Most of us might think we are fairly good at spotting the one who has had a few too many. Slurred speech, red eyes, stumbling. Telltale signs obvious right?

Well, a recent review paper published in the journal Behavioral Sciences and the Law concludes that we are actually pretty awful at detecting the level of intoxication in others.

One study found that police officers covering check points correctly identified only 20 percent of drivers with blood alcohol concentrations (or BACs) over 0.10 percent, a level that typically leaves one's reaction time, motor control, significantly impaired. For perspective the U.S. legal limit is 0.08 percent.

Most of the telltale signs provide surprisingly crude measurement. For instance, slurred speech does separate those above a BAC of 0.08 percent but it cannot accurately predict anything lower than 0.08. Same with stumbling. Great if the person is at a BAC of 0.15 or is pretty much plastered. Not so great if they are around 0.10. And results are similarly inaccurate with the famous finger to nose test, or even when a person stands on one leg. At low BAC levels, these tests lose accuracy.

Curiously, the Gaze Nystagmus test, which looks for jerky eye movements as one follows a smoothly moving object, is the best measurement of BAC at lower levels. 

But overall, it seems no behavioral or physical clue is consistently related to BAC.   And there is a lot variation among individuals. For instance, alcoholics can appear totally straight sober at a BAC that is fatal to many drinkers yet still appear more drunk than a social drinker at a very low BAC.

The author suggests that sobriety tests need to be subject to more peer-reviewed studies and established norms be set. Otherwise we are all in danger of mistaking a drunk for a teetotaler or vice versa.

—Christie Nicholson