Imagine you’re calling a stranger—a possible employer, or someone you’ve admired from a distance—on the telephone for the first time. You want to make a good impression, and you’ve rehearsed your opening lines. What you probably don’t realize is that the person you’re calling is going to size you up the moment you utter “hello.” Psychologists have discovered that the simple, two-syllable sound carries enough information for listeners to draw conclusions about the speaker’s personality, such as how trustworthy he or she is. The discovery may help improve computer-generated and voice-activated technologies, experts say.

“They’ve confirmed that people do make snap judgments when they hear someone’s voice,” says Drew Rendall, a psychologist at the University of Lethbridge in Canada. “And the judgments are made on very slim evidence.”

Psychologists have shown that we can determine a great deal about someone’s personality by listening to them. But these researchers looked at what others hear in someone’s voice when listening to a lengthy speech, says Phil McAleer, a psychologist at the University of Glasgow in the United Kingdom and the lead author of the new study. No one had looked at how short a sentence we need to hear before making an assessment, although other studies had shown that we make quick judgments about people’s personalities from a first glance at their faces. “You can pick up clues about how dominant and trustworthy someone is within the first few minutes of meeting a stranger, based on visual cues,” McAleer says.

To find out if there is similar information in a person’s voice, he and his colleagues decided to test “one of the quickest and shortest of sociable words, ‘Hello.’ ” The team recorded 64 Scottish undergraduate students (32 males and 32 females) reading an unfamiliar passage which included a telephone conversation. They were instructed to read in a neutral tone of voice. He then extracted the single word “hello” from each recording, and let 320 different students (117 males and 203 females) listen to that word over headphones or speakers attached to a computer. The students were asked to rank the voice according to 10 personality traits, including trustworthiness, dominance, attractiveness, and warmth. Each student assessed a single, randomly assigned trait for all the voices, rating them, for instance, as more or less dominant on a scale of one to nine.

Although the voices played for only 300 to 500 milliseconds, the students did not hesitate to form an opinion, McAleer says. More importantly, people converged on these snap judgments; most of the recorded voices elicited the same response from the listeners. (Listen to a trustworthy and untrustworthy "hello.") “It is amazing that from such short bursts of speech you can get such a definite impression of a person,” McAleer says. “And that, irrespective of whether it is accurate, your impression is the same as what the other listeners get.”

McAleer and his colleagues’ analysis, reported online today in PLOS ONE, suggests that people’s impressions are based on the tone of voice. Men who raised the tone of their voices, and women who alternated the pitch of their voices were rated as more trustworthy. Men with lower pitched voices were generally perceived as more dominant. But the opposite was true for women: Those with higher average pitch were rated as more dominant.

The speed with which the students made their judgments makes evolutionary sense, McAleer says. “Deciding who to trust and who to approach can be crucial to your survival. There’s no point to listening to someone talk for 5 minutes to figure out if they’re trustworthy or not—you could be dead already.”

Most intriguingly, there is a strong overlap between how we perceive facial and vocal cues, McAleer says. The trustworthy signals in our faces come from “moveable features,” such as our eyes and mouths. But signs of dominance are conveyed from rigid, immobile ones, such as the distance between our eyes or the width of our jaw. Similarly, trustworthiness in a voice is linked to such things as pitch and glide, elements that we can adjust. But dominance is influenced by morphology, such as the length of one’s vocal tract, over which we have no control. Thus, one either has the facial and vocal features that convey dominance—or not. “It’s striking that we find these same features when we look at the voices—and remarkable that the same assessment system applies to both voices and faces,” says Pascal Belin, a neuroscientist also at the University of Glasgow and one of the authors of the paper.

The study adds to ongoing research about the type of attributions people are ready to make based on very little evidence, Rendall says. “What’s most interesting here is that people are very consistent in their ratings. It makes you think there is something to this.” What remains to be seen, he adds, “is if the attributions are on or off the mark. Are the people that have been rated as trustworthy really trustworthy?” Consider what goes through your mind after you’ve taken your seat on an airplane. You’ve not seen the captain, but his or her voice comes over the intercom. “Hello, this is your captain speaking.” Are you about to fly the friendly skies?