The myth that when women live together their menstrual cycles become synchronised has long persisted - despite research showing there's no evidence that this so-called McClintock effect exists. Any overlap that does happen is probably just down to chance .
Author
- Adam Taylor
Professor of Anatomy, Lancaster University
However, just because this isn't true doesn't mean our bodies don't sync with those around us in other ways. In fact, more and more scientific evidence of this happening is being discovered.
There are two key ways our bodies can "sync up" with people we're close to. The first is synchronisation , where physiological or behavioural processes show coordinated patterns. The second is co-regulation, where the emotional or physiological state of one person influences another.
The heart
Our hearts have a remarkable ability to synchronise when we do things together.
For instance, a small study of 27 couples showed co-regulation of both heart rate and heart rate variability - a metric that represents the time between heartbeats. These metrics have also been shown to be synchronised when couples sleep in the same bed .
But it isn't just couples this happens to. Research has found that choral singers also exhibit synchronised fluctuations in their heart rate when they sing together. Their heart rate increases and decreases at the same time as they breath in and out. This is caused by their breathing becoming synchronised - a phenomenon called respiratory sinus arrhythmia .
The eyes
Another interesting area of overlap involves our eyes.
Research has shown that when two people are involved in the same task or stimulus (such as watching a video interview), their pupils can spontaneously synchronise - becoming wider or narrower at similar points in time.
This is particularly interesting because pupil diameter is largely regulated automatically by the autonomic nervous system, in response to emotions, arousal and light conditions .
Although this synchrony could partly arise because the same stimulus produces similar responses in both people, a 2026 study investigated whether this phenomenon happens even when two people aren't receiving the same stimulus.
One participant listened to emotionally evocative sounds (the study didn't specify exactly what that entailed) while the other did not. Both could see only the other person's eyes.
Even though only one heard the auditory stimuli, their eye behaviour still showed coordination. Both participants blinked in sync, regardless of how their attention changed during the interaction.
This suggests that some forms of synchrony cannot be explained solely by both people responding independently to the same external stimulus. Instead, different aspects of our behaviour and physiology can become coupled through the dynamics of social interaction.
Brain activity
People's brains can show similarity in the electrical activity patterns they produce when imitating each other .
One study demonstrated that while one person spoke and another listened, both exhibited synchronisation in various different brain waves. This synchronisation didn't just occur in brain waves important for processing speech - it also occurred in those relating to focus and even daydreaming.
Researchers have also found that as two people interact, their patterns of neural activity gradually become more closely aligned. This phenomenon is known as interpersonal neural synchrony .
Pain tolerance
Our experiences of pain can also be influenced by social synchrony.
For instance, one study found that rowers who trained together had a higher pain threshold than when they trained on their own. Similar results have been observed for people who sing together.
It isn't clear what the exact reason for this is, but it could be that the camaraderie of the shared experience affects the body's endorphins (our natural painkillers) - either by increasing their levels, or amplifying their effects.
Stress levels
Not all examples of synchrony are good, however. Even our stress levels can sync up with those we're close to.
One of the keys ways that stress impacts the body is that it causes cortisol levels to rise. Cortisol is a hormone produced by the adrenal glands which helps boost blood sugar so the brain and muscles can react quicker.
But over time, if cortisol levels remain too high, this is linked to weight gain , poor sleep and high blood pressure .
Studies have shown that couples often display synchronised cortisol patterns . Although the cortisol concentrations in the blood aren't identical, couples often show similar day-to-day patterns of cortisol rise and decline. As a result, when one partner has higher-than-usual cortisol levels, the other partner is also more likely to show elevated cortisol levels.
This synchrony may be due to external factors that impact one partner directly causing a change in the other member of the couple. Or, it may be due to partners experiencing a shared source of stress .
Siblings also show linked stress profiles when going through a shared stressful experience. One study of 66 pairs of siblings demonstrated that during stressful shared experiences (in this case, caring for a parent), siblings may become biologically connected, with the fluctuations in their stress hormones becoming synchronised.
Humans appear to be biologically connected in ways we rarely appreciate. In an increasingly digital world that may be making us more disconnected from each other, there are many reminders that human connection is not only psychological but physiological too.
Adam Taylor does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.
