 In 1943, a famous paper was published in which a child psychiatrist at Hopkins described a series of children with so-called fascinating peculiarities. He thought these characteristics formed a unique syndrome. He called autism, which seemed rare at the time, but who knew how many were out there undiagnosed? What causes it? Autism is currently considered a multifactorial disorder resulting from both genetic and non-genetic risk factors. Yes, it can run in families, but genetic factors may account for only 10 to 20% of autism cases. This is based in part on the fact that you can have identical twins with identical DNA, the same genes. And one identical twin may have autism, and the other not. So while genetic susceptibility may be a key contributor to these autism spectrum disorders, it may just load the gun, so to speak. With prenatal, perinatal, and postnatal environmental exposures, some kind of exposures during, around, or after pregnancy being the events that pulled the trigger may actually give rise to the disease. This is good news. I mean, the larger role these non-genetic factors play in causing autism, the more modifiable the risk factors may be, potentially opening up avenues for the primary prevention of autism in the first place. Since autism as a medical condition was first described, the prevalence of autism has apparently exploded from like 1 in 5,000 individuals to 1 in 68 now, more than 1% of the population. That would be like a 7,000% increase. And indeed, you'll see graphs like this, showing an exponential increase in the prevalence of autism from like no diagnosed cases in the early 1900s, to the prevalence shooting through the roof in the 80s and 90s. And so, that immediately gets you thinking, what happened around that time that could account for the explosion? But wait a second. Of course, there were no diagnosed cases in the early 1900s. It didn't even have a name until 1943. As Kanner said in the original paper, there's probably more cases out there, but they just hadn't been looking. So this isn't a graph of the prevalence of autism. It's a graph of the prevalence of autism diagnoses, and that depends on what diagnostic criteria you're using and whether you're out there looking for it or not. Put another way, historical prevalence estimates for autism and its rarity might well have been underestimates of the true prevalence back then, may have just been missing lots of cases. The increased recognition among doctors and society at large the broadening of the diagnostic concept over time and different studies using different criteria may account for much of the apparent increase in prevalence, although this can't be quantified. So before we start speculating about the reason for the explosive increase, maybe we should first make sure the explosion is real. The bottom line is that, well, we may never really know what the prevalence was half a century ago. We do have decent data over the last few decades that really does point to a considerable increase in the true prevalence. So yeah, maybe there wasn't actually a 22-fold increase in autism in the 80s and 90s. Maybe there was actually only an 8-fold increase. So yeah, we may quibble whether the increase was 800% or closer to 2000% by the bottom line. Seems to be autism rates are really increasing. And so the question legitimately then turns to why, which we'll explore next.