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The Misnomer of “High Functioning Autism”: IQ Is a Weak Predictor of Functional Abilities at the Time of Diagnosis of Autism Spectrum Disorder
‘High functioning autism’ is a term often used interchangeably for individuals with autism spectrum disorder (ASD) without an intellectual impairment (or Intellectual Disability, ID). Over time, this term has become synonymous with expectations of greater functional skills and long-term positive outcomes, despite contradictory clinical observations. Because an adaptive functioning assessment is not required for a diagnostic evaluation in many settings, it is often assumed that an individual’s cognitive estimate (intelligence quotient, IQ), particularly those IQs that are >70, approximate functional adaptive ability. While there has been some research investigating the relationship between IQ and adaptive functioning in ASD, very little has focused on individuals at the time of diagnosis. Understanding whether IQ is an appropriate proxy for adaptive functioning has important clinical implications for service provision and funding allocated for supports.
Objectives: This objective of this study was to investigate the relationship between adaptive behavior, IQ and age at diagnosis in ASD.
Methods: Participants part of an ongoing prospective register in Australia (The Western Australian Register for Autism Spectrum Disorder), notified by their diagnosing clinician(s) at the time of diagnosis. Established in 1999, the register currently contains details of more than 6000 cases, with information collected about diagnostic criteria met, co-occurring conditions, basic demographic data, and details of any diagnostic, cognitive, language and adaptive functioning assessments conducted. Cases were selected if they were ≤ 18 years at diagnosis, had a cognitive estimate and adaptive functioning scores recorded. This left a sample of n = 2222, which were then grouped by presence (n = 1039) or absence (n = 1183) of ID. Adaptive functioning was reported using the Vineland Adaptive Behavior Scales (VABS).
Results: As expected, VABS scores were significantly lower in the ID group, and IQ estimates significantly correlated with adaptive functioning scores across the whole sample. However, weaker sized correlation coefficients were observed between IQ and VABS in the group without ID, particularly for the Socialization subscale which fell below the threshold for statistical significance. Regression models suggested that IQ was a weak predictor of VABS composite and subscale scores after controlling for sex for children without ID. When comparing difference scores between IQ and VABS scores, the ID group’s adaptive behavior estimates were close to reported IQs, while VABS scores fell significantly below IQs for children without ID. The gap between IQ and VABS scores increased with increasing age at diagnosis for all children.
Conclusions: These data indicate that estimates from IQ alone are an imprecise proxy for functional abilities when diagnosing ASD, particularly for those without ID. We argue that ‘high functioning autism’ is an inaccurate clinical descriptor when based solely on IQ and this term should be abandoned in research and clinical practice. We also recommend functional adaptive assessments be considered a key component of a comprehensive diagnostic evaluation, to facilitate more targeted service delivery and provision of adequate funding for the level of support and needs required on an individual basis.