Thumb-sucking

Thumb sucking is a behavior found in humans, chimpanzees, captive ring-tailed lemurs, and other primates. It usually involves placing the thumb into the mouth and rhythmically repeating sucking contact for a prolonged duration. It can also be accomplished with any piece of skin within reach (such as the big toe) and is considered to be soothing and therapeutic for the person. As a child develops the habit, it will usually develop a "favorite" finger to suck on.

At birth, a baby will reflexively suck any object placed in its mouth; this is the sucking reflex responsible for breastfeeding. From the very first time they engage in nutritive feeding, infants learn that the habit can not only provide valuable nourishment, but also a great deal of pleasure, comfort, and warmth. Whether from a mother, bottle, or pacifier, this behavior, over time, begins to become associated with a very strong, self-soothing, and pleasurable oral sensation. As the child grows older, and is eventually weaned off the nutritional sucking, they can either develop alternative means for receiving those same feelings of physical and emotional fulfillment, or they can continue experiencing those pleasantly soothing experiences by beginning to suck their thumbs or fingers. This reflex disappears at about 4 months of age; thumb sucking is not purely an instinctive behavior and therefore can last much longer. Moreover, ultrasound scans have revealed that thumb sucking can start before birth, as early as 15 weeks from conception; whether this behavior is voluntary or due to random movements of the fetus in the womb is not conclusively known.

Thumb sucking generally stops by the age of 5 years. Some older children will retain the habit, which can cause severe dental problems. While most dentists would recommend breaking the habit as early as possible, it has been shown that as long as the habit is broken before the onset of permanent teeth, at around 5 years old, the damage is reversible. Thumb sucking is sometimes retained into adulthood and may be due to stereotypic movement disorder, another psychiatric disorder, or simply habit continuation.

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