Drinking enough alcohol to become intoxicated increases aggression significantly in people who have one particular personality trait, according to new research.

But people without that trait don't get any more aggressive when drunk than they would when they're sober.

That trait is the ability to consider the future consequences of current actions.

"People who focus on the here and now, without thinking about the impact on the future, are more aggressive than others when they are sober, but the effect is magnified greatly when they're drunk," said Brad Bushman, lead author of the study and professor of communication and psychology at Ohio State University.

"If you carefully consider the consequences of your actions, it is unlikely getting drunk is going to make you any more aggressive than you usually are."

Peter Giancola, professor of psychology, at the University of Kentucky, co-authored the paper with Bushman and led the experiments used in the study. Other co-authors were Dominic Parrott, associate professor of psychology at of Georgia State University and Robert Roth, associate professor of psychiatry, at Dartmouth Medical School. Their results appear online in the Journal of Experimental Social Psychology and will be published in a future print edition.

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Bushman said it makes sense that alcohol would make present-focused people more aggressive.

"Alcohol has a myopic effect -- it narrows your attention to what is important to you right now. That may be dangerous to someone who already has that tendency to ignore the future consequences of their actions and who is placed in a hostile situation."

The study involved 495 adults, with an average age of 23, who were social drinkers. Before participating, the participants were screened for any past or present drug, alcohol and psychiatric-related problems. Women were tested to ensure they weren't pregnant.

All participants completed the "Consideration of Future Consequences scale." They indicated how much they agreed with statements like "I only act to satisfy immediate concerns, figuring the future will take care of itself." Scores on this measure determined how much participants were present-focused or future-focused.

Half the participants were put in the alcohol group, where they received alcohol mixed with orange juice at a 1:5 ratio. The other half were given orange juice with just a tiny bit of alcohol. The rims of the glasses were also sprayed with alcohol so that they thought they were consuming a full alcoholic beverage.

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Participants in the alcohol group had a mean blood alcohol level of 0.095 just before aggression was measured and 0.105 following, meaning they were legally drunk and that their alcohol levels were rising during the measurement of their aggressive behavior.

Those in the placebo group had mean blood alcohol levels that didn't exceed 0.015, meaning they had very little alcohol in their systems and were well below standards of intoxication.

The aggression measure used in this study was developed in 1967 to test aggressiveness through the use of harmless but somewhat painful electric shocks. The researchers measured the participants' threshold to the electric shock pain before the experiment began to ensure that no one received a shock that exceeded what they could take.

Each of the participants was told that he or she was competing with a same-sex opponent in a computer-based speed reaction test, with the winner delivering an electrical shock to the loser. The winner determined the intensity and the length of the shock delivered to the loser.

In actuality, there was no opponent. There were 34 trials, and the participant "won" half of them (randomly determined). Each time they "lost," the participants received electric shocks that increased in length and intensity over the course of the trials, and the researchers measured if they retaliated in kind.

"The participants were led to believe they were dealing with a real jerk who got more and more nasty as the experiment continued," Bushman said. "We tried to mimic what happens in real life, in that the aggression escalated as time went on."

Results were clear, Bushman said.

"The less people thought about the future, the more likely they were to retaliate, but especially when they were drunk. People who were present-focused and drunk shocked their opponents longer and harder than anyone else in the study," he said.

"Alcohol didn't have much effect on the aggressiveness of people who were future-focused."

Men were more aggressive than women overall, but the effects of alcohol and personality were similar in both sexes. In other words, women who were present-focused were still much more aggressive when drunk than were women who were future-focused, just like men.

Bushman said the results should serve as a warning to people who live only in the moment without thinking too much about the future.

"If you're that kind of person, you really should watch your drinking. Combining alcohol with a focus on the present can be a recipe for disaster."

The study was supported by grants from the National Institute on Alcohol Abuse and Alcoholism and from the National Center for Research Resources.