Your brain predicts your behaviour better than you can!

Falk et al (2010) have an interesting article showing how future behaviour, applying sunscreen, can be predicted by brain activity as people watch persuasive messages.

Activity in the (a priori selected) “region of interest” (ROI), a bit of the frontal cortex (MPFC), explained 24% of variance in the number of days of actual sunscreen use. Self-reported intention to apply sunscreen explained only around 3% of variance in days use. Nice result.

I think some caution is warranted, however, before people testing message persuasiveness, such as health organisations, invest all their money in brain scanning.

Behavioural methods include not only self report. For instance asking people how smart they are (e.g., indirectly, using Need for Cognition) is not as good as testing how smart they are (e.g., using Raven’s Advanced Progressive Matrices). Similarly there might well be behavioural ways to test the effect of persuasion, or to infer who is most likely to be persuaded, which can predict much better than the self-reported intention.

These results clearly demonstrate that the expensive and time consuming method of functional imaging is entirely unnecessary for testing message persuasiveness. Simply show people the message and then ask them later how they actually behaved. Very cheap and easy! Especially nice if you can try out a few different versions and select the one which works best.

There is always the danger that people cannot be trusted to report their actual sunscreen usage, a point acknowledged by the authors (as their data explain variance in a self-report measure): “direct observation of behavior would be preferred in future studies, our measurement of behavior through self-report is unlikely to have artificially enhanced our results.” So here the self-report of actual sunscreen usage is completely trusted.

The imaging might provide some theoretical clues for why people were persuaded. There is little of this theory in the paper. This is it, in fact: “… this region has been associated with selfreferential processing, but our ROI also overlaps with a more ventral portion of MPFC that has been associated with implicit valuation.” Such theoretical depth is sadly common in the function imaging literature.

I am very impressed that they found an ROI that predicts so well and I appreciate that a lot of work must have gone into this study. It will be interesting to see if it replicates and generalises. I also hope some theory is on the way explaining mechanisms of persuasion.


Falk, E. B., Berkman, E. T., Mann, T., Harrison, B., & Lieberman, M. D. (2010). Predicting persuasion-induced behavior change from the brain. Journal of Neuroscience, 30, 8421-8424.