The cause of a lisp can vary.
In some instances, the cause is physiological, and the patient has some sort of deformity or medical condition which causes a lisp. For example, a child with swollen adenoids may tend to lisp, as will people who have recurring stuffy noses. Also, a lisp can be formed when the tongue is bruised or swollen. People with underbites, overbites and/or large tongues.