The clerk at the coffee shop looked concerned.

I had just mentioned how my coffee cake was stale--maybe it had sat out for too long. She apologized and went to the back to find a fresh slice. We chatted for a second about my day ahead, and I went back to my laptop. Then, I couldn't help overhearing her boss.

"You need to smile more no matter what," she said to the clerk, smiling.

"Sorry, what did I do?" the clerk replied.

I put my earbuds back on and didn't hear the rest of the conversation, but the truth is, the boss was wrong about that one. "Say it with a smile" doesn't always work. In that instance, a look of concern would have been more appropriate--she was not happy about my coffee cake being stale. Why smile at that point?

For years, we've heard that smiling can make you feel happier and decreases stress, and how smiling changes your brain in a good way--even if you fake a smile.

That clerk was perfectly happy and in a good mood. She was just showing empathy. Is it appropriate to smile and say "I'm sorry your coffee cake is stale?" as though that's a good thing in all cases? That's what we've been told. But what if it's wrong?

A new book suggests that smiling and other emotions like anger and resentment might not be universal expressions. How Emotions Are Made: The Secret Life of the Brain by Lisa Feldman-Barrett suggests that emotions and how we express them might not be universal, and that the brain interprets and displays emotions differently depending on each person. For years, scientists have believed that the same sections of the brain are used by every person to generate emotions like a smile or frown; they fall into a rigid pattern. Yet, the brain is too complex for that, says Feldman-Barrett. It is a living organism, with neurons firing in different regions and moving around.

For me, smiling in that instance with the coffee cake--which is admittedly a minor issue--would have sent a message: That was trivial. My interpretation of the "always smile" rule is different from the next person. According to Feldman-Barrett, smiling at a customer service counter might not be the perfect reaction for every person.

One example she mentions is related to airport security. She explained how a training program called Screening Passengers for Observation Techniques (or SPOT) was intended to detect deception. It cost American taxpayers $900 million dollars, but she says it didn't work. She says the program had an incorrect view of emotions, that an expression reveals innermost feelings. The truth is more complex than that. Someone faking a smile might be hiding something. Someone who looks concerned and stressed might be hungry or in a hurry. The way people show emotion changes and is not something you can quantity perfectly and apply to everyone.

This made me think back to those studies about smiling. OK, so if everyone expresses emotions in a different way using different parts of the brain, how can we say that forcing a smile will make everyone happy? How would we possibly know that? It might work for that coffee store manager but not the clerk. Maybe the clerk has a higher level of empathy and it's an asset to the store for her to express that rather than smiling in a fake way.

And, telling everyone to fake a smile as a way to find happiness? That's a bit suspect as well. I know there are countless studies, but this new research about how the brain works and how we express emotions sheds new light on the topic. If emotions and how we express them vary by person, it's not 100% accurate to say faking a smile will work in every case. For some, it might be perfectly wrong. It could even be similar to the dangerous (but common) view that says women are more emotional than men. Really? If emotions vary so much, how can we pigeonhole people like that so easily?

Here's my takeaway from the research. Making blanket statements about smiling and happiness could be dangerous, and not even that helpful. Feldman-Barrett makes some excellent points in the book that emotions are too complex for those hard and fast rules.

That's why, before I left the coffee shop, I stopped back up at the counter.

"Thanks for the fresh coffee cake," I said.

The clerk smiled back in a genuine way.