Blood alcohol level, whether determined by "Breathalyzer" measurements, urine or blood samples, yields an objective-sounding number. But that figure tells little about one's ability to drive or function while "legally" drunk.
"Individual differences in response and tolerance for alcohol vary so widely that one person may be incapacitated by a less-than-the-legal-limit alcohol dose, while others show almost no response to a fairly high blood alcohol reading," according to Gene Erwin, director of the Alcohol Research Center and professor of pharmacology.
Both an acquired tolerance for alcohol and individual genetic differences account for the wide variations in response, he explained.
Tolerance for alcohol can be built up both over the long term — through regular drinking — and in each individual encounter with alcohol, according to research Erwin has undertaken with colleagues Robert Plomin and Jim Wilson.
In the laboratory, volunteers were given alcohol mixed with water or a sugar-free mixer (to eliminate body changes produced by sugar) until their blood alcohol level was 0.10 percent. At that level, most people performed poorly in terms of judgment, balance, muscle control arid other physical tests.
When kept at the same level of drunkenness for three hours, however, some gradually improved their performances. After building up their tolerance for alcohol in the laboratory situation, roughly analogous to social drinking, some people were able to perform as well while legally drunk as they had when sober.
Others were not able to match their sober performance level. These individuals, the researchers believe, lack either the inherited or acquired ability to function with large alcohol doses.
"In these tests, we're dealing with normal people, not alcoholics," Robert Plomin said. Much of the information on alcohol has involved alcoholics or individuals with what the scientists call "chronic acquired tolerance," not average folks. Generalizing from acutely tolerant people to the public at large does not allow for the wide variations of individual differences, they note.
"The old 'pull over, buddy, and let's see you walk this straight line' approach to determining drunkenness is a much more accurate way to see how impaired an individual may be," Robert Plomin said. The roadside drunk-or-sober tests of performance are gradually being abandoned by law enforcement officers around the country, however, since the tests require subjective judgment — the officers' perceptions of the drivers' abilities.