Imagine you’re meeting someone for the first time – a hairdresser, an electrician coming to your home, a new colleague on Zoom – and they smile at you. What does the smile mean? It could just indicate that they’re feeling happy, of course, but it could also tell you something about the kind of person they are and their attitude towards you. Research shows that people tend to rate a smiling person as more honest and likeable, and someone they want to cooperate with. When someone says they will cooperate, people are more likely to believe them if it’s said with a smile. In other words, smiles seem to be a sign that says: ‘Trust me.’
The trouble is that smiling is easy to do. If flashing a smile can so easily convey good intent, it could be ‘hacked’ by unscrupulous individuals who want you to think that they’re trustworthy so they can exploit you. These kinds of ‘false smiles’ certainly happen in everyday life, yet we still generally trust smiles. In my research, I wanted to understand why.
Earlier research suggested one explanation: genuine smiles – those that happen spontaneously when a person is feeling happy – involve a subtle crinkling around the eyes, which distinguishes them from fake or posed ones. Smiles that involve this muscle movement are referred to as ‘Duchenne smiles’ after the 19th-century French physiologist who studied them, and they tend to be rated more positively (eg, more genuine and more trustworthy) than fake ones. However, people can and do make Duchenne smiles on demand, when they’re not genuinely experiencing happiness.
To better understand why we trust smiles, psychologists like me have turned to a branch of evolutionary biology called signalling theory for insight. This theory states that a communication signal between animals can remain ‘honest’ – ie, it can be trusted – if it’s effortful for the animal sending it. Consider the way that African springbok jump in the air (what’s called ‘pronking’) to indicate to predators that they’re young and fit, and that chasing them will therefore be too difficult. Jumping is effortful for the springbok, so they’re unlikely to bother with it unless they’re genuinely young and fit – making the signal more believable to predictors.
Something similar happens with human social interaction. In his book Honest Signals (2008), the computer scientist Alex (Sandy) Pentland argues that social interaction actually involves a lot of effort – which will ring true for anyone who’s been to a networking event or drinks party. To pull off a smooth interaction, you need to pay attention and respond appropriately to many different sources of information from the other person: their words, their tone, their body language. Pentland suggests that sensitive social signals have just the sort of cost that signalling theory requires: just like the springbok pronking, paying attention and responding appropriately to another person costs a lot of effort. So when people pay careful attention to us, we trust that they’re genuine – because we know that it’s hard work.
I wondered if Pentland’s idea applied to smiling, but one thing was unclear to me. Smiles might be a reliable, honest indicator of a person’s trustworthiness if it takes some effort to make them – but smiling doesn’t take any effort, does it?
The critical thing is that smiles in the real world aren’t just one-off events, happening without anything coming before or after them. Flashing a smile to take a selfie on your own might be low effort, but most smiles are part of active social interactions, and they need to be integrated into the broader context of the conversation. People can tell when you’re smiling along without really following, and they can tell when you’re smiling in an active, responsive way to what they’re saying. Doing the latter costs attention and effort. Maybe it’s not just any smiling but engaged smiling – smiling that requires actively paying attention – that’s trustworthy.
My colleague Michelle Shiota and I tested out this idea by asking 123 pairs of strangers to have a ‘getting to know you’ conversation. The pairs talked about whatever they wanted for five minutes, and we video-recorded their conversation. We then watched these recordings and carefully measured to what extent each person showed ‘engaged smiling’. To do this, we coded each person’s smile intensity throughout the conversation – meaning whether the smiles were Duchenne and how long they lasted – and also used a statistical model that captured how responsive one person’s smiling was to their partner’s smiling.
We wanted to see whether people who had high levels of engaged, responsive smiling were trusted more by their partner. To do this, after the conversation, the pairs played an economic game called the Prisoner’s Dilemma. In this game, each person must make one decision: to cooperate or compete with their partner. If both cooperate, both get $3. If both compete, both get $1. But if one person cooperates while the other competes, the cooperator gets nothing while the competitor gets $5. This task is essentially a measure of trust: it’s risky to choose to cooperate, so making this choice indicates that a person trusts their partner. Our hypothesis was that people would be more likely to cooperate with partners who were more engaged smilers.
This was exactly what we found. Importantly, it wasn’t just about smiling more – although that predicted more cooperation too. What also mattered was how responsive the partner’s smiling was – when a partner smiled more reactively and appropriately, people cooperated more. Smiling consistently without paying attention to the context, while better than not smiling at all, didn’t foster such trust. The participants were strangers, so perhaps there would be different results for long-term friends or coworkers, but these were genuine conversations, where real money was on the line.
Our results might sound like a life hack for unscrupulous individuals: if you want to dupe someone, just make sure you pay attention to what they’re saying, and smile responsively. But to do this – to follow someone’s words, tone, posture and facial expression, and to respond in a way that they actually believe – is genuinely hard to fake. Smiling is generally an honest signal of someone’s intent and character because to do it convincingly means actually being engaged and attentive. That’s why smiling takes more effort than it first appears to – and why we trust people who do it.
