Much of the public debate, particularly in the United States, has been discussion of what has caused the "epidemic of autism." Various environmental causes have been suggested, often without an accompanying scientific evidence base—most notably thimerosal-containing vaccines (4) and, in the United Kingdom, the measles, mumps, and rubella (MMR) vaccination (5). There may have been a real rise in autism prevalence, and (unknown) environmental factors may have played a role in this rise, but, put simply, we do not know that for sure. In the scientific literature, the possible contributory factors that might explain higher measured prevalence in more recent epidemiological studies have been well rehearsed, and some of them are summarized in the article. In brief, over time the diagnostic criteria have broadened, the diagnosis of ASDs has been applied to children previously excluded (those with severe intellectual disability, above-average intelligence, genetic syndromes, or sensory impairments), and prevalence study designs have improved and therefore capture more cases. One reason why the question "Has there been a real increase in autism prevalence?" remains unanswered is that we cannot quantify whether these factors account for all of the apparent increase in measured prevalence over time. However, do these methodological factors help us understand the high prevalence figure found by Kim et al., and do they help us answer the question of why it's so hard to count autism?