It is generally accepted that<unk>  is strongly influenced by genetic factors. The best evidence comes from the twin study method. Two twins growing up together are exposed to the same home environment, yet may differ radically in their<unk>  skills. Such different outcomes are, however, much more common in fraternal (non-identical) twins, who are genetically different. Identical twins share the same genes and tend to be much more similar in<unk>  ability. There can be some variation in the severity and persistence of<unk>  in identical twins, indicating that non-genetic factors affect the course of disorder, but it is unusual to find a child with<unk>  who has an identical twin with typical<unk> . There was considerable excitement when a large, multigenerational family with a high rate of<unk>  were found to have a mutation of the FOXP2 gene just in the affected family members. However, subsequent studies have found that, though<unk>  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<unk>  learning, and<unk>  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<unk> .<unk>  disorders are associated with aspects of home environment, and it is often assumed that this is a causal link, with poor<unk>  stimulation leading to weak<unk>  skills. Twin studies, however, show that two children in the same home environment can have very different<unk>  outcomes, suggesting we should consider other explanations for the link. Children with<unk>  often grow up into adults who have relatively low educational attainments, and their children may share a genetic risk for<unk>  disorder. One non-genetic factor that is known to have a specific impact on<unk>  development is being a younger sibling in a large family. Associated factors It has long been noted that males are more affected by<unk>  than females, with a sex ratio of affected males: 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. The reason for the sex difference is not well understood. Poor motor skills are commonly found in children with<unk> . Standardized measures of motor ability confirm that children with<unk>  exhibit deficits in fine and gross motor skill, both simple and complex. These difficulties also extend to speech-motor ability, particularly with the control of their articulatory movements. Children with<unk>  have difficulty with motor sequence learning and may show deficits in other procedural motor processes as well. Brain scans do not usually reveal any obvious abnormalities in children with<unk> , although quantitative comparisons have found differences in brain size or relative proportions of white or grey matter in specific regions. In some cases, unusual brain gyri are found. To date, no consistent 'neural signature' for<unk>  has been found, although some studies have noted evidence for involvement of subcortical systems. Differences in the brains of children with<unk>  vs typically developing children are subtle and may overlap with atypical patterns seen in other neurodevelopmental disorders.
Developmental language disorder
Expressive language disorder
Developmental disability
Pervasive developmental disorder
Multiple complex developmental disorder
Nonverbal autism
Low-functioning autism
Mixed receptive-expressive language disorder