Types of Evidence

Evidence represents information used to help establish a fact. It may be inculpatory (supporting guilt) or exculpatory (supporting innocence). There are four broad categories of criminal evidence, each with its own strengths and weaknesses.

Circumstantial evidence involves such things as the suspect's opportunity to commit a crime (alibi), his access to commit a crime (special means or knowledge) and motive (financial or psychological). Even if a particular suspect has opportunity, access and motive to commit a crime, in all likelihood, so too do other suspects. Consequently, circumstantial evidence is the weakest proof of a suspect's guilt.

Testimonial evidence involves human inferences or interpretation. For example, an eye witness who relies on memory to pick out a suspect from a line-up is offering testimonial evidence. Other examples include behavior symptom analysis, polygraph, handwriting analysis, medical and psychiatric opinions and information provided by an informant. While testimonial evidence tends to directly link the suspect to a crime, its accuracy can range from chance levels to within the 90^th percentile.

Forensic evidence describes scientific testing that matches an unknown sample to its source. Examples of forensic evidence include fingerprints, tool marks, DNA, hair and fiber analysis, toxicology reports as well as ballistics. While forensic evidence is extremely accurate, it rarely proves a suspect's guilt. Rather, the evidence may indicate that the suspect was at the crime scene, had sex with the victim or that a bullet was fired from a gun owned by the suspect.

Direct evidence describes evidence that directly links the suspect to a crime. An example is finding property stolen during a burglary in the back seat of the suspect's car that was pulled over two blocks from the home that was burglarized. Other examples include a surveillance video clearly showing the suspect robbing a clerk or an employee caught smoking marijuana in the company washroom. Direct evidence represents the strongest proof of a suspect's guilt, but rarely exists.

The Psychology of Evidence

The suspect who is caught "red-handed" committing a crime (faced with direct evidence) will almost always readily admit their crime. It is important for investigators to understand why suspects who are caught under this circumstance confess. One possibility is that they are caught so soon after the crime that they have not had time to think of ways to excuse away the incriminating evidence. Another possibility is that a suspect experiences the most guilt immediately after committing a crime. Based on more than 50 years of empirical observations, we can state that neither of these explanations are likely. Rather, the primary reason a suspect who is essentially caught committing a crime readily confesses is because he is absolutely convinced that the investigator knows he is guilty. The suspect recognizes that no amount of rationalization or justification can possibly offer a credible excuse for the obvious evidence of his guilt, so he tells the truth. This concept is worth repeating in a different way. A suspect is unlikely to tell the truth during an interview or interrogation unless he is absolutely convinced that the investigator knows he is guilty.

Portraying high confidence in a suspect's guilt starts with the investigator's demeanor and statements. If an investigator casually enters the room and says, "Larry, after talking to you and looking over everything I don't think you're telling the complete truth" the suspect will immediately recognize that the investigator does not have clear evidence (or confidence) of his guilt. It is not human nature for a suspect to increase the investigator's confidence of his guilt by confessing.

Assuming there is no direct evidence linking the suspect to the crime, an investigator must rely on other forms of evidence to persuade the suspect that there is no doubt of his guilt. Of course, circumstantial, testimonial or forensic evidence are never positive proof of a suspect's guilt so the investigator must use whatever evidence is available and present it in such a way as to maximize its persuasive impact. In this regard, investigators must appreciate that interrogation relies extensively on implication and innuendo -- It is not so much the hand you are dealt, but how you play the cards that counts.

The Decision to Introduce Evidence

The first question an investigator must address with respect to presenting evidence to a suspect is whether to present it at all. The danger of presenting evidence is that by doing so it may reveal a weakness in the investigator's case and may also give the suspect something tangible to attack. Many of the interrogations we conduct only involve circumstantial and testimonial evidence against the suspect (opportunity, access, motive, propensity, behavior symptom analysis). Under this circumstance we rarely comment on specific evidence during an interrogation. Rather, we start the interrogation with a statement such as, "Jim, I have in this folder the results of our entire investigation and all of the evidence clearly indicates that you did (issue)." From that point on, often there is no further reference to evidence.

If evidence is presented during an interrogation, it should be used only to overcome barriers such as persistent but weak denials or when the suspect is reluctant to accept the alternative question. Under this circumstance the investigator should not tell the suspect, "Bill, here is all of the evidence we have against you." To do so will often fortify the guilty suspect's belief that he can explain away the evidence and escape consequences. Rather, the investigator should select the most incriminating piece of evidence and present it as if it was the tip of the iceberg. For example, "Bill, I'm not going to sit down with you and go over piece by piece all of the evidence we have collected on this case but let me just show you this one piece. Remember earlier you said you left work that night at 6:30. Here is an affidavit from your supervisor who clearly remembers you leaving work that night at 5:15. That means you had plenty of time to do this."

