Psychologists can play important roles in criminal cases for both the defense and prosecution. During trials, they may focus on issues of eyewitness identification, diminished capacity, or the susceptibility of child witnesses to outside influence. Before trial, they establish or refute a defendant’s competency to stand trial. At sentencing, they educate the judge about the defendant’s cognitive ability, mental impairments, family history, social environment, treatment needs, and potential for rehabilitation. Psychologists who provide mitigating evidence during the death penalty phases of murder trials often make the difference between life and death.
Explaining the Choice to Kill
In his recently published book, Listening to Killers, Dr. James Garbarino recounts the lessons he learned from his twenty years of testifying as an expert witness in murder cases. His book exemplifies the kind of testimony that psychologists provide in criminal cases.
Chapter one (available as a pdf online) explores whether and why murderers choose to kill. Mixing case studies from his own experience with current research findings, Dr. Garbarino explains how the choices that murderers make are shaped by a variety of factors, including brain functions, cultural values, panic, personality disorders, traumatic experiences, fear, misperceptions, addiction, curiosity, peer pressure, and the instinct for self-preservation.
Psychologists often examine the difference between choice and compulsion. They join neuroscientists in asking whether the concept of free will (upon which the criminal justice system’s philosophy of punishment is based) is just an illusion. Behavioral choices that many people perceive as evidence of moral weakness or “bad character” may not be choices at all, in the sense that choices can be driven by unconscious motivations. If the decision to murder is the product of psychological and environmental factors that the killer did not choose, is the killer really choosing to kill?
The Science of Decision-Making
Psychologists are not called upon to justify criminal behavior, but to make it comprehensible. Their goal is to help judges or juries see the defendant as a human being, not as a monster. They focus on “the science of decision-making,” basing testimony on the neuroscience community’s evolving understanding of brain development and its impact on human behavior. They also rely upon studies that have linked murders and violent behavior to damage in parts of the brain that are responsible for moral calculations or empathy.
At the same time, experts caution against attributing criminal or violent behavior solely to underdeveloped or damaged brains. Research suggests that murderers have a “genetic vulnerability” to brain development that fails to control antisocial behavior. Despite that vulnerability, brains may develop normally when children are raised in a safe and nurturing environment. On the other hand, abuse or trauma may act as triggers that prevent vulnerable brains from developing the social controls and empathy that cause most people to behave nonviolently.
Differing Approaches to Expert Testimony
The extent to which psychologists are permitted to testify in criminal cases varies from state to state and from context to context. For example, the admissibility of a psychologist’s pretrial testimony on the issue of competency to stand trial is relatively uncontroversial. Mitigation testimony during sentencing is also generally admissible and, in the death penalty phase of a trial, cannot generally be precluded and may even be required.
Judges typically exercise considerable discretion over the admissibility of a psychologist’s defense testimony in the guilt phase of a murder trial. Some states have adopted evidentiary standards that preclude experts from offering an opinion as to whether a defendant was capable of forming an intent to kill. Judges are even less likely to allow a psychologist to testify that a defendant did not have an intent to kill.
State evidentiary standards also differ as to whether (for example) a “battered woman’s defense” can be raised to explain why an abuse victim killed her abuser. Similarly, states have taken inconsistent approaches to diminished capacity defenses. When they are allowed to testify, however, there is little doubt that psychologists and behavioral experts can be of enormous assistance to a criminal defendant who is charged with murder.
