I'm sure we've all seen it. Kid A is playing with a toy, and the next thing you know, Kid B wants it, too. Even when there are other toys around, Kid B is no longer satisfied. He wants what Kid A has got, a phenomenon called "mimetic desire", in which one person's desire mimics that of another. Mimetic desire isn't limited to children, we've all wanted that new computer once we've seen what someone else has. The selling of luxury items has depended upon this for ages: did you really want ice cream? Or did it look a lot better once everyone else had some?

But the question is, what occurs in the brain to produce this feeling? What is responsible for mimetic desire?

Lebreton et al. "Your Goal IsMine: UnravelingMimetic Desires in the

Human Brain" The Journal of Neuroscience, 2012.

Mimetic desire is more than jealously wanting something because someone else has it. Rather, it's about valuing something because someone else values it. And it's pretty easy to transmit the value. Just writing about Person A's activities and habits and showing it to Person B will make Person B start to think Person A must have seen something good about the Toyota Camry...maybe his next car...

But what is behind this contagion of desires? The authors of this study wanted to look at two brain areas in humans: the mirror neuron system and what they call "the valuation system". First, the mirror neuron system is really two areas, the parietal lobules and the premotor areas (where motor signals go before they hit the actual motor command and your muscles start moving). The neurons in these areas respond under two conditions: when you yourself are performing an action, and when you see someone else performing an action. We call it the mirror neuron system because it "mirrors" the behavior of someone else.

The second system, the "valuation system", overlaps a lot with reward related areas, including the striatum and the medial prefrontal cortex. These are areas which help you determine the value of a given items (say, to a rat, one pellet vs five), and act accordingly.

So where do the mirror neuron system and the valuation system come together? The authors of this study hypothesized that these systems might be linked in cases of mimetic desire. To test this, they took 116 subjects and put them in an fMRI machine, to look at oxygenated blood flow as the participants perform a task.

The task was simple, the participants were given a video showing two items, the only difference being color. In the video, an unseen person selected one of the items (the color was switched between participants to control for things like color preference). At the end of the task, the participants had to rate how much they liked the item.

You can see above the trial, using different colored gummi sharks (I really hope they gave them a sample after! Otherwise I'd have to make a stop by the candy store). After rating how much they liked it, they were tested on recall, putting the object next to another object (say, a red gummi), and asked to show which was familiar.

A simple task. But a powerful one.

Above are the ratios for G (goal) vs NG (no goal). The G was when someone else selected the object, and NG was when the object was left alone. When exposed to the videos, people uniformly preferred the object that had been selected by the unseen person, rating it as more attractive than one that had not been touched, and this persisted whether it was food, toys, clothing, or tools. As you can see, mimetic desire is a pretty easy thing to induce. No wonder keeping up with the Joneses is so prevalent.

But they wanted to know what was going on in the brain.

What you can see here are two of the brain areas which were scanned during the rating phase of the video. You can see on the top right that the parietal lobules show higher activity than at resting, and at bottom right that the ventral striatum and medial prefrontal cortex do as well. So the valuation system and mirror neuron system appear to be acting at the same time during the ratings of the objects (though fMRI is actually pretty slow, you can't capture the very fast responses).

It appears that the induction of mimetic desire involves both of these systems, the mirror neuron system responding to the other person's action with the object, and the valuation system putting an increased value on the object because of mirror neuron input.

And an interesting twist on this finding can be seen above. Because the strength of the mimetic desire in the participants correlated nicely with how strong the joined activity in the two systems was. If you don't get strong mirror neuron activation, or strong valuation system in response to it, well you're not going to value the handled object so much.

All the usual caveats to fMRI studies apply, of course. You can analyze fMRI data to make it look at a lot better than it is (though in this case I think they did all the right comparisons), and what you're looking at is just oxygenated blood flow...what does that really mean? The authors did a lot of probability ratings and modeling of network architecture, which is really nice, but it doesn't actually prove the role of each system in the control of mimetic desire. After all, you can't really go in to a human, remove his parietal lobule, and then make him watch videos. So this isn't really proof, but it's an interesting key, showing how mimetic desire might work.

So the next time you see two kids arguing over the same toy, or suddenly find yourself lusting over an iPad you have no real use for, well...blame your mimetic desires, and blame your mirror neuron system, while you're at it.

Lebreton M, Kawa S, Forgeot d'Arc B, Daunizeau J, & Pessiglione M (2012). Your goal is mine: unraveling mimetic desires in the human brain. The Journal of neuroscience : the official journal of the Society for Neuroscience, 32 (21), 7146-57 PMID: 22623659