Synchrony and the art of signalling

Jorina von Zimmermann and Daniel C. Richardson take us from fireflies to military parades.

As soon as we are in the presence of other human beings, we align our behaviours with them. Though often unintentional, powerful social signals are produced when we synchronise our actions with each other.

The 105th ‘Day of the Sun’ was marked by a military parade in North Korea in 2017. Thousands of soldiers, accompanied by several missiles, moved together in perfect synchrony to celebrate the birth of the state founder, Kim Il-sung. On that day North Koreans sent a message to the world, leaving no doubt that they are one and should be perceived as an inseparable entity representing their nation. Elsewhere on the continent, in the mangrove trees along the riverbanks in Southeast Asia, similar displays of synchrony can be observed. When fireflies flash in perfect unison at a rate of about three times in two seconds, it looks as if stars in a pitch-dark sky appear and disappear in concert with each other.

When Hugh M. Smith, an American biologist, first reported the fireflies’ synchronous flashing in the 1930s, many scientists believed that it was an illusion, or mere coincidence. Today, scientists think that the synchronous flashing is a signal related to mating behaviour. Male fireflies blink in unison to enhance their chances of attracting female fireflies. The emergent synchrony between the insects can be explained with the help of mathematics. No leader or conductor is needed to coordinate the synchronous flashing, but fireflies organise themselves in synchrony like many other biological oscillating systems, such as cells that fire together to control our heartbeat, or crickets all chirping in perfect harmony.

The disco-light fireflies, blinking to attract a mate, seem to be a world apart from the North Korean soldiers, marching to inspire and intimidate. Yet, recent studies show that these two types of behaviour are not just similar in their timing and coordination, they may each serve a very similar function, too.

From fireflies to human beings
We humans do not glow and usually do not chirp, but we do coordinate our behaviour in many other ways. Automatically and often unintentionally, we coordinate our postural sway, walk in lockstep, align our speech patterns and eye gaze, imitate each other’s facial expressions and mimic each other’s movements. As a psychological consequence, we form strong social bonds, we feel closer and more similar to each other, we even remember more information about each other. This happens not only when we coordinate without meaning to, but also after we have engaged in effortful and intentional synchronous activities. Doing something – anything – together at the same time has important prosocial consequences.

How are such far-reaching social impacts produced through synchronous behaviour? One way to explain the underlying mechanisms of interpersonal synchrony and how it affects us relates to our human ability to form shared representations of our social and physical environment, and to the neural processes that correlate with this.

Joint action, as any form of social interaction during which two or more individuals coordinate their action in space and time, depends on the ability of interaction partners to share representations, predict actions, and integrate the predicted effects of one’s own, and the other’s, actions. In order to do this successfully, self- and other-related behaviours need to be effectively integrated. This happens at the neural level through the coupling of perception and action. In those cases in which the joint action is characterised by temporal coordination – by synchrony – an individual’s brain is required to simultaneously represent self- and other-generated actions and to integrate them in real time. With increasing coordination during social interaction, shared representations of a joint action are formed, which improve the ability to predict, anticipate and adapt to another’s movements. Thereby coordination can be realised with greater ease, and Fairhurst and colleagues (2013) have observed a reduction in brain activity in areas related to cognitive control in the process. This reduction of activity in cognitive control areas coincided with an increase in brain activity in brain regions associated with socio-emotional processes, which may explain why synchronisation promotes prosocial thoughts and behaviour. Synchrony seems to be characterised by a state of processing fluency, implying successful social interaction.

For a long time researchers almost exclusively studied synchrony between pairs of people. In recent years, however, studies have also demonstrated that behavioural coordination between groups of people increases cohesion between them (Jackson et al., 2018), boosts liking and perceived social closeness (Tarr et al., 2014, 2015) and enhances cooperation (Reddish et al., 2013).

In our lab, we also found that when a group experienced synchrony they were better at a joint task. In a 2016 study we asked groups of around 20 students to chant in synchrony or to speak out of time with each other, and then asked them to play a video game together. They each had a handset that delivered a tiny nudge to a tightrope walker on screen. Collectively, they had to keep him balanced. We found that individuals not only reported higher levels of affiliation for their group when they had chanted together, but those groups were also better coordinated in the tightrope game. As well as our objective measures of game performance, the experiment also gave us a peculiar subjective experience. It is rare that a psychology experiment has a spiritual vibe, yet that was our experience of being in a room with chanting participants.  

