In the relatively short history of our field, much has been learned about language development and what is considered “normal” at different stages. This research, leading to the creation of a kind of “syllabus” to be followed, has allowed SLTs to carry out much of the work that we do (Paul & Norbury, 2012). However, we are still some way from understanding the precise nature of language disorders and their underlying causes. Here’s what we do know.
There’s a general consensus that most cases of language disorder are likely due to a combination of genetic and environmental factors (Bishop, 2014). Family studies, especially twin studies have revealed that language disorders are highly heritable and tend to run in families.
In 2001, researchers even found that a single disordered gene, FOXP2 caused a severe speech and language disorder in three generations of one family (Paul & Norbury, 2012, p. 3). However, it does not seem to be associated with more common forms of Developmental Language Disorder (DLD). Paul and Norbury (2012) also point out that genes do not directly encode behaviour, so searching for a specific “language gene” makes little sense. It is more likely that several are involved.
Other research has highlighted the importance of environmental factors. The strong association between socio-economic status (SES) and language ability is well-established (Bishop, 2014). Many studies over the years have revealed that individuals in high SES groups are more likely to have strong language abilities, whilst those in disadvantaged, low SES groups are more likely to have weaker language skills (Loban, 1976).
Roy and Chiat (2013) note that those in low SES groups are disproportionately affected by language disorders. They observe that SES status affects children’s outcomes through the quality of the physical and psychological environments that they experience. Whilst SES status is typically based on indices such as the main carers’ education and income levels, other factors such as exposure to toxins and stress, nutritional intake, parenting style and cognitive stimulation often go alongside poverty and SES.
Similarly, the language input we receive when we are young is likely to influence the course of our development. In one study, it was found that children in a low SES “welfare” group heard only a quarter of the words that those growing up in professional families heard in a week (Roy and Chiat, 2013).
However, there may be more to this than meets the eye. Language ability is likely to affect SES categorization in the first place, since those with language disorders are less likely to go on to formal education, and more likely to be unemployed or “underemployed” (Larson and McKinley, 2003). What we assume to be the effects of poverty may in fact be related to undiagnosed parental language disorders. In other words, the heritable aspect of these situations may have been overlooked.
(For more information about whether there is a difference between language disadvantage and language disorder, please see my blog post: https://secondaryschoolslt.wordpress.com/2026/03/06/language-disorders-is-there-a-difference-between-disadvantage-and-disorder/ .)
This is not to say that our surroundings have no influence. We know from fields such as medicine that our environments are capable of influencing gene expression, brain structure and functioning. Leveraging the positive effects of the environment is what allows intervention to work, and changes in the gray matter or the brain have even been observed following Speech and Language Therapy (SALT) (Paul and Norbury, 2012).
Language disorders, then, do not have a single cause. Instead, it seems likely that multiple interacting risk factors co-occur to give rise to the pattern of difficulties seen in DLD (Paul & Norbury, 2012). How these factors affect the brain is another question. But whatever the precise mechanisms involved, it does seem that the brains of people with DLD are wired differently.
Whilst DLD is not associated with gross lesions in the brain, recent neuroimaging studies have identified subtle differences in cerebral volume, symmetry and blood flow in those affected (Abbot and Love, 2023). The research is still limited and often conflicting, but there has been some consensus across a few areas.
Firstly, a part of the brain called the “planum temporale”, located in the temporal lobe has been theorized to be involved in language processing. In typically developing individuals, this region is usually bigger on the left-hand side, but those with DLD tend to have more symmetrical brains, and in some, it is actually larger on the right. Some researchers have attributed this to the brain compensating for some deficiency on the left-hand side (Abbot and Love, 2023).
Differences in the gray and white matter around other regions associated with language such as the inferior frontal gyrus and caudate nucleus have also been observed. Reduced blood flow and activation of these areas was found in tandem. Finally, the brains of individuals with DLD were found to be smaller overall (Abbot and Love, 2023).
Many questions remain. Exactly how do these differences map onto the behaviours that we see? Do structural brain differences cause language disorders, or could a different type of functioning associated with language disorders lead to observable differences over time? Some studies also revealed that whilst most parents of children with DLD had similar atypical brain structures to their offspring, they did not all exhibit the same difficulties, further confusing the issue (Paul and Norbury, 2012). Despite all of these discoveries, the connection between the brain and the difficulties we see in DLD remains unclear.
For a discussion around which terminology to use, please see my post: https://secondaryschoolslt.wordpress.com/2026/02/26/developmental-language-disorder-a-brief-history-of-terminology/
Notes
Abbott, N., & Love, T. (2023). Bridging the Divide: Brain and Behavior in Developmental Language Disorder. Brain Sciences, 13(11), 1606.
Bishop, D.V.M. (2014). Ten questions about terminology for children with unexplained language problems. International Journal of Language & Communication Disorders, 49(4), pp. 381-415.
Larson, V.L. and McKinley, N.L. (2003) Communication solutions for older students. Thinking Pub.
Loban, W. (1976) Language Development: Kindergarten through Grade Twelve. Urbana, IL: National Council of Teachers of English.
Paul, R. and Norbury, C. (2012) Language Disorders from Infancy Through Adolescence: Listening, Speaking, Reading, Writing, and Communicating. 4th edn. St. Louis, MO: Elsevier.
Roy, P. and Chiat, S. (2013). Teasing apart disadvantage from disorder The case of poor language. In: Marshall, C. R. (Ed.), Current Issues in Developmental Disorders. Current Issues in Developmental Psychology. (pp. 125-150). PSYCHOLOGY PRESS.
Thoughts of a secondary school SLT 