MIND Institute study offers insight into intelligence change in autistic children over time

A long-term study by UC Davis MIND Institute researchers confirms that changes in the IQ level of autistic children may help predict their path of communication and behavioural development as adolescents.

The new work builds on a previous MIND Institute study of IQ trajectories in autistic children aged between two and eight years, expanding the findings to include older children and teens. 

The study, published in JCPP Advances, has identified three distinct paths of intellectual development in autistic children: persistent intellectual disability, an increase in IQ, or an IQ that remained average or above.

“Given that IQ is perhaps the strongest predicter of later outcomes in autistic children, we believe that studying IQ trajectories in childhood is very important. It provides clues about their potential different future paths and how we can help individuals to flourish,” lead author Professor Marjorie Solomon said. 

The researchers worked with data from the Institute’s Autism Phenome Project, one of the world’s most comprehensive longitudinal studies of its kind, which is following autistic children aged between two years and adolescence.

The study included 373 (115 females, 258 males) autistic participants with all levels of intellectual ability were part of the sample, in the age bracket outlined above. 

Assessments of behaviour and autistic characteristics were collected across childhood. IQ was evaluated at three timepoints: T1 (mean age of three years), T2 (mean age of 5.6 years) and T3 (mean age of 11.5 years).

Clinical psychologists who specialise in autism evaluated the participants using autism assessment tools including the ADOS (Autism Diagnostic Observation Schedule), ADOS-2, ADI-R (Autism Diagnostic Interview – Revised), and the Vineland Adaptive Behavior Scales (VABS).

Based on these assessments, the participants were divided into three subgroups:

• ‘Changers’ described those who began with low IQs in early childhood, followed by a substantial increase that slowed as they entered middle childhood. ‘Changers’ made up 39 per cent of the participants.
• ‘Persistent Intellectual Disability’ described the individuals who began with a below average IQ that persisted across childhood. Around 45 per cent of the participants were in this group.
• ‘Persistently High IQ’ described the individuals who began with an average or above average IQ and remained relatively stable throughout childhood. 16 per cent belonged to this group.

The researchers analysed changes in autism traits and communication adaptive functioning (the ability to understand language, engage in meaningful verbal expression, and read and write) over time.

They also looked at internalising behaviors, such as anxiety or depression, and externalising behaviours, such as impulsivity or aggressiveness.

Of the 191 participants with assessments at two timepoints or more, 10 lost their autism diagnosis. This included about five per cent of the ‘Changers’, 10 per cent of the ‘Persistently High IQ’ group and none of those in the ‘Persistent Intellectual Disability’ group. Identifying what makes the ‘Changers’ group different from those in the groups with more stable IQs is a major goal of the research.

Individuals with stronger early communication adaptive function and lower autism ‘severity’ scores were more likely to be in the ‘Persistently High IQ’ group versus the ‘Persistent Intellectual Disability’ group by adolescence.

Both the ‘Changers’ and ‘Persistent Intellectual Disability’ groups had lower IQ scores in early childhood. However, those that showed improved communication adaptive function and decreased externalising behaviours into adolescence were more likely to be in the ‘Changers’ group compared to the ‘Persistent Intellectual Disability’ group.

“It is striking that we found so much overlap in individuals following different trajectories of intellectual development when assessed at the early childhood and adolescent time points,” Professor Solomon said. “Of course, many other factors are involved in determining outcomes, but intellectual ability level is a core feature and an important starting point.”

“The findings of both studies provide clues about how brain differences between autistic individuals with and without intellectual disability during early childhood might predict future outcomes.”

Read the full study here.