Witnesses will often testify that a suspect looked angry, or tense, or too calm for someone whose husband or wife was just killed. But do such memories reflect reality?

A recent psychology study shows that when evaluating the emotional state of others, our perceptions can be easily manipulated.

Researchers from the University of California, San Diego, working with psychologists in France and New Zealand used the technique of photo morphing - mixing of different photos to create a composite.

The researchers had the same model give an angry and a happy expression, and then created a spectrum of images of the same face, ranging from happy to angry.

Volunteers were then shown a picture that was exactly halfway between angry and happy. The idea was to make the emotions ambiguous, said lead researcher Piotr Winkielman of UCSD. Some volunteers were told to explain why the subjects were angry while others were asked to suggest reasons they were happy.

The researchers then played movies that displayed the entire spectrum of faces morphing from happy to angry and, after a break, asked the subjects to pick the image they were first shown.

The results: The subjects given the happy suggestion picked happier pictures than the ones they originally saw. The ones with the angry suggestion went the other way.

"They were transforming their perceptions because of what they were being led to think about this person," Winkielman said. He hopes the findings, published in Psychological Science, can help us understand the pitfalls of eyewitness memory.

In addition, the work may have therapeutic applications for helping socially phobic or insecure people, who often wrongly attribute disapproving or hostile expressions to others. In such cases, he said, "people often mistake their own expectations for reality."

- Faye Flam