{"id":565,"date":"2010-05-18T23:11:28","date_gmt":"2010-05-19T05:11:28","guid":{"rendered":"http:\/\/www.talyarkoni.org\/blog\/?p=565"},"modified":"2010-05-18T23:11:28","modified_gmt":"2010-05-19T05:11:28","slug":"a-possible-link-between-pesticides-and-adhd","status":"publish","type":"post","link":"https:\/\/talyarkoni.org\/blog\/2010\/05\/18\/a-possible-link-between-pesticides-and-adhd\/","title":{"rendered":"a possible link between pesticides and ADHD"},"content":{"rendered":"<p>A <a href=\"http:\/\/pediatrics.aappublications.org\/cgi\/content\/abstract\/peds.2009-3058v1\">forthcoming article in the journal Pediatrics<\/a> that&#8217;s been getting a lot of <a href=\"http:\/\/news.google.com\/news\/search?q=adhd+pesticides\">press attention<\/a> suggests that exposure to common pesticides may be associated with a substantially elevated risk of ADHD. More precisely, what the study found was that elevated urinary concentrations of <a href=\"http:\/\/en.wikipedia.org\/wiki\/Organophosphates#Organophosphate_pesticides\">organophosphate metabolites <\/a>were associated with an increased likelihood of meeting criteria for an ADHD diagnosis. One of the nice things about this study is that the authors used archival data from the (very large) <a href=\"http:\/\/www.cdc.gov\/nchs\/nhanes.htm\">National Health and Nutrition Examination Survey<\/a> (NHANES), so they were able to control for a relatively broad range of potential confounds (e.g., gender, age, SES, etc.). The primary finding is, of course, still based on observational data, so you wouldn&#8217;t necessarily want to conclude that exposure to pesticides <em>causes <\/em>ADHD. But it&#8217;s a finding that converges with previous work in animal models demonstrating that high exposure to organophosphate pesticides causes neurodevelopmental changes, so it&#8217;s by no means a crazy hypothesis.<\/p>\n<p>I think it&#8217;s really pleasantly surprising to see how responsibly the  popular press has covered this story (e.g., <a href=\"http:\/\/www.cbsnews.com\/stories\/2010\/05\/17\/tech\/main6492410.shtml\">this<\/a>,  <a href=\"http:\/\/www.theatlantic.com\/science\/archive\/2010\/05\/pesticides-linked-to-adhd\/56831\/\">this<\/a>,  and <a href=\"http:\/\/featuresblogs.chicagotribune.com\/features_julieshealthclub\/2010\/05\/study-common-pesticides-linked-to-adhd.html\">this<\/a>).  Despite the obvious potential for alarmism, very few articles have led  with a headline implying a causal link between pesticides and ADHD. They  all say things like &#8220;associated with&#8221;, &#8220;tied to&#8221;, or &#8220;linked to&#8221;, which  is exactly right. And many even explicitly mention the size of the  effect in question&#8211;namely, approximately a 50% increase in risk of ADHD  per 10-fold increase in concentration of pesticide metabolites. Given  that most of the articles contain cautionary quotes from the study&#8217;s  authors, I&#8217;m guessing the authors really emphasized the study&#8217;s  limitations when dealing with the press, which is great. In any case,  because the basic details of the study have already been amply described  elsewhere (I thought this <a href=\"http:\/\/www.cbsnews.com\/stories\/2010\/05\/17\/tech\/main6492410.shtml\">short  CBS article<\/a> was particularly good), I&#8217;ll just mention a few random  thoughts here:<\/p>\n<ul>\n<li>Often, epidemiological studies suffer from a gaping flaw in the sense   that the more interesting causal story (and the one that prompts media   attention) is far less plausible than other potential explanations (a   nice example of this is the recent work on the <a href=\"http:\/\/www.talyarkoni.org\/blog\/2010\/04\/30\/shalizi-on-the-confounding-of-contagion-and-homophily-in-social-network-studies\/\">social contagion of   everything from obesity to loneliness<\/a>). That doesn&#8217;t seem to be the   case here. Obviously, there are plenty of other reasons you might get a   correlation between pesticide metabolites and ADHD risk&#8211;for instance,   ADHD is substantially heritable, so it could be that parents with a   disposition to ADHD also have systematically different dietary habits   (i.e., parental dispositions are a common cause of both urinary   metabolites and ADHD status in children). But given the aforementioned   experimental evidence, it&#8217;s not obvious that alternative explanations   for the correlation are much more plausible than the causal story   linking pesticide exposure to ADHD, so in that sense this is potentially   a very important finding.