A lisp is a functional defect in the speech which usually occurs in childhood and which is characterized by the child pronouncing s and z as th or sh. Your teeth and tongue control the way you pronounce certain words or letters , it thus stands to reason that even minor defects in the teeth arrangement and shape could cause speech disorders. Crooked teeth is an example of a dental misalignment that could cause lisping in adulthood.
Although by definition, most lisps are almost always functional disorders and not usually caused by physical defects, however in a good number of cases slight defects of the teeth and tongue could give rise to lisping in children, teens, and even young adults. It’s not in any way associated with mental issues and in most cases is usually not caused by a physical defect in the mouth such as injuries or cleft palate. There are four kinds of lisp disorders all of which are caused by errors in tongue placement .
Dental lisp involves incorrect placement of the tongue against the back of the upper front teeth when making the sounds s and z, making it sound like th in thick and this respectively.
InterDental lisp occurs when the tongue is incorrectly placed between the upper and lower front teeth when trying to pronounce these two letters.
The other two kinds of lisp speech are caused by air rushing out of the sides of the tongue due to the child placing the tongue incorrectly against the roof of the mouth (palatal lisp) and the lateral lisp which is the hardest to resolve and which usually requires speech therapy.