However, subsequent studies have found that, though DLD runs in families, it is not usually caused by a mutation in FOXP2 or another specific gene. Current evidence suggests that there are many different genes that can influence language learning, and DLD results when a child inherits a particularly detrimental combination of risk factors, each of which may have only a small effect. Nevertheless, study of the mode of action of the FOXP2 gene has helped identify other common genetic variants involved in the same neural pathways that may play a part in causing DLD.Language disorders are associated with aspects of home environment, and it is often assumed that this is a causal link, with poor language stimulation leading to weak language skills. Twin studies, however, show that two children in the same home environment can have very different language outcomes, suggesting we should consider other explanations for the link. Children with DLD often grow up into adults who have relatively low educational attainments, and their children may share a genetic risk for language disorder.One non-genetic factor that is known to have a specific impact on language development is being a younger sibling in a large family. Associated factors
It has long been noted that males are more affected by DLD than females, with a sex ratio of affected males-to-females around 3 or 4:1. However, the sex difference is much less striking in epidemiological samples, suggesting that similar problems may exist in females but are less likely to be detected.
A major divide is between theories that attribute the difficulties to a low-level problem with auditory temporal processing, and those that propose there is a deficit in a specialised language-learning system. Other accounts emphasise deficits in specific aspects of learning and memory. It can be difficult to choose between theories because they do not always make distinctive predictions, and there is considerable heterogeneity among children with DLD. It has also been suggested that DLD may only arise when more than one underlying deficit is present. Developmental learning disorder in adults
Relatively little research has been conducted to test the outcomes of DLD in adults. In a study comparing 17 men with DLD to siblings without DLD, researchers found that the DLD men had normal intelligence with higher performance IQ than verbal IQ. The participants still exhibited a severe and persisting language disorder, severe literacy impairments, and significant deficits in theory of mind and phonological processing. Within the DLD cohort, higher childhood intelligence and language were associated with superior cognitive and language ability at final adult outcome. In their mid-thirties, the DLD cohort had significantly worse social adaptation (with prolonged unemployment and a paucity of close friendships and love relationships) compared with both their siblings and National Child Development Study (NCDS) control cohorts, matched on childhood IQ and social class. Self-reports showed a higher rate of schizotypal features but not schizoaffective disorder. Four DLD adults had serious mental health problems (two had developed schizophrenia).