In a Pediatrics article published in May, scientists from the Harvard School of Public Health reported a strong connection between common pesticides and the diagnosis of ADHD. In the study, children between the ages of 8 and 15 who had above-average levels of organophosphate traces in their urine were twice as likely to later be diagnosed with ADHD. And kids with high levels of the compound were 93 percent more likely to have the diagnosis than children with undetectable levels.
The study was the largest to date to look at the relationship between pesticides and children’s behavior, but it wasn’t the first to raise concern. Organophosphates are widely used (the researchers say that approximately 40 types are registered with the Environmental Protection Agency, mainly for use in agriculture), and the chemicals can make their way into our bodies through what we eat.
The pesticides work by damaging insect nervous systems. Naturally, scientists have wondered for a while whether these compounds might have a neurological impact on humans as well, and in fact, previous studies have found that they can cause hyperactivity in lab animals.
Studies like this one make a lot of parents nervous about the chemicals their children may be unintentionally ingesting – and with good reason. Organophosphates have been found in produce kids often eat, like conventionally grown blueberries, strawberries, and celery. And it’s not just pesticides that have been linked to ADHD; lead and cigarette smoke, among other factors, have also been singled out.
With all the talk of environmental toxins, however, we sometimes lose sight of the fact that ADHD is one of the most heritable psychiatric disorders there is, meaning that, to a large extent, genes are responsible for whether and how it shows up. ADHD has a heritability of about 76 percent, while depression, for example, is estimated at 50 percent.
And this is where teasing apart the nature-nurture debate over ADHD can make your head spin. As I pointed out in a recent Babble column on the new genetics of autism, psychiatric disorders are not straightforward. There isn’t one single gene for the disorder – scientists believe there are actually many involved, combining in different ways depending on the child.
The environment just adds another layer of complexity to the question of what causes ADHD. An endless number of factors contribute to a child’s particular psychological make-up – not just the chemicals they ingest, but family stressors, their educational environment, etc. And every child will react uniquely to these variables.
Which is why studies of toxins are never neat and tidy. We can’t take one group of kids and give them a daily dose of veggies grown in pesticide-steeped soil, keep the others on a steady diet of farmers market produce, and see how the two groups turn out. Even if we could, the rest of life would cloud the picture. The map of how genes, life events, and personality traits are connected is infinitely complex.
So the quest goes on to piece together the genetic underpinnings of ADHD, as does the push to figure out which chemicals might play a role. In Europe, restrictions are tighter; last year 22 pesticides were prohibited. In this country, it takes an overwhelming amount of evidence to enact a federal ban on a particular compound, so to some extent we have to take matters into our own hands.
In my family, I try to buy organic versions of the fruits and veggies known to hold on to residue (known as the “Dirty Dozen”), and I wash the other ones well. I do this knowing that my son will inevitably be exposed to all kinds of things in the environment that I wish he wasn’t. But I don’t worry too much that any one factor will determine his personality or abilities; I try to remember that the picture is much more complicated than that.