Are Most Autistic People Low Functioning? The Answer is NO!
Which made me wonder - what's the truth? I always figured that high functioning is more common, just like mild cognitive disability is more common than severe cognitive disability, because we're the tail end of a normal spectrum. But does the data back me up on that?
Now, it's important to keep in mind that low functioning autism tends to be diagnosed earlier and more easily than high functioning. So if we look only at already-diagnosed autistic people, low functioning will be over-represented. I will be looking at autism screening studies on a random selection from the general population, finding ones that a) provide some information relevant to functioning level, and b) used a design capable of detecting both extremes of functioning (eg screened children older than toddler-aged, did not select based on a sign of good functioning such as attending mainstream schools). In addition, I will only be looking at studies published in 1990 or later, and they must have found at least 15 autistic people.
Next question is how to define functioning level. I've seen a number of definitions - IQ score, language level, adaptive functioning, even presence or absence of certain behavior problems. It gets complicated. In this analysis, I'll be focusing on IQ score, language level and adaptive functioning, using the following definitions:
High functioning autism (all three of these):
* normal or above average baseline verbal functioning (though may have nonverbal episodes due to overload or other issues)
* normal or above average IQ
* adaptive functioning is mildly impaired or better (note - studies usually find a gap in IQ and adaptive functioning among autistic people with normal IQ)
Low functioning autism (at least two of these):
* baseline minimally verbal or nonverbal (though may use AAC devices)
* IQ score less than 50
* adaptive functioning is moderately to profoundly impaired
Anyone who doesn't meet criteria for either group is medium functioning.
If data is given on only one of these metrics, I'll base my judgments of functioning level on that metric alone, and make it clear that I'm doing so. Unfortunately, I only found three relevant studies, and all reported solely on IQ.
The first study was performed in a South Korean community in 2011. They screened both a random population sample of 7-12 year old children and a high risk sample, but I'll only discuss the general population sample here. They found a prevalence of 2.64% autistic kids in the general population sample, and ascertained 201 children.
In this study, functioning level data was based on IQ. The autistic kids from the general population sample had an average IQ of 98, which is clearly in the normal range. Indeed, an IQ of 98 is not significantly different from the general population average of 100. Only 16% of the children had an IQ less than 70, with the percentage less than 50 not being reported (note - in the general population, 3.5% have an IQ less than 70). Indeed, 12% had IQs over 120, in the high-average to gifted range. Therefore, the proportion of autistic kids in this sample who are high functioning is estimated at 84%.
The next study screened children in two UK communities in 2001. They screened 2.5 to 6.5 year old children. They found a prevalence of .6% and ascertained 97 children.
In this study, functioning level data was based on IQ. They do not report the average IQ, but 25.8% of the children had an IQ (or DQ, for the younger children) less than 70, with the prevalence of IQ less than 50 not being reported. Therefore, the proportion of autistic kids in this sample who are high functioning is estimated at 74.2%.
The last study was performed in Toyota, Japan in 2008. All children were screened for autism at 18 month and 36 month check-ups. They found a prevalence of 1.81% and ascertained 228 autistic children.
In this study, functioning level data was based on IQ. They found that 66.4% had an IQ of 70 or higher, and 16.1% had an IQ of 50 or less. Therefore, the proportion of autistic kids in this sample who are high functioning is estimated at 66.4% and the proportion who are low functioning is estimated at 16.1%.
The three studies all found a very high proportion of high functioning children, with 66-84% of the autistic kids having an IQ over 70. While not all of these children will be high functioning according to my criteria, most probably are. In addition, the older the sample of children, the higher the proportion of high functioning children, suggesting that HFA may be more difficult to diagnose in children under age 4, or that some kids may move from medium-low functioning to high functioning during this period. However, even the Japanese study, which performed its' second screening at 3 years of age, found a majority of children with average IQs.
Only the Japanese study provided data on how many autistic children had an IQ of less than 50, finding 16.1%. However, the other two studies almost certainly had even lower prevalence - particularly the South Korean study, which found 16% with IQs below 70.
Clearly, those commenters are wrong. Descriptions of high functioning autism are actually a far better representation of the majority of autistic people than descriptions of low functioning autism. Despite the scare tactics used by many 'awareness' campaigns, most autistic people have an average IQ. The severe, low functioning end is actually a minority among autistic children.