In most cases the cause is unknown. However, there are various known causes of speech impediments, such as "hearing loss, neurological disorders, brain injury, intellectual disability, drug abuse, physical impairments such as cleft lip and palate, and vocal abuse or misuse."

Pressured speech may also lead to the development of a stutter. The person's need or pressure to speak causes them to involuntarily stutter. Therefore, the person's need to express themselves is greater than their ability to vocalise their thoughts.

Psychostimulants such as cocaine, amphetamines may cause speech resembling pressured speech in individuals with pre-existing psychopathology and produce hypomanic or manic symptoms in general, owing both to the substance's own qualities and the underlying nature of an individual's psyche. In many psychotic disorders, use of certain drugs amplifies certain expressions of symptoms, and stimulant-induced pressured speech is among them.

Stroke-associated AOS is the most common form of acquired AOS, making up about 60% of all reported acquired AOS cases. This is one of the several possible disorders that can result from a stroke, but only about 11% of stroke cases involve this disorder. Brain damage to the neural connections, and especially the neural synapses, during the stroke can lead to acquired AOS. Most cases of stroke-associated AOS are minor, but in the most severe cases, all linguistic motor function can be lost and must be relearned. Since most with this form of AOS are at least fifty years old, few fully recover to their previous level of ability to produce speech.

Other disorders and injuries of the brain that can lead to AOS include (traumatic) dementia, progressive neurological disorders, and traumatic brain injury.

In 2006, the U.S. Department of Education indicated that more than 1.4 million students were served in the public schools' special education programs under the speech or language impairment category of IDEA 2004. This estimate does not include children who have speech/language problems secondary to other conditions such as deafness; this means that if all cases of speech or language impairments were included in the estimates, this category of impairment would be the largest. Another source has estimated that communication disorders—a larger category, which also includes hearing disorders—affect one of every 10 people in the United States.

ASHA has cited that 24.1% of children in school in the fall of 2003 received services for speech or language disorders—this amounts to a total of 1,460,583 children between 3 –21 years of age. Again, this estimate does not include children who have speech/language problems secondary to other conditions. Additional ASHA prevalence figures have suggested the following:

- Stuttering affects approximately 4% to 5% of children between the ages of 2 and 4.

- ASHA has indicated that in 2006:

- Almost 69% of SLPs served individuals with fluency problems.

- Almost 29% of SLPs served individuals with voice or resonance disorders.

- Approximately 61% of speech-language pathologists in schools indicated that they served individuals with SLI

- Almost 91% of SLPs in schools indicated that they servedindividuals with phonological/articulation disorder

- Estimates for language difficulty in preschool children range from 2% to 19%.

- Specific Language Impairment (SLI) is extremely common in children, and affects about 7% of the childhood population.

DVD/CAS is a motor disorder, which means that the problem is located in the brain and its signals, and not in the mouth. In most cases, the cause is unknown. Possible causes include genetic syndromes and disorders.

Recent research has focused on the significance of the FOXP2 gene in both species and individual development.

Research regarding the KE family, where half the members of the extended family, over three generations, exhibited heritable developmental verbal dyspraxia, were found to have a defective copy of the FOXP2 gene. and further studies suggest that the FOXP2 gene as well as other genetic issues could explain DVD/CAS. including 16p11.2 microdeletion syndrome.

Birth/prenatal injuries, as well as stroke, can also be causes of DVD/CAS. Furthermore, DVD/CAS can occur as a secondary characteristic to a variety of other conditions. These include autism, some forms of epilepsy, fragile X syndrome, galactosemia, and chromosome translocations involving duplications or deletions.

Longitudinal studies indicate that problems are largely resolved by 5 years of age in around 40% of 4-year-olds with early language delays who have no other presenting risk factors. However, for children who still have significant language difficulties at school entry, reading problems are common, even for children who receive specialist help, and educational attainments are typically poor. Poor outcomes are most common in cases where comprehension as well as expressive language is affected. There is also evidence that scores on tests of nonverbal ability of children with DLD decrease over the course of development.

DLD is associated with an elevated risk of social, emotional and mental health concerns. For instance, in a UK survey, 64% of a sample of 11-year-olds with DLD scored above a clinical threshold on a questionnaire for psychiatric difficulties, and 36% were regularly bullied, compared with 12% of comparison children. In the longer-term, studies of adult outcomes of children with DLD have found elevated rates of unemployment, social isolation and psychiatric disorder among those with early comprehension difficulties. However, better outcomes are found for children who have milder difficulties and do not require special educational provision.

Cluttering (also called tachyphemia or tachyphrasia) is a speech and communication disorder characterized by a rapid rate of speech, erratic rhythm, and poor syntax or grammar, making speech difficult to understand.