<\/li>\n<li>The use of a dichotomous dependent variable (i.e., children either meet criteria for ADHD or don&#8217;t; there are no shades of ADHD gray here) is a real problem in this kind of study, because it can make the resulting effects seem deceptively large. The intuitive way we think about the members of a category is to think in terms of prototypes, so that when you think about &#8220;ADHD&#8221; and &#8220;Not-ADHD&#8221; categories, you&#8217;re probably mentally representing an extremely hyperactive, inattentive child for the former, and a quiet, conscientious kid for the latter. If that&#8217;s your mental model, and someone comes along and tells you that pesticide exposure increases the risk of ADHD by 50%, you&#8217;re understandably going to freak out, because it&#8217;ll seem quite natural to interpret that as a statement that <em>pesticides have a 50% chance of turning average kids into hyperactive ones<\/em>. But that&#8217;s not the right way to think about it. In all likelihood, pesticides aren&#8217;t causing a small proportion of kids to go from perfectly average to completely hyperactive; instead, what&#8217;s probably happening is that the entire distribution is shifting over slightly. In other words, <em>most<\/em> kids who are exposed to pesticides (if we assume for the sake of argument that there really is a causal link) are becoming <em>slightly<\/em> more hyperactive and\/or inattentive.<\/li>\n<li>Put differently, what happens when you have a strict cut-off for diagnosis is that even small increases in underlying symptoms can result in a qualitative shift in category membership. If ADHD symptoms were measured on a continuous scale (which they actually probably were, before being dichotomized to make things simple and more consistent with previous work), these findings might have been reported as something like &#8220;a 10-fold increase in pesticide exposures is associated with a 2-point increase on a 30-point symptom scale,&#8221; which would have made it much clearer that, at worst, pesticides are only one of many other contributing factors to ADHD, and almost certainly not nearly as big a factor as some others. That&#8217;s not to say we shouldn&#8217;t be concerned if subsequent work supports a causal link, but just that we should retain perspective on what&#8217;s involved. No one&#8217;s suggesting that you&#8217;re going to feed your child an unwashed pear or two and end up with a prescription for Ritalin; the more accurate view would be that you might have a minority of kids who are already at risk for ADHD, and this would be just one more precipitating factor.<\/li>\n<li>It&#8217;s also worth keeping in mind that the relatively large increase in ADHD risk is associated with a <em>ten-fold<\/em> increase in pesticide metabolites. As the authors note, that corresponds to the difference between the 25th and 75th percentiles in the sample. Although we don&#8217;t know <em>exactly <\/em>what that means in terms of real-world exposure to pesticides (because the authors didn&#8217;t have any data on grocery shopping or eating habits), it&#8217;s almost certainly a very sizable difference (I won&#8217;t get into the reasons why, except to note that the rank-order of pesticide metabolites must be relatively stable among children, or else there wouldn&#8217;t be any association with a temporally-extended phenotype like ADHD). So the point is, it&#8217;s probably not so easy to go from the 25th to the 75th percentile just by eating a few more fruits and vegetables here and there. So while it&#8217;s certainly advisable to try and eat better, and potentially to buy organic produce (if you can afford it), you shouldn&#8217;t assume that you can halve your child&#8217;s risk of ADHD simply by changing his or her diet slightly. These are, at the end of the day, small effects.