Our trust in strangers depends on whether they appear like someone we already know, a study has found.

Published in the journal PNAS, the study found that strangers resembling past individuals known to be trustworthy are trusted more, while those similar to others known to be untrustworthy are trusted less.

"Our study reveals that strangers are distrusted even when they only minimally resemble someone previously associated with immoral behaviour," said Oriel FeldmanHall, who led the research as a post-doctoral fellow at New York University (NYU) in the US.

"Like Pavlov's dog, who, despite being conditioned on a single bell, continues to salivate to bells that have similar tones, we use information about a person's moral character, in this case, whether they can be trusted, as a basic Pavlovian learning mechanism in order to make judgements about strangers," said FeldmanHall.

The researchers conducted a series of experiments centring on a trust game in which participants make a series of decisions about their partners' trustworthiness.

In this case, they were deciding whether to entrust their money to three different players who were represented by facial images.

"We make decisions about a stranger's reputation without any direct or explicit information about them based on their similarity to others we've encountered, even when we are unaware of this resemblance," said Elizabeth Phelps, a professor at NYU.

"This shows our brains deploy a learning mechanism in which moral information encoded from past experiences guides future choices," said Phelps.