Credit: Slug Signorino

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What’s going on with autism? When I was growing up, there seemed to be hardly any autistic kids around; now they’re everywhere. Is this an example of better diagnostic methods or are there more autistic kids around now, and if so, does anyone have any ideas as to why?Joanne in Euless, Texas

If I had a dollar for every letter I get asking about the autism epidemicwell, you’d think I could fund a study definitive enough to stop people writing the letters. It so happens I addressed a question much like Joanne’s in March 2014, but both the study of autism and popular interest in it are still going strong, and now’s not a bad time for an update.

As of that earlier column, it looked like the dramatic increase in autism diagnoses seen in recent decades was due substantially to a broadened definition of autismspecifically, to the decision made in the 1990s to include Asperger’s syndrome and other developmental conditions under the umbrella term autism spectrum disorders. Nearly three years later, that’s still what it looks like: a 2015 study, for instance, found that 60 percent of an apparent ASD upswing in Denmark could be attributed to changes in reporting practices. After the American Psychiatric Association tightened up the criteria for ASDs in the fifth edition of its Diagnostic and Statistical Manual of Mental Disorders, published in 2013, experts figured that between 10 and 40 percent of existing diagnoses wouldn’t meet the new standards. Again, recent data seems to bear this out: the diagnosis rate among kids evaluated for ASD at one Buffalo hospital dropped from 50 percent under the old criteria to 39 percent under DSM-5.

Complexities of diagnosis aside, there’s a fairly clear set of traits that has historically been described as autismsignificant trouble with communication and interaction, repetitive or otherwise rigid behaviorbut thus far little solid consensus about what causes it. One ongoing question has been the relative impact of heredity, genetic mutation, and environmental factors; all are seemingly in play, but our understanding of the balance continues to swing back and forth.

Scientists have long studied autism in twinsthe basic idea being that if identical twins (who share all their genes) are significantly more likely to both have ASD than fraternal twins (who share only half), that tells us something about the genetics-environment relationship. At one point such research suggested that ASD risk might be as much as 90 percent the result of inherited genes, but a surprising 2011 study at Stanford concluded it was only 38 percent, with 62 percent attributable to environmental exposure. More recent studies haven’t settled much: a 2014 Swedish report estimated that autism was 50 percent heritable; a UK paper from last year didn’t pin it down any better than “56 percent to 95 percent.” On this front the jury is apparently still way out.

In the wake of since-discredited findings about mercury in vaccines (and resulting dips in vaccination coverage), it’s hard not to be skeptical when you hear about some substance newly linked to autism; as I said in 2014,“If there actually were an environmental cause of autism, with so many false positives being reported we’d never know.” OK, so that’s a little strong. The Centers for Disease Control is confident enough to state that exposure during pregnancy to the pharmaceuticals valproic acid and thalidomide present a heightened ASD risk, for instance, but those are known to cause birth defects too.

Pesticides make a more worrisome potential culprit. A California study published in Environmental Health Perspectives in 2014 reported that pregnant mothers who lived near fields treated with pesticides called organophosphates were more likely to have kids with ASDsurely a connection worth further investigation. A louder version of this claim, though, came from MIT researcher Stephanie Seneff, who that same year attributed high autism rates (and a host of other ills) to the wide use of glyphosate, a related herbicide found in Monsanto’s weedkiller Roundup, and predicted that by 2025 half of all American children would have autism. Beyond attacking her findings on their merits, critics have characterized Seneff as an anti-GMO zealot who’s strayed too far from her field (her doctorate’s in computer science); likewise those critics have found themselves dismissed as shills for Big Agriculture. All this notwithstanding, so far Seneff has merely shown a correlation between ASD prevalence and glyphosate use on corn and soybeans without demonstrating causation.

Among the biggest recent news about autism, one might argue, was a CDC report in March finding that the ASD rate had remained flat since the previous report two years earlier. But the sense that autism is ever on the rise, borne out by statistics or not, feeds into the suspicion of many parents that some external malignancy has warped their child’s development. In her 2014 book On Immunity, the essayist Eula Biss examines the seductive appeal of anti-vaxxer beliefs, suggesting that ours is a culture that fears the unnatural and seeks to blame it whenever something goes wrong. If autism turns out to be as natural as any other genetic mutation, that just goes to show that not everything in nature has our best interests in mind.Cecil Adams