Underextension is when a person uses a word in a very restricted fashion, most commonly seen used by children that are just gaining mastery over language. To give an example of underextension, a young boy that sees a dog in the park will not say it is a dog, for only THEIR dog is a dog. In their mind, they think that a word is referring to a very specific instance of a certain concept other than all types and examples of the concept. (Studies done by: Caplan & Barr, 1989) You may point at the strange dog in the park, but the child would shake their head and tell you no. Upon getting home, they will point at Howard lying in the corner and say “Dog!” Bear with them, they are learning.
Overextension is the direct opposite of underextension, in which they use a word in too broad a manner, often speaking in overgeneralizations. To continue in the example above, but in the form of overextension, the young boy is once again in the park, but this time, he sees a police officer riding on a horse, points, at the horse and says “Dog!” It is obviously not a dog, but to the young boy, anything with four legs and not a person is a “dog”. While this is of course flawed speech, it is a sign that the child is advancing in their thought processes. The child is now starting to develop mental categories and concepts (Studies done by: Behrend, 1988), and will soon be able to distinguish horse from dog.
Lecture notes from my Developmental Psychology class taught by S. Harrison, 9-18-2002, Evergreen Valley College, San Jose, California.
Feldman, S. Robert. Development Across the Life Span (3rd Ed). Upper Saddle River, New Jersey: Pearson Education.