That coworker who smiled at you as you quickly walked by? Yeah, she totally faked it. The grin from your partner because you just busted a move like Napoleon Dynamite? Okay, that one's real. But could you tell? How do you know when someone is giving you a genuine beam of good feelings?
It's all in the muscles
In the mid 1800s, a French neurologist by the name of Duchenne de Boulogne began conducting pioneering electrophysiology research, hoping to understand more about the brain and nervous system. He experimented by stimulating the faces of test subjects with electricity. In doing so, he figured out that two major muscles activate in the face during a sincere smile. These are the zygomatic major muscle, which turns up your lips, and the orbicularis oculi, which contracts the corners of the eyes and creates wrinkles (crow's feet).
When people fake a smile, the orbicularis oculi doesn't activate. That's because it's an involuntary muscle that's controlled by the limbic system, or the emotional center of the brain. By contrast, the zygomatic major is controlled by the motor cortex. It activates both spontaneously and voluntarily. Thus, the longstanding belief based on Duchenne's work has been that closure and wrinkling around the eyes is a good sign that a smile is genuine. Because some people also tend to move the zygomatic major outward rather than up when they fake it, being able to see the bottom teeth also is sometimes viewed as another way of confirming the nature of the expression, although this rule of thumb is more loosely applied.
Why it's (not) okay to fake it
As you go through your day, you might offer some fake smiles for a variety of reasons, such as to seem polite to come across as confident. But faking it can backfire. The Duchenne smile, being based in the limbic system and involving involuntary responses, is associated with genuine happiness and enjoyment. Research also indicates that people attune to positive emotions better than negative ones when forming relationships--that is, people look for and prefer to be around others that make them feel good.
That said, the general public can distinguish between fake and real smiles about 60% of the time. Some professionals, such as journalists and social scientists, can distinguish at an even higher percentage, however. So if you're not actually amused, if you're just going through the motions with your smile, there's a decent chance that the people around you not only can tell, but will be put off by what you're doing, especially if the smile is merely an automated product of habit and it's clear you're distracted. That's not what you want when, for example, you're trying to build a bond with a new team or convince shareholders you're passionate about your projects. And if you do flash a fake smile at someone who can't make the distinction between real and forced, they can misinterpret what you mean, want or need, leaving you stuck in boredom, frustration or depression. You even could get pulled into projects that are too hard or overwhelming for you, creating a precarious career situation in which you're constantly afraid and stressed that your real amount of knowledge or skills will be found out.
Looking at Duchenne smiles in terms of what you see from others, being able to tell when someone is really enjoying themselves lets you change course on the fly. For example, if you suggest a team member stays late to work with you on reports and they fake it, you can shift and make a new suggestion for how to complete the job, taking into consideration that they didn't like the first option.
A note on more recent research
New work led by Sarah Gunnery suggests that some people can produce "fake" Duchenne smiles in situations where they are emotionally neutral but imagining emotional scenarios. But people have mirror neurons that activate not only when they perform an action, but when they see someone else perform that action. These neurons might have contributed to the ease with which people were able to imitate Duchenne smiles, as they trigger emotional recall and empathy. It might be that participants in the study subconsciously were able to tap into their mirror neuron circuitry and emotional experiences well enough to stimulate a genuine smile response. Researchers still don't know how well people can fake it when they're really feeling something bad. So while you might not be able to guarantee that a Duchenne smile translates to enjoyment about the immediate present, it's still a good indicator that someone is drawing on something positive.