The surprise finding, which is to be published in the Journal of Experimental Social Psychology, is the first to assess the effect of mood on memory and human thinking.
People in a positive mood such as happiness were shown under experimental conditions to have relatively unreliable memories, and show poorer judgement and critical thinking skills.
By contrast, those who experienced a negative mood such as sadness were shown to provide more reliable eyewitnesses accounts and exercise superior thinking and communication skills.
In one experiment, researchers at the Sydney-based University of New South Wales school of psychology put different subjects in a positive (happy) or negative (sad) mood state and tested the accuracy of their recall of a staged eyewitness event such as a bag snatch.
"The results showed that eyewitness accounts of people in a negative mood are more likely to be accurate compared to those in a positive mood state," says Professor Forgas.
"It shows that our recollection of past events are more likely to be contaminated by irrelevant information when we are in a positive mood. A positive mood is likely to trigger less careful thinking strategies."
In a second experiment, researchers put different subjects in a positive or negative mood state and asked them to write down an argument in favour of a particular proposition.
When their arguments were analysed for their quality and persuasiveness, subjects in a negative mood were shown to be far more effective in their critical thinking and communication skills.
"The finding makes sense in evolutionary terms," says Professor Forgas. "Animals that are wary of their environment are more likely to perceive threats to their survival.
"This supports the idea that mood states are evolutionary signals about how to deal with threatening situations. That is, a negative mood state triggers more systematic, more attentive, more vigilant information processing.
By contrast, good moods signal a benign, non-threatening environment where we don't need to be so vigilant."
UNSW SCIENTIA PROFESSOR JOSEPH FORGAS -- is a distinguished scholar. Holder of a 2004 Rockefeller Foundation Fellowship, he has won numerous research awards, including an Australian Research Council Special Investigator Award, the Alexander von Humboldt Research Prize, an Australian Psychological Society Distinguished Scientific Contribution Award, and the Australian Psychological Society Early Career Award.