Young's Postulate

One can be reasonably certain if witness accounts of the past are consistent or not consistent with physical evidence in the present, but one cannot reliably surmise past events from physical evidence unless there is only one plausible explanation for that evidence.

This wordy statement seems difficult to understand, but the concepts in it are really very simple. Here is my explanation after breaking down the postulate into its multiple component parts.

First, let’s start with the title.

Young’s Postulate

A postulate is a statement that cannot be proven but is assumed to be true for the sake of further reasoning and discussion. The reason this postulate cannot be proven is because the issues dealt with in the postulate occurred in the past—usually as one-time events that are not ongoing. The past is beyond our ability to observe, touch or measure directly. The past is abstract, so it cannot be studied or manipulated for the purposes of proof.

I (Dr. Thomas Young) am interested in approaches to forensic reasoning that work reliably to give correct answers for past events. I have found over many years of forensic pathology practice that this postulate works reliably.

One can be reasonably certain if…

This means that the postulate can allow anyone to find the truth behind a past event not on the basis of luck or speculation (guessing) but on the basis of precise logical reasoning.

Most courts require that the opinions offered by a scientist in the courtroom are made to a reasonable degree of scientific certainty or scientific probability—the exact terms may vary according to the state or local jurisdiction. As a medical expert, I am required to offer only opinions made to a reasonable degree of medical certainty or probability. This postulate does not deal with the science of probability (i.e. What is the measurable likelihood that my opinion is the truth?). My use of the word, probable, in any opinion I may offer is more of an expression of uncertainty than certainty. It is a way of saying, “I think this is what it is, but I am not sure.” The method described by this postulate allows the expert to be certain of certain things.

...witness accounts of the past…

As mentioned above, the past is abstract. It no longer exists. It cannot be touched, seen, heard, or measured. The past remains with us only in the form of memory or record. Witnesses who were present to see or hear a past event may remember it and relate it to others. Witness accounts of past events—and in some cases, even the past events themselves—may also be documented in the form of a record such as a videotape, an audiotape, or a written statement. These memories or records may not always be entirely accurate, but they may be found to be reliable after comparing them to physical evidence.

The postulate recognizes the importance of witness accounts preserved in memory or record. These items are really the only link to the complex and unique events of the past, and as such they should not be ignored or minimized by scientists or by the court.

...are consistent or not consistent with physical evidence in the present,...

The process described in the postulate involves a comparison of witness statements with physical evidence. Examples of physical evidence include hairs and fibers on clothing, stains with DNA, damage patterns to motor vehicles, and autopsy findings. They are the items that forensic scientists can observe and measure.

The scientist sees if the witness accounts are consistent (they fit) or not consistent with (they do not fit) the physical evidence. Consider the process one goes through in buying a tailored suit. The tailor ensures that the garments will fit the shape and proportions of a person’s body. In a similar fashion, a pathologist or other forensic scientist can compare witness evidence with physical evidence and say whether or not the two “fit” each other. If the two “fit” then there is a consistency. If the two do not “fit” then there is an inconsistency.

For example, a pathologist can look at a gunshot wound in a body and tell something about the range of fire. If the witness gives an account of a gun fired 20 feet away from a victim and the gunshot wound has the physical characteristics of a pressed contact gunshot wound, then the witness account of the distant range gunshot is not consistent with the physical evidence. On the other hand, if the witness sees a person put a gun to his head and shoot himself, then the witness account would be consistent with the pressed contact gunshot wound.

...but one cannot reliably surmise past events from physical evidence…

Sherlock Holmes, a fictional character in multiple novels written by Arthur Conan Doyle, advocated surmising past events. All detective and crime-fighting fiction since Sherlock Holmes use this kind of surmising. First, the detective surmises what happened in the past on the basis of “clues” usually in the form of subtle physical evidence. Later in the story, when the criminal is confronted with the story marvelously derived by the hero, the criminal immediately confesses to the crime, acknowledging the truthfulness of the conclusions of the supersleuth. The numerous detective and forensic science shows that fill the prime time slots on television these days use the very same plot device. Screenwriters apparently find great drama with this approach, but real life is not like the television shows or the detective novels. Unfortunately, numerous people in our society—including ones who should know better (forensic scientists, forensic pathologists, police officers, prosecutors)—are persuaded that this approach works.

