Different Standards Regarding the Admissibility of Scientific Expert Testimony During the Guilt and Penalty Phases of a Capital Trial
In the case of United States v. Montgomery, 635 F.3d 1074 (8th Cir. 2011), a federal jury convicted Lisa Montgomery of kidnapping resulting in death. Upon recommendation by the jury, the district court sentenced her to death. She appealed, arguing that the trial court committed errors in excluding neuroimaging evidence. The Eighth Circuit Court of Appeals examined the standards for admission of positron emission tomography (PET) and magnetic resonance imaging (MRI) evidence during both the guilt and penalty phases of the trial.
Facts of the Case
Mrs. Montgomery and Bobbie Jo Stinnett met in April 2004 through an online message board dedicated to dog breeding. Ms. Stinnett became pregnant in the spring of 2004 and shared the news with her virtual community. Around this time, Mrs. Montgomery also reported being pregnant. However, she was unable to become pregnant due to a sterilization procedure that she had undergone more than a decade before. Mrs. Montgomery contacted Ms. Stinnett in December 2004 when she was eight months pregnant. They arranged a meeting for the following day. Mrs. Montgomery drove to Ms. Stinnett's farm. She brought a kitchen knife and a white cord and attacked Ms. Stinnett.
Mrs. Montgomery strangled Ms. Stinnett with the cord, eventually killing her, and, opening the abdomen with the knife, she removed the fetus from the body and departed with the infant. She drove to another location, called her husband, and told him she had delivered the child at a clinic. The next day law enforcement officers arrived at her home, and she confessed to killing Ms. Stinnett.
Mrs. Montgomery was charged with violating 18 U.S.C. § 1201 (a)(1) (2003), kidnapping resulting in death. She intended to assert an insanity defense and to present evidence regarding mental defect or disease. It was without dispute that she had PTSD, borderline personality disorder, and depression. However, defense and government experts disagreed as to whether she carried a diagnosis of pseudocyesis.
At trial, defense expert Dr. Ruben Gur was prepared to testify that Mrs. Montgomery's brain had abnormalities consistent with pseudocyesis. His diagnosis was based on an MRI showing reduced brain volumes in the right hemisphere and a PET scan showing increased activity throughout the limbic and somatomotor regions. Dr. Gur testified that the purpose of the PET scan was to identify abnormalities in Mrs. Montgomery's brain that might help explain her extreme behavior. Mrs. Montgomery argued that this evidence was probative and met the standards in Federal Rule of Evidence 702 (2009) and the Federal Death Penalty Act (18 U.S.C. § 3593 (2009)).
A government expert, Dr. Alan Evans, opined that the MRI abnormalities were less than one standard deviation from normal and that 50 percent of the population would have comparable results. In response to Dr. Evan's assertion that the deviations were not statistically significant, Dr. Gur testified that, based on his eyeball comparison, they were nonetheless clinically significant.
Dr. Evans noted that the PET scan could be consistent with both pathological and normal neurological states. Another expert, Dr. Helen Mayberg, indicated that a PET scan would not be requested by a treating physician to determine whether a patient had pseudocyesis. She opined that PET scans could not be used to diagnose psychiatric disorders or predict behavior. Furthermore, Drs. Mayberg and Evans took issue with the methodology of Dr. Gur, saying that they could replicate his calculations only if they used one method for a control group and a different method for Mrs. Montgomery. The government therefore argued that Dr. Gur's methods and principles were not reliable and challenged his testimony pursuant to Daubert v. Merrell Dow Pharmaceuticals, Inc., 509 U.S. 579 (1993).
The court initially indicated that it would admit the neuroimaging evidence regarding the abnormalities in Mrs. Montgomery's brain. However, it subsequently excluded the evidence because of questionable underlying methodology and minimal probative value.
Mrs. Montgomery was found guilty. The jury recommended the death penalty and the court sentenced her to death.
Ruling and Reasoning
In regard to the district court rulings, the court of appeals made four rulings related to the admissibility of the neuroimaging. It noted that Federal Rule of Evidence 702 requires the district court to serve as a gatekeeper to “ensure that any and all scientific testimony or evidence admitted is not only relevant, but reliable” (Montgomery, p 1089 (citing Daubert, p 589)).
