Sucking is a natural reflex and infants and young children may use thumbs, fingers, pacifiers, and other objects on which to suck. It may make them feel secure and happy or provide a sense of security at difficult periods. Since thumb sucking is relaxing, it may induce sleep.
Thumb sucking that persists beyond the eruption of the permanent teeth can cause problems with the proper growth of the mouth and tooth alignment. How intensely a child sucks on fingers or thumbs will determine whether or not dental problems may result. Children who rest their thumbs passively in their mouths are less likely to have difficulty than those who vigorously suck their thumbs.