Anger is an interesting emotion for psychologists. On the one hand, it's negative, but then it also has some of the features of positive emotions. For a new study published in Psychological Science, a journal of the Association for Psychological Science, researchers find that associating an object with anger actually makes people want the object -- a kind of motivation that's normally associated with positive emotions.

People usually think of anger as a negative emotion. You're not supposed to get angry. But anger also has some positive features. For example, it activates an area on the left side of the brain that is associated with many positive emotions. And, like positive emotions, it can motivate people to go after something. "People are motivated to do something or obtain a certain object in the world because it's rewarding for them. Usually this means that the object is positive and makes you happy," says Henk Aarts of Utrecht University in the Netherlands, first author of the new study. He and his colleagues wanted to examine whether this also applies to the link between anger as a negative emotion and the desire to get your hands on something.

For the study, each participant watched a computer screen while images of common objects, like a mug or a pen, appeared on the screen. What they didn't realize was that immediately before each object appeared, the screen flashed either a neutral face, an angry face, or a fearful face. This subliminal image tied an emotion to each object. At the end of the experiment, the participants were asked how much they wanted each object. In a second version of the experiment, they had the person squeeze a handgrip to get the desired object -- those who squeezed harder were more likely to win it.

People put more effort in action to obtain objects associated with angry faces. (They did not do this for items associated with fear.) "This makes sense if you think about the evolution of human motivation," says Aarts. For example, say there's limited food in the environment. In such a context those persons that associate food with anger and turn aggression into an attack response to get the food are more likely to survive. "If the food does not make you angry or doesn't produce aggression in your system, you may starve and lose the battle," Aarts says.

Interestingly, the participants in this study had no idea that their desire for the objects had to do with anger, Aarts says. "When you ask people why they work harder to get it, they say, 'It's just because I like it.'" That shows how little we know about our own motivations, he says.