Although this type of surmising might be useful as a heuristic (rule of thumb) that may help during the investigation of a case, it should never be the basis for any settled forensic scientific opinion. This is because in real life, this kind of surmising does not work reliably. For any set of physical findings, there may be more than one possible explanation for the physical evidence. If there is more than one possible explanation for the physical evidence, then selecting one explanation over another would then be a matter of guesswork or speculation. If there is more than one possible explanation, how can a scientist state with reasonable certainty that his or her belief of what happened is the truth?

Another name for this type of thinking is scenario building. It is a fallacy to believe that a scientist can manufacture a “story”—as if he were witnessing the events of the past himself—on the basis of physical evidence. This cannot be done reliably, and opinions of this type should never be admitted as expert testimony. A forensic scientist should not invent scenarios, particularly when witness accounts are available for testing in the manner stated previously.

When witness accounts are not available, a forensic scientist cannot and should not state what happened with any certainty. He or she can only state reliably what is possible, not what actually happened.

There is a universal experience we all share. That experience is known as “jumping to conclusions.” It can also be described as “prejudice.” Certain observations we make about individuals and situations in the present may lead us to conclude that we know the events and circumstances that led to the things we see. For example, we know that judging someone’s character by skin color or the way he or she dresses is wrong and frequently incorrect, but we have all done it. This is the problem with surmising past events. It leads to a form of conclusion jumping that we are prone to do as human beings, and it does not allow us to consider carefully the scientific limitations of our observations and conclusions.

...unless there is only one plausible explanation for that evidence.

The final phrase provides the only exception for this investigative process. Under certain circumstances, when the possibilities are limited to only one plausible explanation, that explanation can be stated as an opinion made to a reasonable degree of certainty.

For example, a medical examiner filing a death certificate has to offer an opinion of the manner of death. A manner of death is a one-word description describing the circumstances surrounding the death. There are only five manners: natural, homicide, suicide, accident, and undetermined.

If a body is discovered in a wooded area with numerous stab wounds to vital organs and multiple devastating blunt force head injuries, it would be reasonable to conclude a manner of homicide. One cannot envision any circumstance where natural, suicide, or accident could plausibly apply. Also, the nature of the injuries and the condition and location of the body are of such a nature to allow one to conclude reasonably that foul play has occurred, so an undetermined manner would not be suitable.

Note, however, that no scenario is proposed for how this homicide came to occur. No scientist can describe in any more detail the complex and unique chain of events that led to that death with any reasonable certainty. Only a witness can do that!

What is the practical bottom line of this postulate? I believe it can be summarized like this:

1. Before offering an opinion on any case, the scientist must first free his or her mind of any preconceived ideas and then read the statements of the witnesses who saw what happened. The scientist must compare those witness statements with the physical evidence.

2. If the witness statements in their entirety fit the physical evidence, those statements should be considered as truthful.

3. If one or more portions of the statements do not fit the physical evidence, those portions should be considered as not truthful.

4. The scientist should never invent a scenario that is not supported by witness statements to explain the physical evidence.

5. If scenarios not supported by witness statements are offered by other people, the scientist should speak of them only with terms of uncertainty.

No scientist wants to be confronted with the consequences of the ill-informed conclusions he or she may have made about forensic evidence! I believe the implications of Young’s Postulate need to be taken into consideration prior to offering opinions. Doing so will prevent mistakes that lead to injustice.

For further explanation of this postulate, see Forensic Science and the Scientific Method. Postulate written on April 5, 2009.