Although one in five children have a language delay, it is surprising how often children are misdiagnosed as having an autism spectrum disorder or low IQ. The reasons are several: greater publicity for and education of professionals about the early signs of autism vs. speech difficulties, the shortage of speech language pathologists in the schools and everywhere, the language-dependence of most social interactions (kids who can't speak well have a harder time making friends) and intelligence testing, school systems and medical professionals who don't diagnose dyslexia, and "assortative mating" in some high tech corridors like Redmond / Microsoft or Silicon Valley where spatial / mathematical / engineering / computer-industry men marry spatial / mathematical / engineering / computer-industry women and have children that are brilliant, creative, and analytical, but also intense, self-sufficient, and content to play for extended times by themselves. Economist Thomas Sowell was driven to write Late-Talking Children and Edison Trait when his son (now an adult) was misdiagnosed as having autism because many of the behavioral traits of late-talking children (reduced socialization, eye contact, sensory seeking) can resemble high functioning autism. These children are often those whose parents say, he's affectionate at home, but has a hard time answering open-ended questions.
This study (figure above) provides insight into some of the whys of speech delay. When late-talking children listened to their mother's voices "recalling relatives, pets, and events; and singing familiar songs", they listened with their right language areas rather than their left (controls). We've blogged on some the characteristics of right-hemispheric language (Searching for the Right Word in the Right Brain); gifted dyslexic storytellers (here) also tend to have a right hemispheric pattern of expression - cinematic (immersive, multisensory, rich in associations), but often non-linear. The fMRI appearance of autistic subjects, on the other hand, is very different from this. Language problems associated with autism showed reduced connectivity with sentence comprehension tasks, but the activity is still on the left hemisphere, not the right. For more on this, look here.
Late-talking children often can be distinguished from autism because of their normal affiliative or social drive and social mirroring ability. Their language strengths and weaknesses are also quite different been non-autistic late talking children and autistic late-talking children. Most mistakes are made when children are diagnosed with behavioral checkslists rather than a detailed professional assessments.
Sensory processing behaviors occur in too many diverse groups (late-talking children, dyslexics, preemie birth, autism) to have any specificity in diagnosis.
Problem school behaviors are also non-specific because children struggling to perceive social cues (visual, auditory, or language-based) won't easily be able to resolve conflicts or other social dilemmas with words alone.
Finally, if you are a parent of a young late-talking child, note this advice from Dr. Rescorla and colleagues: "...it may be that todderlhood represents a grace period for many children with language delay, inasmuch as they do not also manifest significant psychological maladjustment. However, as they get older, those who are unable to communicate through language may develop behavior symptoms, such as withdrawal, anxiety, or aggression, or their parents may start to become more aware that their behavior is problematic..."
Speech-Language Impairment: How to Identify the Most Common and Least Diagnosed Disability of Childhood
Visuo-spatial processing and executive functions in children with specific language impairment
Speech delay processes language in the right hemisphere
American Academy of Pediatrics: How can I tell if my child has language delay?
Blessings and Burdens of High IQ
Spatially Gifted at a Loss for Words pdf
Language Delay and Behavioral / Emotional Problems
Eide Neurolearning Blog: Spatial Cognition