Consider a typical DUI traffic stop that often occurs in Jacksonville, Florida. A police officer will see a person commit a driving infraction such as speeding or running a stop sign late on a Saturday night. The police officer pulls the driver over and immediately suspects the driver of being under the influence of alcohol or drugs, perhaps because of the age of the driver, the fact that it is late on a weekend, the fact that there are certain bars or restaurants down the road or any other factors that may bias the officer. From that point on, the police encounter and the decision as to whether or not to arrest the driver for DUI is very subjective. In other words, whether a DUI arrest is made is not based on concrete, objective factors that can later be confirmed in court; rather, the decision to arrest for DUI will often be based on the perceptions, observations, conclusions and biases of the police officer. Just about every police officer that has made a DUI arrest since the beginning of time will report that the suspect had bloodshot and watery eyes, emitted a strong odor of alcohol, had slurred or mumbled speech and failed the field sobriety tests if the driver submitted to them. However, those conclusions are all very subjective. How bloodshot and watery were the driver’s eyes compared to what they normally look like? What if the driver was in a smoky bar or staring at a computer screen all day? How strong is a “strong odor of alcohol”? What is slurred speech compared to how a person normally speaks? Over the entire time period of the police encounter, how often must the driver slur his/her speech for it to be considered significant? Is the speech slurred due to alcohol or because the person is nervous? How the officer interprets these questions is very subjective.
The word “bias” is not used negatively here but as a natural and normal psychological phenomenon- a cognitive bias, and it is a significant factor. The human brain is wired to see patterns and draw conclusions subconsciously. While we would hope that a police officer would come to a conclusion only after assessing all of the relevant data, humans have a psychological tendency to draw the conclusion and fit the data to conform to that conclusion. The human brain is also wired to avoid conflict. In other words, if we believe something to be true, i.e. we see something we believe conforms to a pattern we assume exists, we challenge ideas or perceptions that are inconsistent with our belief and automatically accept ideas that are consistent with our belief. The human brain is much happier when ideas and perceptions are consistent.
At a DUI stop, if a police officer believes the driver is under the influence of alcohol, i.e. that is the idea he/she perceives that is consistent with the pattern he/she accepts, the officer may interpret the subsequent evidence to conform to that belief. As a result, these subjective factors like bloodshot and watery eyes, slurred speech, an odor of alcohol and performance on field sobriety tests may be interpreted to be consistent with the idea of a drunk driver rather than what the facts actually illustrate.