Dysprosody, which may manifest as pseudo-foreign accent syndrome, refers to a disorder in which one or more of the prosodic functions are either compromised or eliminated completely.

Prosody refers to the variations in melody, intonation, pauses, stresses, intensity, vocal quality, and accents of speech. As a result, prosody has a wide array of functions, including expression on linguistic, attitudinal, pragmatic, affective and personal levels of speech. People diagnosed with dysprosody most commonly experience difficulties in pitch or timing control. Essentially, people diagnosed with the disease can comprehend language and vocalize what they intend to say, however, they are not able to control the way in which the words come out of their mouths. Since dysprosody is the rarest neurological speech disorder discovered, not much is conclusively known or understood about the disorder. The most obvious expression of dysprosody is when a person starts speaking in an accent which is not their own. Speaking in a foreign accent is only one type of dysprosody, as the disease can also manifest itself in other ways, such as changes in pitch, volume, and rhythm of speech. It is still very unclear as to how damage to the brain causes the disruption of prosodic function. The only form of effective treatment developed for dysprosody is speech therapy.

In psychology, alogia (Greek ἀ-, “without”, and λόγος, “speech”), or poverty of speech, is a general lack of additional, unprompted content seen in normal speech. As a symptom, it is commonly seen in patients suffering from schizophrenia, and is considered a negative symptom. It can complicate psychotherapy severely because of the considerable difficulty in holding a fluent conversation.

Alogia is often considered a form of aphasia, which is a general impairment in linguistic ability. It often occurs with intellectual disability and dementia as a result of damage to the left hemisphere of the brain. People can revert to alogia as a way of reverse psychology, or avoiding questions.

Recent research has established the existence of primary progressive apraxia of speech caused by neuroanatomic motor atrophy. For a long time, this disorder was not distinguished from other motor speech disorders such as dysarthria and in particular primary progressive aphasia. Many studies have been done trying to identify areas in the brain in which this particular disorder occurs or at least to show that it occurs in different areas of the brain than other disorders. One study observed 37 patients with neurodegenerative speech disorders to determine whether or not it is distinguishable from other disorders, and if so where in the brain it can be found. Using speech and language, neurological, neuropsychological and neuroimaging testing, the researchers came to the conclusion that PAS does exist and that it correlates to superior lateral premotor and supplementary motor atrophy. However, because PAS is such a rare and recently discovered disorder, many studies do not have enough subjects to observe to make data entirely conclusive.

Epidemiological surveys, in the US and the UK converge in estimating the prevalence of DLD in 5-year-olds at around 7 percent.

Tangential speech is a communication disorder in which the train of thought of the speaker wanders and shows a lack of focus, never returning to the initial topic of the conversation. It is less severe than logorrhea and may be associated with the middle stage in dementia. It is, however, more severe than circumstantial speech in which the speaker wanders, but eventually returns to the topic.

Some adults with right hemisphere brain damage may exhibit behavior that includes tangential speech. Those who exhibit these behaviors may also have related symptoms such as seemingly inappropriate or self-centered social responses, and a deterioration in pragmatic abilities (including appropriate eye contact as well as topic maintenance).

Alogia is characterized by a lack of speech, often caused by a disruption in the thought process. Usually, an injury to the left hemisphere of the brain will cause alogia to appear in an individual. In conversation, alogic patients will reply very sparsely and their answers to questions will lack spontaneous content; sometimes, they will even fail to answer at all. Their responses will be brief, generally only appearing as a response to a question or prompt.

Apart from the lack of content in a reply, the manner in which the person delivers the reply is affected as well. Patients affected by alogia will often slur their responses, and not pronounce the consonants as clearly as usual. The few words spoken usually trail off into a whisper, or are just ended by the second syllable. Studies have shown a correlation between alogic ratings in individuals and the amount and duration of pauses in their speech when responding to a series of questions posed by the researcher.

The inability to speak stems from a deeper mental inability that causes alogic patients to have difficulty grasping the right words mentally, as well as formulating their thoughts. A study investigating alogiacs and their results on the category fluency task showed that people with schizophrenia who exhibit alogia display a more disorganized semantic memory than controls. While both groups produced the same number of words, the words produced by people with schizophrenia were much more disorderly and the results of cluster analysis revealed bizarre coherence in the alogiac group.

Language delays are the most frequent developmental delays, and can occur for many reasons. A delay can be due to being a “late bloomer,” or a more serious problem. The most common causes of speech delay include

- Hearing loss

- Slow development

- Intellectual Disability

Such delays can occur in conjunction with a lack of mirroring of facial responses, unresponsiveness or unawareness of certain noises, a lack of interest in playing with other children or toys, or no pain response to stimuli.

Other causes include:

- Psychosocial deprivation - The child doesn't spend enough time talking with adults. Research on early brain development shows that babies and toddlers have a critical need for direct interactions with parents and other significant care givers for healthy brain growth and the development of appropriate social, emotional, and cognitive skills.

