In particular, a number of speakers expanded on the connections between neuroscience developments and criminal law and procedure. Brain imaging, in fact, has been offered by defendants in criminal cases for many purposes, such as attempting to show incompetence to stand trial or to waive basic constitutional rights like the right to a jury trial. Imaging has also been offered in an effort to rebut the mental element of crimes, called the mens rea (“guilty mind”), such as intent or knowledge, to prove the mental disease or defect element of the insanity defense, and to mitigate the imposition of the death penalty. A few unsuccessful efforts have been made to admit brain scans for the purpose of demonstrating actual innocence. . It is important to note that the issues involving mental states, aside from competence to stand trial or to waive a constitutional right, all involve past mental states – that is, mental states that existed at the time the alleged crime was committed.
In terms of specific types of brain imaging, CT scans and MRI images have been readily admitted in court as proof of brain disease or trauma. Courts have been far more guarded, however, about admitting scans such as PET (positron emission tomography) or fMRI (functional magnetic resonance imaging) when offered as the basis for inferences about broader issues such as competence, insanity, or criminal responsibility in general. Somewhat more liberal standards have been applied to offers of mitigating evidence in death penalty cases, since it is generally acknowledged that death is different. In ruling, courts have focused on reliability standards as well as relevance and probative vs. prejudicial considerations. Other types of brain scans have been developed as well, such as SPECT (single photon emission computed tomography), but also including some types (such as “brain fingerprinting”) that are not supported by most neuroscientists. Criminal trial judges routinely face offers of evidence based on expert testimony on these matters and many – perhaps most – of their decisions are not appealed and, therefore, are not published.
By way of examples of court decisions to date, one of the earliest cases was the 1992 New York case of People v. Weinstein. Herbert Weinstein was a 65-year-old executive who was charged with strangling his wife, Barbara, and then throwing her body from a 12th-story window in order to make her death look like a suicide. In that case, the trial court judge ruled that a PET scan showing reduced brain function in and around an arachnoid cyst in the frontal lobe of Weinstein’s brain was admissible. Weinstein had admitted murdering his wife but claimed that the cyst was evidence that he could not be held criminally responsible. Fearing that the brain scan evidence might unduly influence the jury, the prosecutors offered a bargain to Weinstein: he would plead guilty in exchange for a reduced charge of manslaughter.
In People v. Mezvinsky, a 2002 federal case, however, the trial court refused to admit a PET scan to support a claim that the defendant was incapable of deception in a fraud prosecution. Similarly, in People v. Protsman, a 2001 California case, the court refused to admit a PET scan to demonstrate decreased frontal lobe activity due to traumatic brain injury such that he could not formulate intent to commit murder. These cases are representative of the case law because they show the admissibility on issues of injury or disease but the refusal to admit images to go beyond that – to show the connection between reduced or impaired brain function and the formation of mental states essential to the crimes charged.