Despite what your parents might have taught you, you really can judge a person's trustworthiness based on nothing more than their face, a new study has found, which really throws a wrench into that whole "don't judge a book by its cover" thing. But here's the catch - those judgements are actually what influences a person to become more honest or dishonest.

So how does that work? Well, according to researchers from Columbia University, people internalise the snap judgements the world makes about them, and they start to act accordingly. For example, if a person is viewed as honest, they tend to start acting more honestly. On the other hand, if they are viewed as dishonest, they start to act dishonestly, which means, in a way, the world at large is to blame for your lies.

"Past research is split about whether such a link exists, but we found that people who looked trustworthy were in fact more likely to act trustworthy," said one of the researchers, Michael Slepian. "Of course, not every judgment of every face is right, and people are susceptible to baseless stereotypes in judging others on appearances. But finding evidence of the link between faces and honesty led us to dig deeper into why this link might emerge."

In order to test this hypothesis, the team had 30 students judge pictures of 90 other students. Once this initial judgement was made, the groups played a quick card game where the objective was to get the other player to trust you.

The game was pretty simple: two players were dealt cards that either said "high" or "low" on them, and they had to get the other person to believe which one they had. The point system rewarded liars more than honest players, so deception was key to victory. But before the game even started, the players reported how often they thought the other person would trust them, which gave a benchmark for how they believed the world judged them. 

When all was said and done, the team found that the people who thought they were viewed as more trustworthy and had 'trustworthy' faces to others were, in reality, more trustworthy in their actions, and vice versa.

"People with trustworthy faces acted more honestly, in part because they expected to be trusted, and wanted to live up to those expectations," said study co-author, Daniel Ames. "Those who looked untrustworthy were somewhat more likely to lie seemingly because they sensed that they wouldn't be trusted."

The takeaway from the study, published in Psychological Science this week, is that yes, we can pretty much gauge someone's trustworthiness on their face alone. But be careful - these judgements could be directly influencing the person, which means we really shouldn't judge a book by its cover if we don't want to rewrite the contents.