One of my favorite parts in the development of a good friendship is the moment you realize that you’ve started to mimic each other. Take, for instance, the ridiculous way my high school friends and I started to mispronounce word “beverage” with an extra r: breverage. This didn’t stem from any speech difficulties or our semi-subtle New Jersey accents; it was simply an artifact of being weird teenagers who, for reasons lost to our adolescence, decided to adopt this goofy pronunciation. Since then, I’ve found that old habits do apparently die hard (especially the fun ones), and I carried this one to grad school, where, to my great amusement, this idiosyncrasy born from high schoolers transferred to MS and PhD candidates. In exchange, I also adopted some of their own funny words, like “scramps” for “shrimp” and “terlet” to refer to the commode, and this combined, ridiculous lexicon has remained a delightful component of many friendships over the years.
It turns out that synchrony between people who share a close personal bond extends beyond similar speech patterns. Individuals with a strong interpersonal relationship have been shown to synchronize their breathing, heart rate, and other behaviors. There’s even some truth in the saying that some people are functioning “on the same wavelength”: individuals in a close relationship will even start exhibiting similar patterns of electrical brain activity while in each other’s presence.
Considering the importance of social bonds in maintaining mental well-being, it makes sense to want to study the synchronized physiological patterns, or interpersonal physiological synchrony (also abbreviated to IPS), of people who share a strong personal connection. IPS studies not only grant us a way to measure the strength of relationships; they are also informative about the underlying psychological processes that are involved in these relationships.
IPS manifests in multiple systems throughout the body, which means there are multiple ways of studying it. In a recent paper published in the Psychonomic Society journal Cognitive, Affective, & Behavioral Neuroscience, entitled “Measurement of interpersonal physiological synchrony across dyads: A review of timing parameters used in the literature,” Analia Marzoratti and Tanya M. Evans review several of these methods, including electrodermal activity (which is essentially a measure of how sweaty our skin gets in response to changing emotions), heartrate measures, breathing rate patterns, and brain activity.
However, there isn’t a fully standardized approach to conducting IPS studies yet and given the multiple ways that people can synchronize with each other, as well as the multiple technologies available to study these behaviors, it’s easy to see how the field could potentially get a bit chaotic if researchers aren’t careful. Each of these methods has a set of different advantages and limitations that can be combined or balanced in different ways, and Marzoratti and Evans emphasize that researchers ought to carefully consider these qualities when designing IPS studies.
At the heart of the authors’ extensive review is a multilayered consideration of timing. Each of the methods that they discuss operates on a different timescale and with a different level of sensitivity to different psychological processes. For example, it takes one to three seconds for sweat glands under the skin to respond to an experiment stimulus, and these responses exclusively reflect activity from the sympathetic nervous system, which controls the body’s “fight or flight” response. Electrical brain activity, on the other hand, fluctuates constantly, and we can record brain activity responses from multiple psychological processes on the scale of milliseconds. Designing an IPS study therefore requires researchers to have a sense of how much time is needed for a psychological process to unfold and then choose a measure that can adequately capture the ensuing physiological activity.
Timing is also critical for appropriate analysis procedures, and the authors provide detailed discussions for analyzing data from each type of measure. On a general level, they recommend analyzing snippets of data from short time intervals, since analyzing across too large of a time window could lead to important data patterns being washed out or false correlations between psychological processes and otherwise unrelated physiological activity. However, time windows that are too short might not fully capture all of the activity underlying a particular psychological process. Data analysis in IPS studies therefore requires researchers to strike a delicate balance in their considerations of psychological and physiological timing.
Ultimately, the benefit of having so many different ways of studying IPS is their joint capacity to provide multiple lines of evidence supporting a causal relationship between psychological processes and physiological behaviors. Knowing the direction of causality—whether it’s the psychological state that causes physiological patterns to change, or the other way around, or even that they influence each other—ultimately helps researchers better understand the nuances of the connections between our minds and bodies. However, since every kind of measure has a unique set of strengths and limitations to consider, researchers will also have to make sure that their experiments are well-designed for drawing the kinds of conclusions they are interested in.
So next time you notice that you and a friend seem to be “in sync,” know that you very much are, down to the level of your brain activity! With a rich variety of tools available to study interpersonal physiological synchrony, it’s only a matter of time (and timing!) before we fully understand how our relationships influence both our minds and our bodies.
