The first is characterized by phonological and fluency problems thus causing delays in the appearances of intelligible words and sentences, articulation problems, and slow speech rates.
For example, some adults have the difficulty of pronouncing longer words and have inconsistent articulation errors. Thus, stuttering occurs on longer and unfamiliar words.
The second one is directly linked to difficulties in voluntary control of muscles of respiration. That is why children who stutter most likely have asthma, allergies, and upper respiratory distress. This disorder is characterized by blocks and unvoiced prolongations.
Some theorists also consider the linguistic processing problems as main etiologic factor. Many children show delayed language development. This general category is called linguistic stuttering. There are three sub-types of group. The first one involves the developmental delays in aspects of linguistic processing. The second one is associated with problems in word finding or retrieval. The third is associated with problems in generation of complex grammatical forms and the last involves problems with auditory processing.
Another sub-group involves the cognitive processes. Problems in this area lead to disfluent speech production. However, one theorist says most likely a person who stutter has cognitive abilities that are superior to their linguistic and speech motor abilities.
Psychologically related factors cannot be eliminated in the sub-groupings of the causes stuttering. Anxiety is an important factor in stuttering. This is a critical factor with respect to severity of stuttering.
Theorists have based these groupings mainly on present results of studies. The sub-groupings of stuttering are yet to be tested and experimented.