- Television viewing is associated with delayed language development. Children who watched television alone were 8.47 times more likely to have language delay when compared to children who interacted with their caregivers during television viewing. As recommended by the American Academy of Pediatrics (AAP), children under the age of 2 should watch no television at all, and after age 2 watch no more than one to two hours of quality programming a day. Therefore, exposing such young children to television programs should be discouraged. Parents should engage children in more conversational activities to avoid television-related delays to their children language development, which could impair their intellectual performance.

- Stress during pregnancy is associated with language delay.

- Being a twin

- Attention deficit hyperactivity disorder

- Autism (a developmental disorder) - There is strong evidence that autism is commonly associated with language delay. Asperger syndrome, which is on the autistic spectrum, however, is not associated with language delay.

- Selective mutism (the child just doesn't want to talk)

- Cerebral palsy (a movement disorder caused by brain damage)

- Genetic abnormalities - In 2005, researchers found a connection between expressive language delay and a genetic abnormality: a duplicate set of the same genes that are missing in sufferers of Williams-Beuren syndrome. Also so called XYY syndrome can often cause speech delay.

- Correlation with male sex, previous family history, and maternal education has been demonstrated.

Speech disorders or speech impediments are a type of communication disorder where 'normal' speech is disrupted. This can mean stuttering, lisps, etc. Someone who is unable to speak due to a speech disorder is considered mute.

Tachylalia or tachylogia is extremely rapid speech. Tachylalia occurs in many clutterers and many people who have speech disorders.

Tachylalia is a generic term for speaking fast, and does not need to coincide with other speech problems. Tachylalia by itself is not considered a speech disorder.

Tachylalia may be exhibited as a single stream of rapid speech without prosody, and can be delivered quietly or mumbled. Tachylalia can be simulated by stimulating the brain electronically.

Tachylalia can occur with the following:

- Normal speech

- Cluttering

- Parkinson's disease

- Cluttered speech

- Pressure of speech

Studies have failed to find clear evidence that language delay can be prevented by training or educating health care professionals in the subject. Overall, some of the reviews show positive results regarding interventions in language delay, but are not curative. (Commentary - Early Identification of Language Delays, 2005)

In psychiatry, stilted speech or pedantic speech is communication characterized by situationally inappropriate formality. This formality can be expressed both through abnormal prosody as well as speech content that is "inappropriately pompous, legalistic, philosophical, or quaint". Often, such speech can act as evidence for autism spectrum disorder (ASD) or a thought disorder, a common symptom in schizophrenia or schizotypal personality disorder.

To diagnose stilted speech, researchers have previously looked for the following characteristics:

- speech conveying more information than necessary

- vocabulary and grammar expected from formal writing rather than conversational speech

- unneeded repetition or corrections

While literal and long-winded word content is often the most identifiable feature of stilted speech, such speech often displays irregular prosody, especially in resonance. Often, the loudness, pitch, rate, and nasality of pedantic speech vary from normal speech, resulting in the perception of pedantic or stilted speaking. For example, overly loud or high-pitched speech can come across to listeners as overly forceful while slow or nasal speech creates an impression of condescension.

These attributions, which are commonly found in patients with ASD, partially account for why stilted speech has been considered a diagnostic criteria for the disorder. Stilted speech, along with atypical intonation, semantic drift, terseness, and perseveration, are all qualities known to be commonly impaired during conversation with adolescents on the autistic spectrum. Often, stilted speech found in children with ASD will also be especially stereotypic or rehearsed.

Patients with schizophrenia are also known to experience stilted speech. This symptom is attributed to both an inability to access more commonly used words and a difficulty understanding pragmatics, or the relationship between language and context. However, stilted speech appears as less common symptom compared to a certain number of other symptoms of the psychosis (Adler "et al" 1999). This element of cognitive disorder is also exhibited as a symptom in the narcissistic personality disorder (Akhtar & Thomson 1982).

In its more common usage, "stilted speech" is a term used to describe overly-formal, unnatural-sounding speech.

There are three significant features that differentiate DVD/CAS from other childhood speech sound disorders. These features are:

- "Inconsistent errors on consonants and vowels in repeated productions of syllables and words

- Lengthened coarticulatory transitions between sounds and syllables

- Inappropriate prosody, especially in the realization of lexical or phrasal stress"

Even though DVD/CAS is a "developmental" disorder, it will not simply disappear when children grow older. Children with this disorder do not follow typical patterns of language acquisition and will need treatment in order to make progress.

There are many potential causes of dysarthria. They include toxic, metabolic, degenerative diseases, traumatic brain injury, or thrombotic or embolic stroke.

