If you are predisposed to trust a stranger because he belongs to the same club as you, for example, a happy mood makes you even more likely to trust him. But if you are predisposed to not trust him, a positive mood will make you even less trusting than normal.
"A person's mood may determine how much they rely on subtle – or not so subtle -- cues when evaluating whether to trust someone," said Robert Lount, author of the study and assistant professor of management and human resources at Ohio State University.
In five separate experiments, researchers found that people in a positive mood were more likely than those in a neutral mood to follow cues or stereotypes when determining whether they should trust someone.
All five experiments involved undergraduate students who took part in various scenarios in which they were put into positive or neutral moods, and were then given the opportunity to show trust or distrust toward a stranger.
In one study, for example, participants were first asked to write one of two short essays. Some wrote about an experience that made them happy while others wrote about what they did in a typical day. Those writing tasks were previously shown to put people in a happy or neutral mood.
The participants were then shown a picture of a person and asked a variety of questions designed to find out how much they would trust him. For example, one question asked how likely the participants thought it would be that the person would intentionally misrepresent their point of view to others.
All the pictures were created by a software program that made the faces appear trustworthy or untrustworthy to most people. A trustworthy person had a round face, round eyes and was clean shaven. An untrustworthy person had a narrow face, narrow eyes and facial hair.
The results were striking: participants in a positive mood evaluated the person with the trustworthy features as more trustworthy than did those in a neutral mood.
"I think the assumption is that if you make someone happy, they are going to be more likely to trust you. But that only works if they are already predisposed to trust you," Lount said.
"If you're a professional meeting new clients, you may think if you buy them a nice lunch and make them happy, you're building trust. But that can actually backfire if the client has some reason to be suspicious of you," he said.
Conversely, the happy people were less trusting of the person with untrustworthy features than were those in the neutral mood. "For those in a good mood, it is all depended on the cues that the pictured person gave that suggested whether he was trustworthy or not," Lount said.
But why would happy people rely more on stereotypes and cues to evaluate a person's trustworthiness? Research suggests the answer relies on motivation.
"When you're happy, you're less motivated to carefully process information," he said. "You feel like everything is going OK, so there is no reason to search out new information. You can rely on your previous expectations to guide you through a situation."
Another one of the experiments provided evidence for that theory. In this experiment, the participants were put in a happy or neutral mood. They were then asked to memorize a nine-digit number, which they would be asked to repeat in a few minutes.
Then, they were shown pictures of untrustworthy faces and asked to rate how trustworthy each face looked.
In this case, people in a neutral mood responded much as did the happy people in the previous experiments – they rated untrustworthy faces as even more untrustworthy.
"In this experiment, people's minds were busy trying to remember the number so they processed information differently than they normally did," Lount said.
"They relied more on the cues, just like happy people did." People aren't aware of this process and don't even know how their mood is affecting how they evaluate others. "You need to be careful, especially when you're happy. You should ask yourself how your mood may be affecting your willingness to trust or distrust another person."
Citation: RB Lount, 'The impact of positive mood on trust in interpersonal and intergroup interactions', Journal of Personality and Social Psychology, March 2010, 98(3):420-33