Psychologists from the University of Glasgow have found humans make judgments on someone's trustworthiness within the first 500 milliseconds of hearing their voice.
And decisions appear to be made on sound alone, with people making snap decisions based on hearing a simple greeting.
The researchers played recordings of people saying hello and asked subjects to rank them on 10 personality traits including trustworthiness, dominance, attractiveness and warmth.
The study found that most of the recorded voices elicited the same response from participants and these opinions were formed rapidly, after hearing the voices for only 300-500 milliseconds.
The most important traits identified were trustworthiness and dominance, with men who raised their tone and women who alternated the pitch of their voices seen as more trustworthy.
Dr Phil McAleer, who is from the Voice Neurocognition Laboratory, University of Glasgow, said: "It is amazing that from such short bursts of speech you can get such a definite impression of a person.
"And more so that, irrespective of whether it is accurate, your impression is the same as what the other listeners get."