Degenerative diseases include parkinsonism, amyotrophic lateral sclerosis (ALS), multiple sclerosis, Huntington's disease, Niemann-Pick disease, and Friedreich ataxia.

Toxic and metabolic conditions include: Wilson's disease, hypoxic encephalopathy such as in drowning, and central pontine myelinolysis.

These result in lesions to key areas of the brain involved in planning, executing, or regulating motor operations in skeletal muscles (i.e. muscles of the limbs), including muscles of the head and neck (dysfunction of which characterises dysarthria). These can result in dysfunction, or failure of: the motor or somatosensory cortex of the brain, corticobulbar pathways, the cerebellum, basal nuclei (consisting of the putamen, globus pallidus, caudate nucleus, substantia nigra etc.), brainstem (from which the cranial nerves originate), or the neuro-muscular junction (in diseases such as myasthenia gravis) which block the nervous system's ability to activate motor units and effect correct range and strength of movements.

Causes:

- Brain tumor

- Cerebral palsy

- Guillain–Barré syndrome

- Hypothermia

- Lyme disease

- Stroke

- Intracranial hypertension (formerly known as pseudotumor cerebri)

- Tay-Sachs, and late onset Tay-Sachs (LOTS), disease

Most speech sound disorders occur without a known cause. A child may not learn how to produce sounds correctly or may not learn the rules of speech sounds on his or her own. These children may have a problem with speech development, which does not always mean that they will simply outgrow it by themselves. Many children do develop speech sounds over time but those who do not often need the services of a Speech-Language Pathologist to learn correct speech sounds.

Some speech sound errors can result from other syndromes or disorders such as:

- developmental disorders (e.g. autism)

- genetic disorders (e.g. Down syndrome)

- hearing loss, including temporary hearing loss, such as from ear infections

- cleft palate or other physical anomalies of the mouth

- illness

- neurological disorders (e.g. cerebral palsy)

Stuttering, also known as stammering, is a speech disorder in which the flow of speech is disrupted by involuntary repetitions and prolongations of sounds, syllables, words or phrases as well as involuntary silent pauses or blocks in which the person who stutters is unable to produce sounds. The term "stuttering" is most commonly associated with involuntary sound repetition, but it also encompasses the abnormal hesitation or pausing before speech, referred to by people who stutter as "blocks", and the prolongation of certain sounds, usually vowels or semivowels. According to Watkins et al., stuttering is a disorder of "selection, initiation, and execution of motor sequences necessary for fluent speech production." For many people who stutter, repetition is the primary problem. The term "stuttering" covers a wide range of severity, encompassing barely perceptible impediments that are largely cosmetic to severe symptoms that effectively prevent oral communication. In the world, approximately four times as many men as women stutter, encompassing 70 million people worldwide, or about 1% of the world's population.

The impact of stuttering on a person's functioning and emotional state can be severe. This may include fears of having to enunciate specific vowels or consonants, fears of being caught stuttering in social situations, self-imposed isolation, anxiety, stress, shame, being a possible target of bullying (especially in children), having to use word substitution and rearrange words in a sentence to hide stuttering, or a feeling of "loss of control" during speech. Stuttering is sometimes popularly seen as a symptom of anxiety, but there is actually no direct correlation in that direction (though as mentioned the inverse can be true, as social anxiety may actually develop in individuals as a result of their stuttering).

Stuttering is generally not a problem with the physical production of speech sounds or putting thoughts into words. Acute nervousness and stress do not cause stuttering, but they can trigger stuttering in people who have the speech disorder, and living with a stigmatized disability can result in anxiety and high allostatic stress load (chronic nervousness and stress) that reduce the amount of acute stress necessary to trigger stuttering in any given person who stutters, exacerbating the problem in the manner of a positive feedback system; the name 'stuttered speech syndrome' has been proposed for this condition. Neither acute nor chronic stress, however, itself creates any predisposition to stuttering.

The disorder is also "variable", which means that in certain situations, such as talking on the telephone or in a large group, the stuttering might be more severe or less, depending on whether or not the stutterer is self-conscious about their stuttering. Stutterers often find that their stuttering fluctuates and that they have "good" days, "bad" days and "stutter-free" days. The times in which their stuttering fluctuates can be random. Although the exact etiology, or cause, of stuttering is unknown, both genetics and neurophysiology are thought to contribute. There are many treatments and speech therapy techniques available that may help decrease speech disfluency in some people who stutter to the point where an untrained ear cannot identify a problem; however, there is essentially no cure for the disorder at present. The severity of the person's stuttering would correspond to the amount of speech therapy needed to decrease disfluency. For severe stuttering, long-term therapy and hard work is required to decrease disfluency.

Developmental verbal dyspraxia is a developmental inability to motor plan volitional movement for the production of speech in the absence of muscular weakness. Research has suggested links to the FOXP2 gene.