Prevalence of Autism Spectrum Disorders Among Children With Intellectual Disability.

Tonnsen et al. (2016) counted how many children with intellectual disability also meet autism criteria. They looked at records for over 2,000 kids in South Carolina who already had an ID label.

The team used standard autism screeners and diagnostic rules. They wanted to know the overlap rate and whether the kids with both labels looked different from kids with autism only.

Eighteen out of every 100 children with ID also had autism. That is almost one in five.

These kids showed more severe social and behavior problems than peers who had autism without ID. Expect a more complex profile when the two labels occur together.

Tsakanikos et al. (2006) and Porter et al. (2008) asked the same question in adults. They found no extra mental-health risk once ability level was held constant. The adult data extend the child count across the lifespan and calm fears that autism plus ID always means more psychiatric illness.

Ratcliffe et al. (2015) zoomed in on school-age kids. They showed that poor social skills, common in the ASD-plus-ID group, strongly predict later mental-health problems. Together the studies say: watch social skills, but do not assume extra psychiatric disease is inevitable.

Jin et al. (2018) ran a similar head-count in Shanghai. Most children they caught had very low IQ, echoing the severe profile seen in L et al. The two papers give clinicians worldwide benchmarks: expect roughly one-fifth of kids with ID to also meet ASD rules, and plan for greater symptom intensity.

When you assess a child already labeled with ID, keep autism on your checklist. One in five will meet criteria, and they will likely need stronger supports for language, social, and behavior goals. Build social-skills teaching into the plan, track progress closely, and share the adult-study comfort with families: having both labels does not doom the person to extra mental-health trouble.