Pennell v. Delaware: Criminal Signature Testimony at Trial

Introduction and Case Background

In Pennell v. State of Delaware, the court addressed issues surrounding an expert's testimony about serial killers. This testimony was crucial because the defendant was indicted and tried for the murders of three prostitutes and was convicted of murdering two of those victims. The defendant attempted to argue that the admission of the expert's testimony violated Frye v. United States. However, in Bass v. State, the court had previously determined that the Frye test was inapplicable when an expert's opinion was based solely on his own knowledge and experience, as opposed to scientific information. Therefore, the court determined that Delaware Rule of Evidence 702 governed the admissibility of the expert's opinion. The expert's experience was outside the realm of average knowledge and could have assisted the jury's understanding of signature crimes and crime analysis. In Pennell's case, such information was considered helpful to the jury because the jury had to determine whether the three charged murders were committed by the same person.

The court supported its decision to admit expert testimony regarding serial murders because the injuries suffered by two of the victims were extremely similar, and the victims had identical causes of death. Furthermore, there were additional victims who exhibited similar injuries, but the defendant was not charged with those crimes. Therefore, the signature aspects of those crimes were relevant to the issue of guilt or innocence. The trial court also limited the admissibility of the expert's testimony: while it permitted the expert to testify regarding the signature aspects of the crime, it refused to allow the expert to testify regarding his criminal profile. The court determined that "profile evidence is that which attempts to link the general characteristics of serial murderers to specific characteristics of the defendant. Such evidence is of little probative value and extremely prejudicial to the defendant since he is, in a sense, being accused by a witness who was not present at any of the crimes" (Pennell v. State of Delaware, 1991).

Because of their frequent use in television, books, and movies, the general public may believe that criminal profilers are involved in all aspects of crime detection and prosecution. This perception is inaccurate, but criminal profiling does play an important role in the investigation of serial criminals. While the Pennell court did not allow the expert to testify regarding the criminal profile used to narrow down the range of suspects, it did permit the expert to testify regarding those elements of the crime that helped the expert develop his signature analysis. The elements that help an expert develop a profile are elements of a crime that appear somewhat unique and allow the crime to stand out from similar crimes. These distinctive elements are referred to as a criminal signature. The Pennell court made the correct decision to allow the expert to testify regarding the signature elements of the murders, because these elements were crucial in linking the defendant to two of the murders. Furthermore, the fact that the defendant was found not guilty of one of the murders demonstrates that juries use signature elements as tools for decision making, but do not substitute the presence of certain criminal signatures for a lack of reasonable doubt.

The Role of Criminal Signatures in Crime Detection

The use of signature elements is important in crime detection. According to Special Agent John Douglas — the expert who testified in Pennell's criminal trial — the use of signature elements plays an important role in helping detectives solve crimes (Douglas & Munn, 1992). He believes that "most crime scenes tell a story. And like most stories, crime scenes have characters, a plot, a beginning, a middle, and hopefully, a conclusion" (Douglas & Munn, 1992). Furthermore, because an individual's core behavioral characteristics do not change regardless of whether that individual is engaged in legal or illegal activity, Douglas and Munn believe that "learning to recognize crime scene manifestations of behavioral patterns enables investigators to discover much about the offender. It also provides a means by which investigators can distinguish between different offenders committing the same types of offense" (1992).

Clearly, a criminal's signature can serve several important roles in crime detection. It can link crimes that might otherwise appear unrelated, give investigators insight into a criminal's behavior, and help narrow down the field of potential suspects. However, some argue that testimony regarding criminal signatures should not be permitted at trial.

Modus Operandi, Signature, and Staging Distinguished

Douglas and Munn distinguish between modus operandi, personation or signature, and staging. Modus operandi refers to an "offender's actions while committing the crime" (Douglas & Munn, 1992). Furthermore, the modus operandi "is a learned behavior that is dynamic and malleable. Developed over time, the M.O. continuously evolves as offenders gain experience and confidence" (Douglas & Munn, 1992). Repeat violent offenders often leave signatures, which refers to criminal conduct that is "a unique and integral part of the offender's behavior and goes beyond the actions needed to commit the crime" (Douglas & Munn, 1992). Staging may be committed by the criminal or a third party and "occurs when someone purposely alters the crime scene prior to the arrival of the police" (Douglas & Munn, 1992). While these distinctions are important for psychologists and investigators alike, one can see how the Pennell decision would allow an expert to testify about any of these three possible manifestations of offender behavior, even though the differences between them would make signature testimony more relevant than testimony regarding M.O. or staging alone.

It is the signature that is most relevant with violent, repetitive offenders. As Douglas and Munn explain:

"Fantasies of offenders often give birth to violent crime. As offenders brood and daydream, they develop a need to express these violent fantasies. When they are finally acted out, some aspect of each crime demonstrates a unique, personal expression or ritual based on these fantasies. However, committing the crime does not satisfy the needs of offenders, and this insufficiency compels them to go beyond the scope of the offense and perform a ritual. When offenders display rituals at the crime scene, they have left their individualized 'calling card'" (Douglas & Munn, 1992).