The University of NSW psychologists also tested the effect of the shooters' mood on the choice of people they mowed down, and these findings were surprising - those who shot the greatest number of unarmed turban-wearers were the happy people. In our society happiness is a prized commodity. Feeling upbeat is supposed to make us more creative, co-operative and successful - a positive state we are encouraged to strive for. Yet the most deeply ingrained human emotions - fear, anger, disgust and sadness - are negative ones, points out Professor Joseph Forgas, of the University of NSW. "It seems intriguing that despite our apparently never-ending quest for happiness and satisfaction, the human emotional repertoire is nevertheless heavily skewed towards negative feelings," he says. The reason why fear, anger and disgust evolved in the dangerous environment our ancestors faced is easiest to understand. These emotions would have helped prepare people for flight or fight in the face of threats.

But the advantages of feeling sad are more puzzling, he says. Many ideas have been proposed for why sadness might have evolved. Sitting in a corner feeling glum after defeat in a fight, rather than challenging the victor, may have been a lifesaving strategy for our ancestors - a trait they could then pass on to their offspring. It could also have signalled to other members of the group that an individual needed extra support and protection. Sadness, and time out to think, may have helped people learn from their mistakes as well as deterred them from being reckless, suggests Professor Allan Horwitz and Jerome Wakefield in their book The Loss of Sadness: How Psychiatry Transformed Normal Sorrow into Depressive Disorder.

There have also been suggestions that sadness can affect the way people think and process the information around them, and this is where the University of NSW research fits in. Forgas and his colleagues have shown not only that being happy can have its drawbacks, but that there are also some important upsides to being down. Mild sadness can improve people's recall of everyday events, make them more sceptical and better at detecting deception by others, and less likely to make snap judgments about people, based on stereotypes. "It seems counterintuitive, but a little bit of sadness in some situations can turn out to be a good thing," Forgas says. The findings are particularly relevant in legal and forensic settings, where the negative or positive mood of a witness, a police officer or a judge might influence their recall or decisions.

Mild sadness tends to make people more attentive to the details of their surroundings and have a more careful, thorough thinking style. People who are expecting to participate in a demanding personal event, such as meeting a stranger, intuitively know this and prefer to reduce their level of happiness first by reading sad articles, research shows. Forgas and his colleagues chose a suburban newsagency in Sydney last year to test the influence of positive and negative mood on memory. They placed 10 little items on the counter, including a pink pig, a London bus and a toy cannon, and then quizzed 73 unsuspecting customers about them as they left the shop. On cold, wet days, when sombre music such as a requiem was playing, people could list three times as many items as on a sunny day when they heard cheery music, such as Gilbert and Sullivan tunes, the study, published in April, found. "Accurately remembering mundane, everyday scenes is a difficult and demanding task, yet such memories can be of crucial importance in everyday life, as well as in forensic and legal practice," Forgas says.

Feeling happy cannot only reduce the accuracy of a witness's memory, it can also make them more susceptible to believing false information, the Sydney psychologists have found. In another study people looked at pictures of a car-crash scene and a wedding, and later were given misleading suggestions, for example, that a firefighter had used a hose at the crash. People in a happy mood were more likely to include this misinformation in their description when they recalled the events. Despite being wrong, they were also more confident they were right. Those in a gloomy state, on the other hand, were less likely to be fooled. "In fact, negative mood almost completely eliminated this common misinformation effect," Forgas wrote in a review of their research to be published soon.

Deciding whether someone is a liar or telling the truth is also vital in police investigations and court cases, so the team looked at the effect of mood on this human ability. People were told to either steal a movie ticket from an empty room or leave it there. They then had to deny taking it in a videotaped interrogation. When other people were asked to judge the guilt of a defendant they were more likely to accept the denials as truthful if they were in a happy mood. "Sad judges made significantly more guilty judgments and were significantly better at correctly detecting deceptive targets," Forgas says. And as the turban study showed, happy people also tend to rely more on pre-existing stereotypes. The Sydney team used music, the weather, watching movies and remembering happy or sad events to induce mild positive and negative moods in the participants.

In real life, other factors than mood could be at play, for example whether people are focusing their attention closely on events. But eventually courts may need to take the mood of a witness during an event into account, Forgas says. In the Loss of Sadness, Horwitz, a professor of sociology at Rutgers University, and Wakefield, a professor of social work at New York University, suggest intense sadness after a major loss may also have evolved because it had some sort of survival value. "As Darwin noted, apes and humans show similar facial expressions in situations that are associated with sadness, including elevated eyebrows, drooping eyelids, horizontal wrinkles across the forehead, and outward extension and drawing down of the lips," they say. Like us, apes also withdraw socially, stop playing, slouch and even adopt foetal-like postures to cope with the painful emotion. For these behaviours to have been naturally selected, there must have been special circumstances where the benefits of intense sadness outweighed the costs, Horwitz and Wakefield say.

They liken it to acute pain after an injury, which immobilises people and prevents them doing further damage. Not only might it have led to a defeated person withdrawing from physical battle in the past, it might help people today accept that some other life goals are unattainable, they say. Sadness is a natural response to life crises, such as the death of a loved one, loss of a job, the end of a love affair or failure to achieve a dream. When people are forced to re-evaluate their futures, a period of sadness might "allow the individual to emerge properly motivated by newly selected goals". Loading Psychiatrists caution that sadness can lead to depression and people have the right to seek treatment for symptoms that concern them. But the two authors argue that a clear distinction needs to be made between depressive illness and a period of normal sadness after a loss.

Although drugs to treat depression can lift sadness, there may be some benefits to experiencing this emotion, they say.