We investigated the relationship of discrepancies between VIQ and NVIQ (IQ split) to autism symptoms and adaptive behavior in a sample of high-functioning school-age children with ASDs. We used two methods: comparison of extreme groups of children with VIQ > NVIQ and NVIQ > VIQ; and using the full sample, examination of correlations of the IQ indices (VIQ, NVIQ, and VIQ–NVIQ discrepancy) with autism symptoms and adaptive behavior. Using both methods, we find that discrepantly high NVIQ is associated with greater social symptoms in ASD. This result replicates and extends the findings of Joseph et al. (2002)
to a higher IQ sample of children with ASDs (mean FSIQ = 98.5 in the current study compared to 84.5 in the Joseph et al. school-age sample).
Our findings potentially qualify Joseph et al.’s (2002)
original findings that discrepantly higher NVIQ was associated with more autism social symptoms. We found the same pattern in our overall findings, but when using a scatterplot to examine the relationship between VIQ–NVIQ discrepancy and autism social symptoms within each of the IQ discrepancy groups, we found that the discrepancy between VIQ and NVIQ in either direction (favoring either VIQ or NVIQ) was associated with more autism social symptoms. These findings suggest that although strong verbal abilities are often associated with better overall functioning in autism (particularly for autism communication symptoms), as VIQ becomes increasingly discrepant from NVIQ it is also associated with considerable social difficulties as well.
Our group analysis was complicated by the fact that FSIQ, driven by VIQ, was significantly higher in the VIQ > NVIQ group than in the NVIQ > VIQ group. To address differences in FSIQ as a possible explanation for our primary findings, we re-ran our primary analyses in a subsample with all groups matched on FSIQ. These findings were comparable to our original findings and suggest that group differences in social symptoms were not due to differences in FSIQ. In addition, it was the split between VIQ and NVIQ that was related to social symptoms in the correlations within the full sample, underscoring the importance of the discrepancy in IQ scores to social symptoms in this sample. That IQ split was not correlated with autism communication symptoms, while both VIQ alone and NVIQ alone were, suggests that VIQ–NVIQ discrepancy is uniquely related to autism social symptoms.
Similar to Joseph et al.’s (2002)
findings but in contrast to Klin et al.’s (2007)
more recent report on two separate samples, we find that higher VIQ and NVIQ were associated with less severe communication symptoms. Reasons for discrepant findings among these four samples are not immediately clear. Both Klin et al.’s samples have IQ ranges similar to the current study whereas mean IQ in Joseph et al.’s school-age sample is about one standard deviation lower. In comparing symptom severity (reflected in the ADOS scores), Joseph et al.’s sample may have somewhat more symptoms while the study presented here was more similar to Klin et al.’s samples. The lack of consensus among these studies concerning the relationship between IQ and autism communication symptoms requires additional research, perhaps with a longitudinal design to more definitively address this question.
Our investigation of social, communication, and daily living abilities, as measured by the Vineland Adaptive Behavior scales (VABS), reveals a different pattern. IQ split was not related to socialization skills, nor were the extreme IQ split groups different in their social abilities. Consistent with previous reports (Bolte and Poustka 2002
; Klin et al. 2007
), VIQ and NVIQ were strongly correlated with adaptive communication abilities. Taken together, these findings suggest that both VIQ and NVIQ are related to adaptive communication, arguing that communication skills are related to overall cognitive ability among children with ASDs and average range IQ.
There are a number of limitations to the current study. Specifically, differences in FSIQ across the three IQ split groups qualifies interpretation of our primary finding that as the discrepancy between VIQ and NVIQ increases autism social symptoms increase. We have tried to address this by re-analyzing our data using a subset of subjects from each group that is matched on FSIQ. Because this was a smaller sample, limiting statistical power, we compared the effect size of these results to the original findings as described above. Second, this is a cross-sectional study, which limits any speculation about causal mechanisms. Rather, we can only conclude that among children with ASDs, those with a significant discrepancy between VIQ and NVIQ are also likely to have more social symptoms. Future studies using prospective longitudinal designs quantitatively assessing autism symptoms and neurocognitive profiles over time are needed to address questions concerning causality. Finally, our reliance on a clinical sample for this investigation introduced three different versions of the Wechsler Intelligence Scales into the data set, each with somewhat different subtests. As a result, we relied on two subtests to determine VIQ and NVIQ, respectively. Although these abbreviated scores are highly correlated with more comprehensive Wechsler IQ scores (verbal measures correlate with VCI/VIQ > .7 and non-verbal measures correlate with PRI/PIQ > .56 across Wechsler scales [Wechsler 1991
]), future investigations using one comprehensive IQ measure may be useful.
These findings have several implications. They support earlier investigations proposing IQ split as a potentially informative marker for subgroups in ASD. The findings also raise the importance of scrutinizing social skills in children with ASD and high VIQ who, due to large vocabularies and fluent speech, may appear to be more competent in social settings than is actually the case. Finally, these data support Klin et al.’s (2007)
argument that social ability and disability are independent constructs and should both be studied in ASD.