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Is slurred speech a good indicator of intoxication?

Yes and no. Yes in the limited sense that intoxication is indeed a common cause of slurred speech. If someone has been drinking heavily, it is likely that it will affect his or her speaking fluency. And, yes, it goes the other way: if someone is heavily slurring his or her speech—and in the absence of a speech impediment or neurological disorder—the person has probably been drinking. Research has indeed shown that most people can differentiate between sober and intoxicated speech when listening to recordings of people talking.

However, this does not mean that it is possible to judge, with a high degree of reliability, that a person is above the legal limit simply based on listening to them talk. Drinking and driving, remember, is not against the law. What is against the law is to operate a vehicle with a certain blood alcohol content or while being significantly impaired. So let’s put the question a different way. Can a person accurately distinguish between someone who is too drunk to drive versus someone who consumed alcohol but can nonetheless drive legally—all based on how they talk? After all, that’s what police officers implicitly claim to do when they put “slurred speech” on the police report.

To that question the answer is decidedly “No.” Studies have shown that even experts in speech—called phoneticians—aren’t that much better than the average person, or the average police officer for that matter, at making this kind of judgment. That is to say, they’re not that good at all. It is true that both experts and non-experts can usually tell a person who has been drinking heavily from someone who hasn’t, but they cannot consistently determine the relative amount of alcohol a person has consumed.

Here’s an excerpt from an article in Discover magazine.

Bartenders, police officers and hospital workers routinely identify drunks by their slurred speech. Several investigative groups judged the captain of the Exxon Valdez oil tanker to be intoxicated based solely on the sound of his voice in his radio transmissions. But a team led by Harry Holien, a phonetician at the University of Florida, has found that even self-proclaimed experts are pretty bad at estimating people’s alcohol levels by the way they talk.

Hollien asked clinicians who treat chemical dependency, along with a group of everyday people, to listen to recordings made by volunteers when they were sober, then mildly intoxicated, legally impaired, and finally, completely smashed. Listeners consistently overestimated the drunkeness of mildly intoxicated subjects. Conversely, they underestimated the alcohol levels of those who were most inebriated. Professionals were little better at perceiving the truth than the ordinary Joes….

He thinks his research could encourage police to be more wary of making snap judgments: Mild drinkers might come under needless suspicion."
(Saunders, “News of Science, Medicine and Technology: Straight Talk”, 21(1) Discover Oct. 2000)

The other problem with using speech as an indicator of intoxication is that alcohol is not the only thing that causes someone to slur their speech. The first, which is rare but too interesting not to mention, is a condition called Ataxic dysarthria—a neurological disorder that causes a person to sound drunk. The most common, and the most likely to lead a police officer to make an unfair assumption, is stress. Stress can have a host of different effects on speech, such as a higher pitch, stuttering, and, yes, slurring. And as we all know, being pulled over is always a stressful experience. When we are extremely preoccupied with saying the right thing—as we are when talking to a police officer—we often can’t seem to form a normal English sentence, much less speak eloquently.

None of this is to say that it will never be possible to determine how drunk a person is from their speech, or to distinguish between a stressed-out person and a drunk person. A Georgia Institute of Technology researcher is working with computer analyses of speech to see if it is possible to teach a computer to reliably make just this kind of judgment. Though it may be possible, even this is far off in the future. Teaching police officers to make this kind of judgment, however, is a whole different story.