There is no single cause of stuttering to date. Many researchers have come out with varied results—some psychological basis and others neurological causes. There is another field that says stuttering is a homogenous disorder but this is yet to be disputed. It says that stutterers suffer from one underlying problem. Nevertheless, popular theories are based on heterogeneity of the disorder.
One theorist has argued that since there no measurement and causes of fluency, it is harder to define the causes of abnormality. For a long time, theorists believed that the concept of stuttering was an outgrowth or exacerbation of normal disfuency. Yet, these premises and models are still subjected to further experiments and studies.
To further aid in the study of stuttering, theorists tried to categorize and make a sub-group of people who stutters. First is the severity as a grouping variable. Many studies used this sub-grouping with so far mixed results of analyses. Meanwhile, the intriguing sub-grouping of Van Riper described four tracks in the development of shuttering.
The model of fluent speech production presents two important points in understanding the categories of stuttering. First, stuttering shows a failure in temporal processing. Second, stuttering shows an imbalance between the capacities of the fluency generating system and demands of the environment.
Following the model of fluent speech production, hypothetical types or sub groups in stuttering are formulated. First, the speech motor control sub-groups which have two distinct groups called dyspraxic stuttering and respiratory control stuttering.