Science magazine has a series of three (1, 2, 3) articles by Greg Miller over the past few days covering an interesting trial in Tennessee. The case itself seems like garden variety fraud, but the novel twist is that the defense is trying to introduce fMRI scans into the courtroom in order to establish the defendant’s innocent. As far as I can tell from Miller’s articles, the only scientists defending the use of fMRI as a lie detector are those employed by Cephos (the company that provides the scanning service); the other expert witnesses (including Marc Raichle!) seem pretty adamant that admitting fMRI scans as evidence would be a colossal mistake. Personally, I think there are several good reasons why it’d be a terrible, terrible, idea to let fMRI scans into the courtroom. In one way or another, they all boil down to the fact that just isn’t any shred of evidence to support the use of fMRI as a lie detector in real-world (i.e, non-contrived) situations. Greg Miller has a quote from Martha Farah (who’s a spectator at the trial) that sums it up eloquently:
Farah sounds like she would have liked to chime in at this point about some things that weren’t getting enough attention. “No one asked me, but the thing we have not a drop of data on is [the situation] where people have their liberty at stake and have been living with a lie for a long time,” she says. She notes that the only published studies on fMRI lie detection involve people telling trivial lies with no threat of consequences. No peer-reviewed studies exist on real world situations like the case before the Tennessee court. Moreover, subjects in the published studies typically had their brains scanned within a few days of lying about a fake crime, whereas Semrau’s alleged crimes began nearly 10 years before he was scanned.
I’d go even further than this, and point out that even if there were studies that looked at ecologically valid lying, it’s unlikely that we’d be able to make any reasonable determination as to whether or not a particular individual was lying about a particular event. For one thing, most studies deal with group averages and not single-subject prediction; you might think that a highly statistically significant difference between two conditions (e.g., lying and not lying) necessarily implies a reasonable ability to make predictions at the single-subject level, but you’d be surprised. Prediction intervals for individual observations are typically extremely wide even when there’s a clear pattern at the group level. It’s just easier to make general statements about differences between conditions or groups than it is about what state a particular person is likely to be in given a certain set of conditions.
There is, admittedly, an emerging body of literature that uses pattern classification to make predictions about mental states at the level of individual subjects, and accuracy in these types of application can sometimes be quite high. But these studies invariably operate on relatively restrictive sets of stimuli within well-characterized domains (e.g., predicting which word out of a set of 60 subjects are looking at). This really isn’t “mind reading” in the sense that most people (including most judges and jurors) tend to think of it. And of course, even if you could make individual-level predictions reasonably accurately, it’s not clear that that’s good enough for the courtroom. As a scientist, I might be thrilled if I could predict which of 10 words you’re looking at with 80% accuracy (which, to be clear, is currently a pipe dream in the context of studies of ecologically valid lying). But as a lawyer, I’d probably be very skeptical of another lawyer who claimed my predictions vindicated their client. The fact that increased anterior cingulate activation tends to accompany lying on average isn’t a good reason to convict someone unless you can be reasonably certain that increased ACC activation accompanies lying for that person in that context when presented with that bit of information. At the moment, that’s a pretty hard sell.
As an aside, the thing I find perhaps most curious about the whole movement to use fMRI scanners as lie detectors is that there are very few studies that directly pit fMRI against more conventional lie detection techniques–namely, the polygraph. You can say what you like about the polygraph–and many people don’t think polygraph evidence should be admissible in court either–but at least it’s been around for a long time, and people know more or less what to expect from it. It’s easy to forget that it only makes sense to introduce fMRI scans (which are decidedly costly) as evidence if they do substantially better than polygraphs. Otherwise you’re just wasting a lot of money for a fancy brain image, and you could have gotten just as much information by simply measuring someone’s arousal level as you yell at them about that bloodstained Cadillac that was found parked in their driveway on the night of January 7th. But then, maybe that’s the whole point of trying to introduce fMRI to the courtroom; maybe lawyers know that the polygraph has a tainted reputation, and are hoping that fancy new brain scanning techniques that come with pretty pictures don’t carry the same baggage. I hope that’s not true, but I’ve learned to be cynical about these things.
At any rate, the Science articles are well worth a read, and since the judge hasn’t yet decided whether or not to allow fMRI or not, the next couple of weeks should be interesting…
[hat-tip: Thomas Nadelhoffer]