Simply the belief that you are drinking alcohol can impair judgement and dent memory, say researchers.
By Dr David Whitehouse
BBC News Online science editor
According to Seema Assefi and Maryanne Garry, two psychologists at Victoria University in New Zealand, memory can be affected by an alcohol placebo.
Just thinking about it can make you tipsy
Tests showed that participants in an experiment who were told they were drinking vodka, but were not, were more swayed by misleading information and more certain their memory was correct than those who were told they were drinking tonic water.
Dr Garry says the research has given new insights into how human memory works and how both social and non-social influences can affect a person's recall of events.
"What we have done is that we have made people's memory worse by telling them that they were intoxicated even though they had drunken nothing stronger than plain flat tonic water with limes," he adds.
Thinking yourself tipsy
For the study, 148 students were split into two groups, half being told they were getting vodka and tonic and the rest told they were getting just tonic. In reality, all were getting just plain tonic.
The research was carried out in a bar-like room equipped with bartenders, vodka bottles, tonic bottles, and glasses.
We found people who thought they were intoxicated were more suggestible and made worse eyewitnesses compared to those who thought they were sober
Flat tonic water was poured from sealed vodka bottles to appear genuine. The deception was completed by rimming glasses with limes dunked in vodka.
After consuming their drinks, the students watched a sequence of slides depicting a crime. They also read a summary of the crime that contained misleading information.
"We found people who thought they were intoxicated were more suggestible and made worse eyewitnesses compared with those who thought they were sober," Seema Assefi says.
"In fact the 'vodka and tonic' students acted drunk, some even showing physical signs of intoxication," she adds.
When told, the sober students reacted with disbelief.
"When students were told the true nature of the experiment at the completion of the study, many were amazed that they had only received plain tonic, insisting that they had felt drunk at the time," she comments.
Dr Garry concludes: "It showed that even thinking you've been drinking affects your behaviour.
"Even on plain tonic water, the male students flirted with Seema as she conducted the experiment and the girls giggled a lot."
The serious point behind the research is that it demonstrates that memory is not just about filing away information like a computer does. It is what we use to understand and remember events in a social setting, such as witnessing a crime.
The research is published in Psychological Science, published by the American Psychological Association.