmental cause for thumb-sucking should be sought and eliminated; for cases where thumb-sucking is an empty habit, parents are urged to remind the child gently only if the child wants to quit. Other cures involve bitter tasting liquids applied to the thumb, and palatal bars. In all cases children require emotional support.

Kristen L. Zacharias

Bibliography

Hardyment, C. (1983). Dream babies. Three centuries of good advice on child care. Harper and Row, New York.
Leung, A. K. C. and and Robson, W. L. M. (1991). Thumb sucking. American Family Physician, 44, 1724–8.