The incidence of autism has increased from 1 in 10,000 in the 1970s to 1 in 150 today, an increase of over 6,000%. Many more children have been diagnosed with other neurodevelopmental disorders all considered to be on the same spectrum including Asperger's, ADHD/ADD, speech delay, and many other developmental delays and learning disabilities.
The refutation of this argument is well-known and straight forward: The prevalence of 1 in 150 includes Asperger's syndrome (no speech delay) whereas the definition of autism in the 1970s did not. Another important point is that these numbers represent prevalence, not incidence. In general, a formal way to refute it is to point out that the groups are not equivalent.
Pointing out that the groups are not equivalent means that the prevalence of traits in one group is not comparable to that of the other group. For example, in one group, prevalence of speech delay is much higher than in the other. Prevalence of mental retardation (according to Weschler) is probably much higher in the 1970s group.
In simpler terms, you can't compare the prevalence of apples to the prevalence of oranges, and then claim that the incidence of fruits has changed.
General application of this principle
I believe it's possible to make this argument from quarter to quarter, and from year to year. For example, the group of autistics in the CDDS in December 2002 is not equivalent to the group of autistics in the CDDS in December 2003. And this is demonstrably true. I'll list four groups below indicating trait proportions; namely, incidence of epilepsy and profound mental retardation (PMR) among autistics:
Group 1 - Dec. 2002: Epilepsy: 7.80%, PMR: 3.23%
Group 2 - Dec. 2003: Epilepsy: 7.42%, PMR: 2.86%
Group 3 - Dec. 2004: Epilepsy: 7.10%, PMR: 2.59%
Group 4 - Dec. 2005: Epilepsy: 6.73%, PMR: 2.35%
Not only is it crystal clear that these 4 groups are not equivalent, it would appear that autistics in the CDDS keep getting higher and higher functioning with every year that passes, with no end in sight. If this continues indefinitely, the CDDS will have to rename the 'Autism' category to 'NT'.
Claiming that the incidence of autism is increasing based on CDDS prevalence data suffers from a critical flaw. No such conclusion can be derived from the data, given that the group of autistics in one quarter is not equivalent to the group of autistics the next quarter.