<\/li>\n<li>The authors report that fully 12% of children in this nationally representative sample met criteria for ADHD (mostly of the inattentive subtype). This, frankly, says a lot more about how silly the diagnostic criteria for ADHD are than about the state of the nation&#8217;s children. It&#8217;s frankly not plausible to suppose that <em>1 in 8<\/em> children really suffer from what is, in theory at least, a severe, potentially disabling disorder. I&#8217;m not trying to trivialize ADHD or argue that there&#8217;s no such thing, but simply to point out the dangers of <a href=\"http:\/\/neuroskeptic.blogspot.com\/2010\/05\/commercialization-vs-medicalization.html\">medicalization<\/a>. Once you&#8217;ve reached the point where 1 in every 8 people meet criteria for a serious disorder, the label is in danger of losing all meaning.<\/li>\n<\/ul>\n<p><span style=\"float: left; padding: 5px;\"><a href=\"http:\/\/www.researchblogging.org\"><img decoding=\"async\" style=\"border:0;\" src=\"https:\/\/i0.wp.com\/www.researchblogging.org\/public\/citation_icons\/rb2_large_gray.png?w=660\" alt=\"ResearchBlogging.org\" data-recalc-dims=\"1\" \/><\/a><\/span><span class=\"Z3988\" title=\"ctx_ver=Z39.88-2004&amp;rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&amp;rft.jtitle=PEDIATRICS&amp;rft_id=info%3Adoi%2F10.1542%2Fpeds.2009-3058&amp;rfr_id=info%3Asid%2Fresearchblogging.org&amp;rft.atitle=Attention-Deficit%2FHyperactivity+Disorder+and+Urinary+Metabolites+of+Organophosphate+Pesticides&amp;rft.issn=0031-4005&amp;rft.date=2010&amp;rft.volume=&amp;rft.issue=&amp;rft.spage=&amp;rft.epage=&amp;rft.artnum=http%3A%2F%2Fpediatrics.aappublications.org%2Fcgi%2Fdoi%2F10.1542%2Fpeds.2009-3058&amp;rft.au=Bouchard%2C+M.&amp;rft.au=Bellinger%2C+D.&amp;rft.au=Wright%2C+R.&amp;rft.au=Weisskopf%2C+M.&amp;rfe_dat=bpr3.included=1;bpr3.tags=Psychology%2CHealth%2CAbnormal+Psychology%2C+Clinical+Psychology%2C+Psychiatry%2C+Epidemiology%2C+Public+Health%2C+Nutrition\">Bouchard, M., Bellinger, D., Wright, R., &amp; Weisskopf, M. (2010). Attention-Deficit\/Hyperactivity Disorder and Urinary Metabolites of Organophosphate Pesticides <span style=\"font-style: italic;\">PEDIATRICS<\/span> DOI: <a rev=\"review\" href=\"http:\/\/dx.doi.org\/10.1542\/peds.2009-3058\">10.1542\/peds.2009-3058<\/a><\/span><\/p>\n","protected":false},"excerpt":{"rendered":"<p>A forthcoming article in the journal Pediatrics that&#8217;s been getting a lot of press attention suggests that exposure to common pesticides may be associated with a substantially elevated risk of ADHD. More precisely, what the study found was that elevated urinary concentrations of organophosphate metabolites were associated with an increased likelihood of meeting criteria for &hellip; <a href=\"https:\/\/talyarkoni.org\/blog\/2010\/05\/18\/a-possible-link-between-pesticides-and-adhd\/\" class=\"more-link\">Continue reading <span class=\"screen-reader-text\">a possible link between pesticides and ADHD<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"jetpack_post_was_ever_published":false,"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"footnotes":"","_jetpack_memberships_contains_paid_content":false,"jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":false,"jetpack_social_options":{"image_generator_settings":{"template":"highway","enabled":false},"version":2}},"categories":[339,6,137],"tags":[340,720,342,690,341,691],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_shortlink":"https:\/\/wp.me\/pEZxN-97","jetpack_sharing_enabled":true,"jetpack-related-posts":[],"_links":{"self":[{"href":"https:\/\/talyarkoni.org\/blog\/wp-json\/wp\/v2\/posts\/565"}],"collection":[{"href":"https:\/\/talyarkoni.org\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/talyarkoni.org\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/talyarkoni.org\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/talyarkoni.org\/blog\/wp-json\/wp\/v2\/comments?post=565"}],"version-history":[{"count":5,"href":"https:\/\/talyarkoni.org\/blog\/wp-json\/wp\/v2\/posts\/565\/revisions"}],"predecessor-version":[{"id":570,"href":"https:\/\/talyarkoni.org\/blog\/wp-json\/wp\/v2\/posts\/565\/revisions\/570"}],"wp:attachment":[{"href":"https:\/\/talyarkoni.org\/blog\/wp-json\/wp\/v2\/media?parent=565"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/talyarkoni.org\/blog\/wp-json\/wp\/v2\/categories?post=565"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/talyarkoni.org\/blog\/wp-json\/wp\/v2\/tags?post=565"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}