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."

According to the St. Louis system for the diagnosis of schizophrenia, tangentiality is significantly associated with a low IQ prior to diagnosis (AU Parnas "et al" 2007).

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

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.

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.

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)

The term refers simplistically to a thought disorder shown from speech with a lack of observance to the main subject of discourse, such that a person whilst speaking on a topic deviates from the topic. Further definition is of speech that deviates from an answer to a question that is relevant in the first instance but deviates from the relevancy to related subjects not involved in a direct answering of the question. In the context of a conversation or discussion the communication is a response that is ineffective in that the form is inappropriate for adequate understanding. The person's speech seems to indicate that their attention to their own speech has perhaps in some way been overcome during the occurrence of cognition whilst speaking, causing the vocalized content to follow thought that is apparently without reference to the original idea or question; or the person's speech is considered evasive in that the person has decided to provide an answer to a question that is an avoidance of a direct answer.

Specific language impairment (SLI) is diagnosed when a child has delayed or disordered language development for no apparent reason. Usually the first indication of SLI is that the child is later than usual in starting to speak and subsequently is delayed in putting words together to form sentences. Spoken language may be immature. In many children with SLI, understanding of language, or "receptive" language, is also impaired, though this may not be obvious unless the child is given a formal assessment.

Although difficulties with use and understanding of complex sentences are a common feature of SLI, the diagnostic criteria encompass a wide range of problems, and for some children other aspects of language are problematic (see below). In general, the term SLI is reserved for children whose language difficulties persist into school age, and so it would not be applied to toddlers who are late to start talking, most of whom catch up with their peer group after a late start.

Specific language impairment (SLI) is diagnosed when a child's language does not develop normally and the difficulties cannot be accounted for by generally slow development, physical abnormality of the speech apparatus, autism spectrum disorder, apraxia, acquired brain damage or hearing loss. Twin studies have shown that it is under genetic

influence. Although language impairment can result from a single-gene mutation, this is unusual. More commonly SLI results from the combined influence of multiple genetic variants, each of which is found in the general population, as well as environmental influences.

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.

To the untrained ear, those with the syndrome sound as though they speak their native languages with a foreign accent; for example, an American native speaker of English might sound as though he spoke with a south-eastern English accent, or a native English speaker from Britain might speak with a New York American accent. However, researchers at Oxford University have found that certain specific parts of the brain were injured in some foreign accent syndrome cases, indicating that particular parts of the brain control various linguistic functions, and damage could result in altered pitch and/or mispronounced syllables, causing speech patterns to be distorted in a non-specific manner. Contrary to popular beliefs that individuals with FAS exhibit their accent without any effort, these individuals feel as if they are suffering from a speech disorder. More recently, there is mounting evidence that the cerebellum, which controls motor function, may be crucially involved in some cases of foreign accent syndrome, reinforcing the notion that speech pattern alteration is mechanical, and thus non-specific.

The perception of a foreign accent is likely to be a case of pareidolia on the part of the listener. Nick Miller, Professor of Motor Speech Disorders at Newcastle University has explained: "The notion that sufferers speak in a foreign language is something that is in the ear of the listener, rather than the mouth of the speaker. It is simply that the rhythm and pronunciation of speech has changed."

The late British singer George Michael claimed in 2012 that, after waking from a three-week long coma, he temporarily had a West Country accent.

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.

An example of circumstantial speech is that when asked about the age of a person's mother at death, the speaker responds by talking at length about accidents and how too many people die in accidents, then eventually says what the mother's age was at death.

The most common cause of asemia is brain damage, such as a stroke or a brain tumor. Other possible causes include Alzheimer's disease and infection. Roger Wolcott Sperry, through his research of split-brain patients, had found out that the human brain lateralizes functions, meaning that the two hemispheres of the brain have different functions. Brain damage, specifically to the left hemisphere, can impair our ability to speak or understand language. This led Sperry to conclude that due to the lateralization of brain function, language is based in the left hemisphere. Therefore, any kind of brain damage to the left hemisphere will greatly impact language, whether it is expressive or receptive.

Foreign accent syndrome is a rare medical condition in which patients develop speech patterns that are perceived as a foreign accent that is different from their native accent, without having acquired it in the perceived accent's place of origin.

