Two Hearts Beating As One: An Introduction to Physiological Synchrony
How we regulate each other's bodies & why it matters
We all know the poets and artists have a lot to say about love, but what about the psychologists?
This was the opening line from a reading performed during my wedding ceremony. Yes, I wrote a summary of my favorite psychological findings related to love for my wedding. No, you should not be surprised.
Here’s another excerpt:
“Loved ones help us regulate our bodies […] they affect our heart rates, our breathing patterns, our sleep, and many other bodily functions that keep us healthier, stronger, and more resilient to illness.”
Physiological synchrony (a.k.a. physiological concordance or physiological attunement) occurs when physical processes between two or more people become coordinated, or “in sync,” with each other. This can be as obvious as matching someone else’s voice volume during a conversation or as subtle as coordinating the timing of our eye blinks when watching a movie together. If you’ve ever had a period and suddenly become in sync with a friend’s menstrual cycle, you are well aware of physiological synchrony. We do this all the time without thinking about it, and how much our body is coordinated with someone else’s has large implications for our lives that we often are entirely unaware of. We might be more attracted to someone on a date or more successful achieving a group task if our hearts beat with the same rhythm or we end up sweating during the same moments as someone else.
Physiological synchrony becomes even more visible as a parent. A pregnant person’s body has to be in sync with their fetus as they are literally responsible for regulating so many bodily functions (breathings, nutrition, waste removal, etc.) This regulation becomes more subtle, yet no less important, after birth. Whether it’s rocking your child to sleep, calming a crying infant, or keeping your cool in front of a tantruming toddler, your main job as a parent is to help this little human learn how to regulate themself.
Physiological synchrony appears to be part of this process. In a 2022 review by Di Lorenzo and colleagues published in Neuroscience & Biobehavioral Reviews, they summarize the evidence on physiological synchrony on a variety of biological processes between mothers and young children during and after a stressful situation.1 They found the most evidence for physiological synchrony between mothers and children in cortisol responses (a hormone typically associated with stress that I’ve previously written about) before and after experiencing a stressful situation. Additionally, a smaller subset of these studies found that the relationship in cortisol responses between mother and children (ages 6-17 months) was bidirectional: Mothers’ cortisol levels affected their children’s levels, and children’s levels affected their mothers’.2
In other words, when my baby’s stress is high, my stress is high, and when my stress is high, my baby’s stress is also high. We, as parents and children, appear to be co-regulating each other.
Of course we, as adults, have more control over our thoughts, feelings, and behaviors compared to a newborn, and your physical processes will most likely affect your baby more than your baby’s affect your own. However, what what this research shows, and what we know about physiological synchrony, is that how well I regulate your body is going to affect how well you regulate mine, and vice versa. So much of the research on parent-child interactions focuses solely on how the child is affected by the parent. What I like about this research is that it doesn’t ignore the other side of that coin: How a baby’s physical processes have direct effects on their parent.
Scientists theorize that one of the main functions of physiological synchrony is to help reduce the burden of regulating our physical processes by ourselves. Taking care of a body (including a brain!) is hard work. As social animals, we’ve evolved to rely on each other to help us with that task. It’s why we want to call a friend when we’re feeling stressed or cuddle with our child (or partner, or pet) when we’re feeling sad: We want someone else to help us regulate ourselves. This is one reason why we think social isolation is related to a shorter life expectancy: If you’re alone, you don’t have someone else to help you regulate your body, and doing it alone over time causes more strain on your body compared to other people who consistently have others to help them. If we’re all running a marathon and some of us have friends cheering us on the sidelines and giving us water, and some of us don’t, the people who don’t are going to put more strain on their bodies than the people who do. It’s nice to remember that our children are part of those cheering bystanders, too.
As a new parent, I’m still figuring out how not to get too down on myself in moments when, let’s face it, I’m a bad co-regulator: When I get snappy or withdrawn or generally respond to my child in a way that I wish I hadn’t. Reading this research, it might be easy to start spiraling into anxiety about things that aren’t even in your control (“Oh God, what if my cortisol levels aren’t matching my baby’s? Am I a bad mother?!”) Instead, I find it comforting to know that, well, it’s not entirely just up to me. My child and I are in this together, along with many other people. Physiological synchrony is a dance done with many partners across a lifetime. Yes, you’re a major co-regulator in your baby’s life now, but so is your co-parent (if you have one), your best friend, your baby’s daycare teacher, your dog, or that nice cashier that always smile at your baby at the grocery store. As they grow up, they will develop more co-regulators to help them on this marathon we call life.
In other words, just as you help calm your child down from a tantrum, your child has this amazing ability to regulate you, too. We just need to remember to let them.
Such as, I’m not kidding, a task where the mother waves an attractive toy at a baby and then takes it away, only to be placed in an inaccessible, transparent container. This is called the Toy Frustration Procedure and, yes, it sounds very stressful!
Research is complicated caveat: Measuring physiological synchrony is an incredibly complicated procedure, even if you’re only measuring one biological process. Take cortisol response, for example. There are multiple ways a scientist might measure it (From saliva? From blood?), when they might measure it (20 minutes after a stressful situation ends? 2 hours after it ends? In the morning? At night?), and how they might analyze their data (Do I treat mother and infant cortisol responses as potentially bidirectional or do I assume that an infant’s cortisol response precedes a mother’s response)? All of this is to say, as always, know that I am simplifying these results substantially.