Using Weak Evidence to Imply Further Evidence

From the outset of an interrogation, the guilty suspect is asking himself the question, "Does the investigator really know that I did this?", e.g., "Is there really evidence of my guilt." One way to imply the existence of strong evidence against a suspect is for the investigator to attack the evidence that does exists. Consider a homicide where it is known that the suspect acknowledged being with the victim shortly before her death and that they recently broke off their relationship. Under this circumstance it may be very persuasive to tell the suspect the following:

"Mike, I've talked to a lot of people who have done the same thing that you did. I know you're sitting in that chair and asking yourself, eWhat proof do they have?" We talked earlier about the fact that you saw her the day this happened and have no alibi. You know what. There are probably 100,000 people in this city who have no alibi at the time she died. You told me about your breakup with her. During our investigation we identified a number of people who were upset with her and also had a motive to do this. The evidence we have has nothing to do with your opportunity or motive. I wish I could lay everything we have out on the table so you could see where I am coming from but at this stage of the investigation I can't do that. The only reason I'm talking to you is to find out the circumstances that led up to your decision to do this... [continue with theme].

Using Evidence to Attack the Suspect's Credibility

Consider a hit and run case where there is circumstantial evidence indicating that the suspect's vehicle (which matched the description of the vehicle involved in the accident) had front end repair work done the day after the accident. During the interview the investigator could show the suspect the work order from the body shop and ask for an explanation. The deceptive suspect is likely to offer an explanation such as saying that his car was damaged in a parking lot the day of the hit and run so he took it in for repairs. The innocent suspect, of course could tell the same story. An axiom of interviewing is to never reveal evidence to a suspect until first giving the suspect a chance to volunteer the information.

In the case of the hit and run, the investigator should first ask whether the suspect's car had sustained any recent front end damage and whether the vehicle had any recent repairs. An innocent suspect will truthfully acknowledge this information. On the other hand, the deceptive suspect, who has not been told about the work order, may lie and deny having any repair done to his vehicle. During an interview the investigator should allow the suspect to tell this lie and accept it. Later, during the interrogation, if the investigator needs to introduce evidence the work order can be brought up and used to attack the suspect's credibility. The dialogue may be as follows:

"Tom, I wouldn't be talking to you this way unless I knew for sure that you did this. A lot of the questions I asked you earlier I already knew the answer to. For example, you said that your car had no front end damage and had not been repaired. Look at this work order dated the day after the accident. I'm sure you recognize it. The biggest problem you've got right now is your credibility. At some point there may be a concern that this wasn't an accident at all and that maybe you purposely aimed your car at that man because maybe you thought he was having an affair with your wife. Or maybe someone might claim that you were paid money to hit that man. Now, I don't think either of those things happened but if you don't start telling the truth no one is going to believe anything you say!"

Establishing the Credibility of Testimonial Evidence

When the primary evidence against a suspect is testimonial, the investigator may need to bolster the strength of that evidence in the suspect's mind. This is best accomplished through the use of statements that appeal to the suspect's logic and common sense. For example, if a victim or witness identified the suspect as the perpetrator of the crime the investigator may comment that the victim or witness has no motive to lie. In the case where the witness or victim knows the suspect personally, the investigator may state that this is clearly not a case of mistaken identity. Similarly, the details of an informant's statements could be revealed to bolster that evidence.

For legal reasons an investigator must be cautious about overstating the accuracy of detection of deception procedures such as the polygraph technique, CVSA or behavior symptom analysis. Some courts have ruled that, for example, attempting to convince a suspect that the CVSA was infallible constitutes a deceptive statement that shocks the conscience of the court or community. Rather, the investigator should rely on logical arguments in supporting the strength of testimonial evidence as the following dialogue illustrates:

I: "Linda, I'm sure that over the last 25 years you've had friends or relatives lie to you and you could usually tell when they were lying right?"

S: "Yeah."

I: "Would you agree that when you were younger it was harder to tell when people lied to you than when you got older and had more life experiences?"

S: "Sure."

I: "Well, I've had specific training in evaluating behavior symptoms. That's what I do in my job. I interview people and evaluate their behavior. And I've been doing this for almost 15 years. I wouldn't be talking to you this way if I wasn't convinced that you did this."

Next month we will discuss additional strategies relating to the use of evidence during an interrogation.