The Misleading Concept of Alcohol Tolerance in Michigan DUI Cases

Many judges, prosecutors, and defense attorneys have mistaken beliefs about alcohol tolerance.  In a drunk driving trial, these mistaken beliefs can lead to misleading arguments which might further lead to wrongful convictions.  A recent article from the peer-reviewed science journal article Psychopharmacology[i] helps support a defense lawyer’s attempt to preclude a wrongful conviction based on these misleading arguments.

Generally speaking, most drunk driving trials include observations of the police leading up to the arrest and a subsequent breath or blood test, generically called a “chemical test.”  Therefore, winning a drunk driving trial requires reasonable doubt as to both.

Issue With Alcohol Tolerance

There are many ways to approach the defense of a drunk driving crime, and a defense should always be matched to the facts and science of a particular case.  However, one way to explain to jurors that there is a reason to doubt the chemical test is to show that there is a disparity between the breath or blood test result and the physical and mental condition of the driver.  This might occur in cases where the driver performs well on the field tasks, such as the one-leg stand or walk and turn.  This evidence, usually collected at the roadside, is often referred to as the “observational evidence.”

The argument that follows is that there is an inconsistency, sometimes called a “disconnect” between the chemical test and observational evidence.  In other words, one would not expect a drunk person to be able to walk a straight line or stand on one leg

Let’s look at a hypothetical case to help elucidate the point.  Assume a driver is arrested and a later chemical test suggests a bodily alcohol level (BAC) of .19.  The driver is charged with high BAC or super drunk driving.  The video from the patrol vehicle on the night of the arrest shows that this same driver could stand on one leg for 30 seconds, and was able to walk 9 heel-to-toe steps forward, turn in the prescribed manner using a series of small steps, and then walks 9 heel-to-toe steps back.  The driver has bloodshot eyes, and a strong odor of “intoxicants” on their breath.  The driver also admits to drinking “a couple” before being stopped.  A roadside breath test shows a BAC of .15.  Because the evidentiary test is a .19, it’s charged as a high BAC case, and the prosecutor says there is no plea bargaining.  The driver does not wish to plead guilty as charged, so the case is set for trial.  What is the defense?

One possible approach to this case would be to argue to the jury that it is not possible for a driver to both have a BAC of .19 and perform well on the field sobriety tasks.  This makes logical and intuitive sense.  However, this argument will be met by a prosecutor’s counter-argument that the driver was an experienced drinker and could clearly “hold their liquor.”  The prosecutor will prepare the jury for this argument in voir dire (jury selection) by asking the jury questions about the person they know who can really put away the drinks.  The prosecutor will ask the jurors to think of a person they know who can drink everyone else under the table and never appear to be drunk.  In other words, the prosecutor wants the jury to think the .19 driver is just such a person. This is a powerful argument because it makes intuitive sense.  We’ve all seen such a thing, right?  The trouble with this misleading argument is it is so clearly wrong.

In fact, there are many problems with such prosecutorial arguments.  First, it assumes that the driver is a heavy drinker.  Since there will be no proof of this, the argument must assume facts not in evidence.  And the argument that the driver is heavy drinker is not one that logically follows from facts that will be in evidence.  An even bigger problem, however, is that it is contrary to the science.  Because of this, such arguments should never be allowed, yet they are frequently made by prosecutors, allowed by judges, and not objected to by defense attorneys.

Understanding why these arguments are wrong requires the defense attorney to understand the science well enough to be able to articulate it to the judge.  Because of the complexity of this task, it should always be done in the form of a pretrial motion to preclude the prosecutor from making such arguments or engaging in this questing of prospective jurors during voir dire.

The Science of Alcohol Tolerance

Pharmacology and toxicology are the two branches of science that address the issue of alcohol intoxication and tolerance.  Pharmacokinetics is the science addressing how alcohol moves through the body while Pharmacodynamics involves how the alcohol impacts the brain.  An understanding of both is required.

As it relates to Pharmacokinetics, suffice to say the range of alcohol metabolism within the human population, that is the “burn rate,” is about .009 per hour for naïve or inexperienced drinkers to about .036 per hour for people regularly exposed to large quantities of alcohol.  That’s an almost four-fold difference.  This means that it takes four times as many drinks for a heavy drinker to reach the same BAC as a naïve drinker.  But this supports the prosecutor’s arguments, doesn’t it?  Not so fast!  This means that one driver with a BAC of .10 may have consumed four times as many drinks as another driver, just to reach the same BAC.  But once each driver gets to the same BAC, do they behave differently?  This is no longer a Pharmacokinetics argument but instead is a Pharmacodynamics argument.

A recent study addressing this very issue is entitled Alcohol-induced performance impairment: a 5-year re-examination study in heavy and light drinkers. Interestingly, this study found that:

“it appears that chronic excessive drinking can yield increased pharmacokinetic tolerance over a 5-year period. BAC was a significant predictor of both performance impairment and perceived impairment in both groups as expected; however, HDs’ increased speed of absorption and metabolism did not result in an advantage in performance on the more complex psychomotor task.”

In other words, once a person gets to a given BAC, it doesn’t matter how much it took them to get there, both heavy and naïve drinkers will behave essentially the same.

Returning now to our hypothetical example, the walk and turn and the one leg stand are “complex psychomotor tasks.” One would not be expected to be able to perform well on these tasks with a BAC of .19. A prosecutor who is allowed to argue opposite this is misleading the jury.   We know this is true because according to the above article:

“this study provided evidence of heavy drinkers’ acquired behavioral tolerance over a 5-year period on a task of fine motor speed but not on a cognitively more complex task requiring motor speed but also executive functioning skills of encoding, set-shifting, and short-term memory.”

Field sobriety tasks are specifically intended to test this kind of executive functioning because a police officer administering them gives the driver a complex set of instructions that the driver must them remember as he or she performs the physical “motor” functions of balancing on one leg or walking heel-to-toe a prescribed number of steps, turning in a prescribed fashion, then walking back a prescribed number of steps.  The standardized instructions are as follows:

(1)“When I tell you to start, take nine heel-to-toe steps, turn, and take nine heel-to-toe steps back.”  (2) “When you turn, keep the front foot on the line, and turn by taking a series of small steps with the other foot, like this.”  (3) “While you are walking, keep your arms at your sides, watch your feet at all times, and count your steps out loud.”(4)“Once you start walking, don’t stop until you have completed the test.”(5) “Do you understand the instructions?”  (6) “Begin, and count your first step from the heel-to-toe position as “One”.

The conclusion to all this is that a disconnect argument is supported by science while a “we all know a guy who can hold his liquor” argument is not.  Prosecutors should never be allowed to mislead juries in drunk driving trials using such arguments.

[i] June 2017, Volume 234, Issue 11, pp 1749–1759