Research Reveals What Your Smile Says About Your Relationship

New research reveals the telling nature of couples' 'smile synchrony' in terms of relationship satisfaction.

By Mark Travers, Ph.D. | February 17, 2025

If you were to try and determine how happy a couple was, what would be the first thing you'd look for? Would you pay attention to the way they speak to each other—the nicknames they use, the tone of their voice or their choice in language? Or would you look at the way they physically interact with each other—how closely they sit together, their public displays of affection and their body language?

According to a December 2024 study from Emotion, these may not be the factors you should be keeping an eye on. Rather, Reut Machluf-Ruttner, the lead author of the study, suggests "emotional synchrony" to be far more telling of a couple's relationship quality. Here's what this entails, as well as its single greatest indicator.

What Is 'Emotional Synchrony'?

Enacting behaviors simultaneously with others is an electrifying feeling; it's something you can experience in a variety of ways, and probably already have. Dancing to the same routine, marching to the same beat or singing the same song with a group of people—no matter the behavior, there's something inherently thrilling about synchronizing your thoughts and actions with those around you.

This concept of synchrony applies to our emotions too. As a 2015 study from the Journal of Personality and Social Psychology explains, human emotions are deeply intensified when experienced collectively. The authors of the study explain this eloquently: "Both sadness and joy are amplified by echoing from consciousness to consciousness: Everyone drives everyone."

Humans have an innate desire to connect with those around them. As such, sensing that your feelings are mirrored by someone dear to you can feel inexplicably rewarding—yet, the converse applies too. When someone you love is unable to empathize with your joy or your sorrow, it's easy to feel put off. This was the premise of Machluf-Ruttner's study.

As he explained in an interview with PsyPost, "The inspiration for this research stemmed from a deep curiosity about the world of nonverbal communication between romantic partners." Machluf-Ruttner and his co-authors sought to assess the visibility of couples' emotional synchrony. More specifically, the researchers analyzed their "smile synchrony"—that is, how often and quickly the couples smiled at each other, in a similar way or at the same time. Notably, they were able to correlate this synchrony to their relationship satisfaction.

What 'Smile Synchrony' Says About Couples

In total, Machluf-Ruttner and his research team studied 61 couples who live together, and 31 of these couples were seeking couples' counselling at the time of the study. They had clinically, significantly low relationship satisfaction, which made them the "distressed" group. The remaining 30 couples were the "non-distressed" group; none were seeking therapy, and were characterized by having very high relationship satisfaction.

The researchers observed a six-minute discussion between each of the 61 couples. The conversations switched between both negative and positive topics—such as their strengths as a couple, as well as their arguments.

Specifically, the researchers recorded the couples' facial expressions throughout the discussion by means of high-definition cameras. Thereafter, the research team used state-of-the-art facial recognition software to intricately analyze their expressions—namely, the presence, intensity and frequency of their smiles.

In terms of synchrony, the team assessed the timing of the individual partners' smiles—specifically, how closely aligned the timing of one partner's smile was to the other's. All of these different measures of smiling were compared between the distressed and non-distressed couples. Unsurprisingly, the findings were plain as day.

Researchers found that couples with lower relationship quality and satisfaction had lower levels of smile synchrony—on a scale from 0 to 1, they had an average of 0.32. Happier couples, on the other hand, had far greater smile synchrony—scoring an average of 0.46. Moreover, these happier couples returned their smiles to one another at a much faster rate; it only took them an average of 0.62 seconds to reciprocate smiles, whereas it took distressed couples around 1.07 seconds.

Reflecting on these findings, Machluf-Ruttner explained, "Despite being automatic and largely uncontrollable, findings from this study—and others in the field—highlight that these signals are highly indicative and can serve as markers of relationship quality."

The Significance Of 'Smile Synchrony'

At first glance, it might seem almost trivial—how much can something as small as a shared smile really say about a relationship? But when you consider the science behind it, the findings make perfect sense. This is because smiling is a fundamental social cue amongst humans—one that's tightly tied to our capacity for empathy and connection. It's so much more than just an expression.

Mirror neurons, the specialized cells in our brains that help us instinctively mimic and understand others' emotions, play a major role in this phenomenon—as 2017 research from Emotion explains.

When you see your partner smile, your brain responds automatically; it triggers a similar emotional state within you. This subconscious mirroring may be subtle, but it's a striking sign of emotional attunement—it shows that you're permanently mindful of each other's experiences, and that you feel joy together in real time.

Beyond neuroscience, there's still something profoundly human about this kind of synchrony. A smile isn't always something we consciously coordinate, nor is it something we can effectively force when we aren't truly happy—it happens naturally, without pretense. That's what makes it so revealing. In a healthy, connected relationship, happiness becomes an exchange.

A genuine smile from one partner sparks a response in the other—and, in turn, warmth and reassurance becomes unconsciously cyclical. And while a grin may last only a fraction of a second, these micro-moments of connection accumulate over time and form the ties that hold a couple together.

Perhaps this is why couples who have been together for years often develop an effortless rhythm with one another—for them, acute empathy is the baseline. Finishing each other's sentences, shared looks, instinctive laughs, the warmth of a smile given and received—these are where the essence of a thriving relationship reveals itself.

Is your relationship visibly thriving? Take this science-backed test to find out where your relationship stands: Relationship Flourishing Scale

A similar version of this article can also be found on Forbes.com, here.