Friday, January 18, 2008

In Defense of (Some) Neuroeconomics

Earlier this week, Alex Tabarrok was not too impressed by a recent neuroeconomics paper that examined the neural responses to typical marketing actions for consumed goods. In this case, the authors showed that telling subjects that a wine was expensive changed the way the brain processed the experience. (Here's a popular press article on the paper. Alex provides a link to the published paper.)

I saw one of the paper's authors, Antonio Rangel, present the paper at Econopalooza this month. I had a similar reaction to Alex--this is probably not the most critical use of fMRI technology to enhance our understanding of economic behavior. However, I should also point out that there are hundreds of sessions to attend at the ASSA meetings, and two of the ones I chose to attend were the ones on neuroeconomics that featured Antonio and David Laibson. They are two of the most creative and insightful researchers in our profession, and I would make similar choices about which sessions to attend given the same menu next year.

Seeing a number of papers presented, I think David's work on hyperbolic discounting provides a better illustration of how the new technology can enhance the field of economics. In several theoretical and empirical papers over the last decade, David and his co-authors have investigated the tendency of people to act as if they have very high discount rates for choices in the near future, such as a preference for $1 today versus $1.20 tomorrow, but fairly low discount rates for choices in the more distant future, such as a preference for $1.05 in 11 years versus $1 in 10 years. This introduces a time-inconsistency problem, since what I will actually do in year 10 changes when it arrives. This is a departure from the classical model of consumer behavior. (I think the most important consequence is that it makes illiquidity a feature, not a bug, of a long-term savings account like a 401(k)). David has argued his case very persuasively, and his research is having a large impact on the way academics and policy makers think about saving.

Can neuroeconomics help him make the case? In this article in Science, he and his co-authors show:
When humans are offered the choice between rewards available at different points in time, the relative values of the options are discounted according to their expected delays until delivery. Using functional magnetic resonance imaging, we examined the neural correlates of time discounting while subjects made a series of choices between monetary reward options that varied by delay to delivery. We demonstrate that two separate systems are involved in such decisions. Parts of the limbic system associated with the midbrain dopamine system, including paralimbic cortex, are preferentially activated by decisions involving immediately available rewards. In contrast, regions of the lateral prefrontal cortex and posterior parietal cortex are engaged uniformly by intertemporal choices irrespective of delay. Furthermore, the relative engagement of the two systems is directly associated with subjects’ choices, with greater relative fronto-parietal activity when subjects choose longer term options.

The research shows that two different neural systems, which evolved for very different purposes in the human brain, deal with the two decisions. When a part of the brain is activated during a particular decision, we can infer that the decision is similar to other choices or behaviors that activate that part of the brain. The more primitive part of the brain is activated with the near-term choice. This is what gives Laibson's argument credibility. We have already learned from observation of individual choices that behavior departed from the classical model. Without the brain imaging, there could have been a number of competing theories for why this is so, many of which would not cause us to dramatically rethink the underlying model. With the brain imaging, we give substantially greater weight to the theories like Laibson's that are predicated on different decision frameworks for different types of intertemporal choices.

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