Previously in this series:
N=1: Introduction
N=1: Single-Subject Research
N=1: Hidden Variables and Superstition
N=1: Why the Gender Gap in Chronic Illness? 
N=1: Symptom vs. Syndrome
N=1: Latency and Half-Life
N=1: n of Small

I. 

In the old days, psychology was dominated by the school of behaviorism.

Behaviorism taught that mental states like thoughts and feelings are unworthy of study, and possibly don’t exist. 

Behaviorists also thought that animals are born without anything at all in their brains, that the mind at birth is a blank slate, and that everything an animal learns to do comes from pure stimulus-response learning built up over time. Turns out, this is wrong.

At some point in the 1950s, a guy named John Garcia was irradiating Sprague-Dawley rats for his job at the U.S. Naval Radiological Defense Lab, like you do, when he noticed something weird. The rats who had been exposed to low levels of gamma radiation were eating and drinking less than usual, and groups that had been exposed to radiation the most times ate and drank the least. 

Garcia thought that the rats might be learning to associate their food and water with the nausea from radiation exposure. After all, rats have no concept of ionizing radiation, so from their point of view, they were going about their day as normal when they suddenly started feeling nauseous for no clear reason. They might reasonably wonder if it was something they ate. In particular, he noticed that the rats wouldn’t drink out of the plastic bottles they were used to, but were happy to drink out of unfamiliar glass bottles. Garcia thought that maybe the plastic bottles gave the water a particular taste that the rats had learned to avoid. 

So in a series of experiments, Garcia tried exposing rats to different kinds of stimuli to see what they would learn. He discovered two surprises that called the whole behaviorist concept into question. 

First, he discovered that if a rat was exposed to radiation (making it nauseous) after encountering a new food, it would quickly learn to reject the food, even if the radiation came hours later. 

This contradicted the understanding at the time of how conditioning worked — behaviorists thought that you had to present the unconditioned stimulus (nausea) immediately after the conditioned stimulus (the new food), or the animal wouldn’t learn to associate the two. But Garcia found that learning could occur even if the rat got sick well after eating a new food. 

Rats would instantly associate nausea with whatever food they had most recently eaten, and had no problem doing so. If he made them sick after giving them Cheetos, they would learn to reject Cheetos forever. But the rats simply could not learn to associate their nausea with any other kind of stimulus. It didn’t matter if the stimulus was bright lights, or an annoying buzzer. No matter how many times Garcia flashed lights at them, the rats never learned to associate their nausea with the lights.

On the flipside, when he gave the rats electric shocks instead of exposing them to radiation, they would learn to be afraid of the lights and sounds. But no matter how many times he shocked them after eating, the rats would never learn to associate food or water with getting shocked.

This was another big pie in the face of behaviorism. Learning was supposed to be purely stimulus-response, and you were supposed to be able to teach an animal to do just about anything by pairing a behavior with the right reward or punishment. But Garcia’s rats seemed to be hard-wired to associate nausea (from radiation) with what they ate or drank, and similarly hard-wired to associate pain (from electric shocks) with what they saw or heard, and not to associate these things with anything else.

This was confusing to the behaviorists, but makes perfect sense if you think about evolution for even one second. In the real world, rats become nauseous when they eat spoiled food, so it’s important for a rat to associate nausea with things they recently ate. Any rat that doesn’t learn this will be dead, so eventually all rats are born prepared to make these food-nausea associations. Even though Garcia’s rats had been born in a laboratory and had never eaten a bit of ham left out in the sun for too long, they still came with an overwhelming bias to associate a feeling of nausea with whatever they most recently ate.

Similarly, pain is associated with sights and sounds, like the sight of an owl or the sound of a fox; or specific locations, like parts of the forest where predators are common. So rats are born ready to associate pain with things like weird noises or flashing lights. The idea that pain might be related to food, on the other hand, never crosses their minds. 

As you may have guessed, these predispositions aren’t limited to rats. In his review of John Bradshaw’s book on domes⁣tic cat psychology, Cat Sense, Gwern mentions that cats have a similar tendency to associate food with nausea: 

…[cats’] lack of trainability apparently has an exception, Bradshaw states: food can trigger learning of powerful associations even hours after consumption. This would make sense as an anti-bad-food defense, but unfortunately, this is yet another maladaptation in the modern context: “…this mechanism occasionally has unexpected consequences: a cat that succumbs to a virus may then go off its regular food even after it has recovered, because it has incorrectly associated the illness with the meal that happened to precede it.”

More generally this is called conditioned taste aversion, and it occurs in most mammals — though maybe not vampire bats, since they eat only one thing that never spoils, and being put off their food would be a guaranteed death sentence. 

(Some researchers did a version of Garcia’s study where they compared vampire bats with closely related species of bats that eat more than one thing, and while the other bats learned to avoid new flavors that were paired with nausea, vampire bats didn’t learn to associate new flavors with nausea when they were fed different kinds of flavored blood. Just imagine being that researcher on a first date; “Oh, what do I do at work? Yeah, I’m the guy who injects vampire bats with a 1% weight/volume lithium chloride solution to make them nauseous, it’s not much but it’s a living!”)

II.

Humans are also mammals, so we might have the same tendency. Maybe when we feel nauseous, or sick, or even just kind of weird, we assume it’s something we ate or drank. 

Wikipedia thinks this is the case, claiming, “even something as obvious as riding a roller coaster (causing nausea) after eating the sushi will influence the development of taste aversion to sushi,” but doesn’t offer any citations. We suppose you could run this study on your own with a few sushi meals and a season’s pass to INSERT LOCAL THEME PARK.

People often suspect that their chronic illnesses have food triggers, different kinds of food or drink that will bring on an attack or generally make them feel like crap. But if our brains are hard-wired to pick out food-based explanations for feeling ill, maybe we tend to latch onto the idea of some food trigger causing our illness, even when food has nothing to do with it. 

When our ancestors felt nauseous, it was usually because they had eaten the wrong kind of frog, so we come with a strong bias towards assuming that a random feeling of sickness is connected to something we ate. We don’t assume it has anything to do with the awesome glowing rocks we found in that sweet cave.

This worked well up until 3000 BC, but since then humans have discovered and invented lots of new things that can make you sick, most of which are not foods.

In general this should make us more skeptical of food triggers (and food-related triggers like packaging), especially if your chronic complaint is anything related to nausea, anything that feels like an illness, or anything digestive.

Food can still make you sick, and there are for sure some real food triggers out there. But the lesson here is that your instincts will tell you that your random sickness is caused by what you ate, even if it’s actually caused by something completely different. If you were one of Dr. Garcia’s rats, you would never have guessed that you were being hit with gamma radiation. You’d be all like, “it must be some chemical in those nasty plastic bottles.”