I.

The New Yorker  has an article about Kent Kiehl, a researcher who studies psychopaths using neuroimaging techniques; he has a mobile MRI scanner that he takes to prisons.

Brain imaging is rarely used in criminal trials (no reference, but I believe the number is <100.)  But when it is used, and it will be more used, it has fairly important effects not just on the case, but on society, e.g.,

1. Ask any resident about the anatomic correlates of schizophrenia, and they inevitably mention widened sulci-- which isn't actually specific or useful.  What most residents don't know is that this "finding" was made real by the trial of John Hinckley Jr: the defense showed that the cortical atrophy and widened sulci was similar to the brains of schizophrenics (and neglected to mention it was similar to findings in a dozen other non-psychotic conditions.)  Hinckley was found NGRI, which means hospital, then free; because of this case, we now have  "guilty but mentally ill" which means hospital, then jail.
2. Brain scans were also used in the (failed) attempt at banning violent video games, on the assertion that it caused patterns in the frontal lobe similar to clinically violent adolescents (never mind that "patterns" doesn't mean anything, let alone predict thought.) 
   
3. We can't execute juveniles anymore (Roper v. Simmons) because of the evidence of ongoing myelination of neurons in the DFC in adolescents as they aged.  (Never mind that all the kids in this study were relatively homogeneous anyway, so differences are not to be expected; never mind that this evidence is a long way from saying they lack the ability to take moral responsibility; never mind that, oh, never mind.)  Not executing juveniles is a wonderful idea; not executing them because of a lack of myelin is dangerously absurd.  So if science discovers their brains are completely mature after all, then what?

These imaging techniques only tell you one story, and it's not even a very good story.  They cannot tell you the impact of other forces, including genetics, lifestyle habits, and even the effect of chronic thought patters on brain function or anatomy.

Neither are scans themselves showing what you think they are showing.  They are showing something (e.g. glucose utilization) that we interpret as function ("region is activated") which is then used to say, "the amygdala is involved in..."  You may as well say Nebraska is involved in. 

They are, in essence, a videotape of someone's house while it's raining, from which you are supposed to deduce based on the amount of water collecting on the second floor bedroom window, that he was unable to avoid masturbating at work.

Unfortunately, juries love videotapes.

II.

But more importantly, forensic brain scans represent the worst kind of classism.  They say, essentially, that because your brain looks very similar to other brains, then you belong in that class; and you take on their other qualities.  They may be appropriate for scientific investigations-- as early clues directing further research, "hey, this is interesting, let's look over here from now on"-- but they have no place in the justice system.  I'd argue any association studies have no place in court, especially when they are read backwards ("because he has this, he is this.")

Social scientists are hoping that these anatomic associations to psychopathy will show it to be a mental illness just like any other.  Consider the quote from Jean Decety, a professor of social neuroscience at the University of Chicago:

"We still basically work out of a Biblical system of punishment--we don't consider, in most cases, to what extent the offender's actions were intentional or unintentional. But what neuroscience is showing us is that a great many crimes are committed out of compulsion--the offenders couldn't help it. Once that is clear, and science proves it, what will the justice system do?"

Not withstanding that nearly every single proposition in this paragraph is flawed, let's take it to its conclusion: he wants us to consider psychopathy the way we consider, say, bipolar.

Currently, psychopathy is considered an aggravating factor-- if you have it, it's worse for you.  This is opposite to bipolar, for example, which can be a mitigating factor.  (For example, the coming exemption from execution for "mental illness" won't include psychopathy.)  But take a step back: both psychopathy and bipolar are heuristics.  They both have about as much and as good anatomical and genetic evidence.  In other words, they are formally equivalent.  They shouldn't be considered any differently in court.  Right?  Well, don't get excited: society could just as easily decide that bipolar should become an aggravating factor.

And, importantly, society--psychiatry-- can then change its mind every ten years or so.

Do you want to live in a world where the criminality of an act-- the culpability of a person-- is based essentially on the moral ADHD of a bunch of doctors with third hand understanding of  MRI results?  Strike that-- on a very select bunch of doctors who purport to speak for these other doctors, who claim to have the greatest grasp of the science, and who claim in complete and absolute seriousness to have the best understanding of the ethical progressivism of society-- better than lawyers or engineers or teachers or priests or anyone else-- ethics which they admit are constantly evolving-- but have thankfully, magically, and conveniently reached their culmination with today.  The end of history.   4000 years of ethics, useless; they got it all right with a grant from the NIH and a subscription to The New Yorker.

As if the science really mattered anyway:  no matter what those brain scans show, they'd find a way to make them support whatever ethical position is fashionable at the time.