Foreign accent syndrome usually results from a stroke, but can also develop from head trauma, migraines or developmental problems. The condition was first reported in 1907, and between 1941 and 2009 there were 62 recorded cases.

Its symptoms result from distorted articulatory planning and coordination processes and although popular news articles commonly attempt to identify the closest regional accent, speakers suffering from foreign accent syndrome acquire neither a specific foreign accent nor any additional fluency in a foreign language. Despite an unconfirmed news report in 2010 that a Croatian speaker had gained the ability to speak fluent German after emergence from a coma, there has been no verified case where a patient's foreign language skills have improved after a brain injury. There have been a few reported cases of children and siblings picking up the new accent from someone with foreign accent syndrome.

A person afflicted with circumstantiality has slowed thinking and invariably talks at length about irrelevant and trivial details (i.e. circumstances). Eliciting information from such a person can be difficult since circumstantiality makes it hard for the individual to stay on topic. In most instances however, the relevant details are eventually achieved.

The disorder is often associated with schizophrenia and obsessive-compulsive disorder.

The prognosis for children with LKS varies. Some affected children may have a permanent severe language disorder, while others may regain much of their language abilities (although it may take months or years). In some cases, remission and relapse may occur. The prognosis is improved when the onset of the disorder is after age 6 and when speech therapy is started early. Seizures generally disappear by adulthood. Short-term remissions are not uncommon in LKS but they create difficulties in evaluating a patient's response to various therapeutic modalities.

The following table demonstrate the Long-Term Follow-up of Acquired Epileptic Aphasia across many different instrumental studies:.

Lower rates of good outcomes have been reported, ranging between 14% to 50%. Duran "et al." used 7 patients in his study (all males, aged 8–27 years of age) with LKS. On long-term followup, most of his patients did not demonstrate total epilepsy remission and language problems continued. Out of the seven patients, one reported a normal quality of life while the other six reported aphasia to be a substantial struggle. The Duran "et al." study is one of few that features long-term follow up reports of LKS and utilizes EEG testing, MRIs, the Vineland Adaptive Behavior Scales, the Connor's Rating Scales-revised, and a Short-Form Health Survey to analyze its patients.

Globally, more than 200 cases of acquired epileptic aphasia have been described in the literature. Between 1957 and 1980, 81 cases of acquired epileptic aphasia were reported, with 100 cases generally being diagnosed every 10 years.

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.

Pragmatic language impairment (PLI), or social (pragmatic) communication disorder (SCD), is an impairment in understanding pragmatic aspects of language. This type of impairment was previously called semantic-pragmatic disorder (SPD). People with these impairments have special challenges with the semantic aspect of language (the meaning of what is being said) and the pragmatics of language (using language appropriately in social situations). It is assumed that those with autism have difficulty with "the meaning of what is being said" due to different ways of responding to social situations.

PLI is now a diagnosis in DSM-5, and is called social (pragmatic) communication disorder. Communication problems are also part of the autism spectrum disorders (ASD); however, the latter also show a restricted pattern of behavior, according to behavioral psychology. The diagnosis SCD can only be given if ASD has been ruled out.

Among preschoolers, the prognosis for recovery is good. Based on research, about 65% of preschoolers who stutter recover spontaneously in the first two years of stuttering, and about 74% recover by their early teens. In particular, girls seem to recover well. For others, early intervention is effective in helping the child overcome disfluency.

Once stuttering has become established, and the child has developed secondary behaviors, the prognosis is more guarded, and only 18% of children who stutter after five years recover spontaneously. However, with treatment young children may be left with little evidence of stuttering.

With adult people who stutter, there is no known cure, though they may make partial recovery or even complete recovery with intervention. People who stutter often learn to stutter less severely, though others may make no progress with therapy.

Emotional sequelae associated with stuttering primarily relates to state-dependent anxiety related to the speech disorder itself. However, this is typically isolated to social contexts that require speaking, is not a trait anxiety, and this anxiety does not persist if stuttering remits spontaneously. Research attempting to correlate stuttering with generalized or state anxiety, personality profiles, trauma history, or decreased IQ have failed to find adequate empirical support for any of these claims.

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.

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).

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)

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.