With this is mind, first the court held that the testimony of Dr. Gur regarding the fact that the PET scan showed abnormalities in the somatomotor and limbic regions of Mrs. Montgomery's brain was reliable enough to have been admitted because there is “no question that the PET scan is scientifically reliable for measuring brain function” (Montgomery, p 1090 (citing Hose v. Chi. Nw. Transp. Co., 70 F.3d 968 (8th Cir. 1995)). In other words, if the scan had not been used to contend that the abnormalities found on it could aid in the diagnosis of pseudocyesis or predict Mrs. Montgomery's behavior, then it would have been admissible. Nonetheless, the court found that, in this case, the PET findings were of minimal probative value and that any error arising from their exclusion was harmless.
Second, the court of appeals concluded that Dr. Gur's opinion that the PET scan revealed abnormalities consistent with pseudocyesis did not meet the reliability requirement of Federal Rule of Evidence 702 and that the district court did not abuse its discretion in excluding the testimony. The court noted that Dr. Gur's opinion “was at most a working hypothesis, not admissible scientific ‘knowledge’” (Montgomery, p 1090 (citing Tamraz v. Lincoln Elec. Co., 620 F.3d 665, p 670 (6th Cir. 2010)). The court concluded that the defense failed to demonstrate that Dr. Gur's opinion was based on scientifically valid principles, noting “a hypothesis without support, like the one posited here, is no more than a subjective belief or an exercise in speculation” (Montgomery, p 1091).
Third, with respect to the structural MRI findings, the appellate court upheld the district court's exclusion. Because the exclusion was based on the “unreliable…methodology underlying Dr. Gur's opinion” (Montgomery, p 1093), the appellate court ruled that the district court had not abused its discretion.
Finally, the court acknowledged that the threshold for admissibility under the Federal Death Penalty Act is low and noted that the exclusion of Dr. Gur's testimony from the penalty phase represented a reversible error. It noted that Dr. Gur's interpretation of the PET scan as it pertained to a diagnosis of pseudocyesis was arguably admissible when this lower standard was used. However, it concluded that any error resulting from its exclusion was ultimately harmless. Furthermore, because the MRI evidence was both methodologically unreliable and, in this case, “had no scientifically recognized significance,” the district court acted within its authority when it concluded that “the results were irrelevant to Montgomery's insanity defense and the mitigating factors she pleaded” (Montgomery, p 1093).
Discussion
In this case, the Eighth Circuit addressed the evidentiary rules regarding the admissibility of brain imaging and scientific testimony during the guilt and penalty phases of a capital trial. The court noted that during the guilt phase, Federal Rule of Evidence 702, informed by Daubert, governs the admission of scientific testimony. The earlier “general acceptance test,” originally established in Frye v. United States, 293 F. 1013 (D.C. Cir. 1923), held that scientific evidence is admissible at trial so long as it is “sufficiently established to have gained general acceptance in the particular field in which it belongs” (Frye, p 1014). The stricter Daubert standard requires that the district court act as gatekeeper and “ensure that any and all scientific testimony or evidence admitted is not only relevant, but reliable” (Daubert, p 589). The decision in Daubert stressed that the inquiry “must be solely on principles and methodology, not on the conclusions that they generate” (Daubert. p 595). In the present case, the court of appeals went on to explain that the threshold governing admissibility of scientific expert testimony was lower during the sentencing phase of a capital trial.
In Daubert, the court outlined five guidelines to assist judges in determining scientific validity. It noted that judges should consider whether the methodology is falsifiable, subject to publication and peer review, based on a technique with a known or potential error rate, standardized, and generally accepted within the scientific community.
Although the court in Daubert assigned the status of gatekeeper to district judges, best practice requires that forensic psychiatrists do the same. Ideally, the scientific community should hold the evidence and testimony it puts forward to standards as high or higher than those of judges. Whether it is neuroimaging as it pertains to psychiatric diagnoses or any other scientific assertion, it is paramount that the methodology on which the expert's assertions are based be relevant, reliable, and grounded in science. Holding experts to such high standards is vital to engendering the trust of both the public and the legal system as to the value of their testimony. It is imperative that testimony by forensic psychiatrists carry more weight than “subjective belief or unsupported speculation” (Montgomery, p 1090 (citing Tamraz, p 670)).
Footnotes
Disclosures of financial or other potential conflicts of interest: None.
- © 2013 American Academy of Psychiatry and the Law