Is Your Brain A Communist?

Capitalists beware. No less a journal than Nature has just published a paper proving conclusively that the human brain is a Communist, and that it's plotting the overthrow of the bourgeois order and its replacement by the revolutionary Dictatorship of the Proletariat even as we speak.

Kind of. The article, Neural evidence for inequality-averse social preferences, doesn't mention the C word, but it does claim to have found evidence that people's brains display more egalitarianism than people themselves admit to.

Tricomi et al took 20 pairs of men. At the start of the study, both men got a $30 payment, but one member of each pair was then randomly chosen to get a $50 bonus. Thus, one guy was "rich", while the other was "poor". Both men then had fMRI scans, during which they were offered various sums of money and saw their partner being offered money too. They rated how "appealing" these money transfers were on a 10 point scale.

What happened? Unsurprisingly both "rich" and "poor" said that they were pleased at the prospect of getting more cash for themselves, the poor somewhat more so, but people also had opinions about payments to the other guy:

the low-pay group disliked falling farther behind the high-pay group (‘disadvantageous inequality aversion’), because they rated positive transfers to the high-pay participants negatively, even though these transfers had no effect on their own earnings. Conversely, the high-pay group seemed to value transfers [to the poor person] that closed the gap between their earnings and those of the low-pay group (‘advantageous inequality aversion’)

What about the brain? When people received money for themselves, activity in the ventromedial prefrontal cortex (vmPFC) and the ventral striatum correlated with the size of their gain.

However, when presented with a payment to the other person, these areas seemed to be rather egalitarian. Activity rose in rich people when their poor colleagues got money. In fact, it was greater in that case than when they got money themselves, which means the "rich" people's neural activity was more egalitarian than their subjective ratings were. Whereas in "poor" people, the vmPFC and the ventral striatum only responded to getting money, not to seeing the rich getting even richer.


The authors conclude that this

indicates that basic reward structures in the brain may reflect even stronger equity considerations than is necessarily expressed or acted on at the behavioural level... Our results provide direct neurobiological evidence in support of the existence of inequality-averse social preferences in the human brain.

Notice that this is essentially a claim about psychology, not neuroscience, even though the authors used neuroimaging in this study. They started out by assuming some neuroscience - in this case, that activity in the vmPFC and the ventral striatum indicates reward i.e. pleasure or liking - and then used this to investigate psychology, in this case, the idea that people value equality per se, as opposed to the alternative idea, that "dislike for unequal outcomes could also be explained by concerns for social image or reciprocity, which do not require a direct aversion towards inequality."

This is known as reverse inference, i.e. inference from data about the brain to theories about the mind. It's very common in neuroimaging papers - we've all done it - but it is problematic. In this case, the problem is that the argument relies on the idea that activity in the vmPFC and ventral striatum is evidence for liking.

But while there's certainly plenty of evidence that these areas are activated by reward, and the authors confirmed that activity here correlated with monetary gain, that doesn't mean that they only respond to reward. They could also respond to other things. For example, there's evidence that the vmPFC is also activated by looking at angry and sad faces.

Or to put it another way: seeing someone you find attractive makes your pupils dilate. If you were to be confronted by a lion, your pupils would dilate. Fortunately, that doesn't mean you find lions attractive - because fear also causes pupil dilation.

So while Tricomi et al argue that people, or brains, like equality, on the basis of these results, I remain to be fully convinced. As Russell Poldrack noted in 2006

caution should be exercised in the use of reverse inference... In my opinion, reverse inference should be viewed as another tool (albeit an imperfect one) with which to advance our understanding of the mind and brain. In particular, reverse inferences can suggest novel hypotheses that can then be tested in subsequent experiments.

Tricomi E, Rangel A, Camerer CF, & O'Doherty JP (2010). Neural evidence for inequality-averse social preferences. Nature, 463 (7284), 1089-91 PMID: 20182511