Confirmation Bias and DUI Drug Allegations by Jon Scott Fox   September 2010

The Washington State Patrol and many other police departments are on patrol looking for citizens who might be driving while under the influence of drugs. This not what you might think. A citizen can be arrested and prosecuted for DUI due to illegal or legal drugs, pursuant to RCW 46.61.502 . It is not uncommon for a citizen to be prosecuted for DUI after taking drugs that a doctor has prescibed. For alcohol related DUI charges there is a "legal limit" of .08 but no such limit exists for drugs that might impair driving. There is no law against driivng with ".08 marijuana" in the system but there is a law against driving when one's ability to drive is impaired by alcohol or any other drug.

The absence of a "legal limit" for driving under the influence of drugs means that a DUI officer who suspects such conduct must investigate and attempt to prove that the driver's ability to drive was indeed impaired by the drug. To address this issue, the police came up with a "drug recognition" procedure to investigate whether a person is impaired by drugs and if so, to determine which drug might be involved. This protocol is used by Seattle and King County DUI officers and the State Patrol. The protocol begins with an interview with the DUI suspect, who is asked which drug he or she has consumed (or smoked). Thus, if a driver says "I was smoking marijuana just before I drove," it seems logical that this "clue" might influence the officer to investigate and conclude that it was, indeed, marijuana. The same is true, of course, when the driver tells the officer that he was drinking alcohol before driving.

In a recent court case involving DUI "drug recognition experts" the court observed that modern psychology has a term for the bias inherent in this investigatory methodology. It is called "confirmation bias." Confirmation bias was described by the court as:

"The tendency to bolster a hypothesis by seeking consistent evidence while minimizing inconsistent evidence. Confirmation bias involves nonconscious information processing rather than deliberate case building. Someone intentionally preparing a one-sided argument, such as a debater preparing for a match, would not be said to display confirmation bias. Rather, it involves unwittingly selecting and interpreting evidence to support a previously held belief."

The court continued:

"Confirmation bias" is a form of tunnel vision, and it can happen in one or more ways. See Keith A. Findley & Michael S. Scott, The Multiple Dimensions of Tunnel Vision in Criminal Cases, 2006 WIS. L.REV. 291. People seek out evidence to confirm their hypothesis, id. at 308-09; people search their memories in biased ways, preferring information that tends to confirm a presented hypothesis or belief, id. at 312; and people also tend to give greater weight to information that supports existing beliefs than to information that runs counter to them; that is to say, people tend to interpret data in ways that support their prior beliefs. Id. at 312-13. Empirical research demonstrates that people are "incapable of evaluating the strength of evidence independently of their prior beliefs." Id.

The court then applied the theory to the case before it:

In this case, Moertl knew from his briefing by Selk and from Haynor's own admissions that Haynor had taken three different CNS depressants before driving. "Confirmation bias" suggests that when Moertl performed the Drug Recognition Protocol, he would nonconsciously look for evidence to support his hypothesis that Haynor was operating under the influence of drugs.

Applied to a DUI "drug recognition" investigaiton, or any DUI stop, the theory of confirmation bias holds that once an officer hears from the accused that he smoked marijuana or drank alcohol at some point earlier in time, the task is unconsciously transformed into finding evidence to confirm this information.

No Washington court case known to this author has presented "confirmation bias" evidence in court but that won't be true for long. The psychologists have now only given a name to a logical trait of human behavior that almost certainly influences, to some degree, every DUI investigation.