Thumb sucking is common in babies and is said to actually begin in utero. Meant as a comforting gesture that mimics the endorphin rush brought on by breastfeeding, the habit should gradually subside in the first year of the baby's life. However, in some, the habit persists even into adulthood.
Chris Hart, a counselling psychologist says, "Adult thumb sucking is actually quite common. Around 1 in 10 adults do it, more women than men. In fact, double the number of women compared to men and even some celebrities like Rihanna have been spotted doing it."