The evidence shows that synchronous behaviour not only affects how we feel about one person, but also how we relate to a whole group. This is perhaps why large-scale coordination can be observed in many aspects of social life, such as sports, dance or music, and why it has been an essential and enduring part of human ritual.

The costs and benefits of synchrony
We feel attached to the people we know – those in our immediate social communities with whom we live, work and socialise. But we also often feel a strong connection with larger numbers of people, more than we could possibly engage with in meaningful interaction. Human beings are prone to quickly develop shared social identities: research in the field of Henri Tajfel and John Turner’s social identity theory has shown that we often form and feel attached to groups in a heartbeat even when those are based on fairly arbitrary criteria, such as the preference for one painting over another. Researchers have recently argued that synchrony could be an adaptive mechanism to maintain larger social networks, to feel connected to whole communities rather than just individuals, and to increase group cohesion (Launay et al., 2016).

From a cognitive perspective, it has been claimed that the amount of social contacts we can realistically sustain is limited to about 150 (the so-called Dunbar number). This is about the size of villages and human groups through much of human history, and today approximately corresponds to the median number of Facebook friends. Other primates, in comparison, can only pick fleas off one person at a time, which means that creating social bonds is time-consuming and restricted in scope. But human beings do feel a sense of connection with groups much larger in numbers than they could sustain through grooming. Launay and colleagues argue that through dance and music, rituals and sports, bonding can take place between multiple individuals simultaneously, and studies from Bronwyn Tarr and Robin Dunbar’s group and others have shown that moving together in unison releases endorphins and activates the brain’s reward system. These physiological processes potentially help to reinforce large-scale, rhythmical human movement. Instead of only ever grooming individuals directly and establishing close social contacts, we may have developed mechanisms that allow us to bond with high numbers of people and to maintain these bonds over time. Mass coordination becomes the ‘social glue’ through which social communities were and still are sustained and strengthened.

If this is so, one important question about human rituals can be answered. All human societies that we know about have always danced and made music together, and human rituals often involve complicated coordinated movement and speech. The latter are difficult to achieve and require a lot of energy and training. This could therefore be regarded as costly behaviour. The time and energy needed to dance and make music together could even be considered a luxury. However, if joint activities that involve coordinated behaviour really have the important function of establishing and maintaining meaningful social bonds between people, as Launay and colleagues suggest, then, all of a sudden, the benefits of coordinated behaviour possibly outweigh its costs.

Marching together and feeling together
Military parades are some of the most dramatic and fascinating displays of human synchrony. When thousands of soldiers march together in unison, indistinguishable from each other, we pause in awe and admiration. To this day, drill is part of a rigorous training regime for soldiers the world over. And yet, since the invention of the cannon and the machine gun, lining up in ordered rows and walking slowly towards the enemy is recognised as a poor stratagem.

Why are soldiers still required to march together today, when – to quote the historian William H. McNeill – ‘a more useless exercise would be hard to imagine’? One possible explanation is that marching together creates obedience to a relevant authority, a behavioural mode that is certainly considered critical in the military. One of Wiltermuth’s experiments from 2012 supports this assumption. He asked participants to walk around campus a few steps behind an experimenter. In one condition, they were told just to follow him, while in another they were told to match his footsteps, walking in time with him. Then the experimenter requested that they help out with a different experiment that involved placing as many sow bugs (woodlice) into an ‘extermination machine’ as they could in 30 seconds. Of course, no sow bugs were ever killed during these experiments, but the participants themselves did not know this. The researchers found that those participants who had previously marched in synchrony with the experimenter sent approximately 54 per cent more bugs to their death than the participants who had walked at their own rate.

In his 1995 book Keeping Together in Time: Dance and Drill in Human History, McNeill offers a less chilling answer to his own question. Similarly to Launay and colleagues, he proposes that synchronous activities have an important social function and that rhythmically moving together in unison leads to ‘muscular bonding’ and alters human feelings to create enhanced group solidarity and cohesion. However, especially when it comes to drill, the creation of strong bonds between group members is probably not the sole purpose and effect of movement in unison. Displaying synchronous behaviour does not only have prosocial consequences for actors, but synchronicity also functions as a signal to observers.

During the military parade in North Korea in 2017, the soldiers who marched in perfect unison signalled to outsiders that they are highly disciplined and committed to a lager goal. The display of highly skilled synchronisation awes the viewer, because it is apparent that only through mentally and physically costly, time-consuming training and devotion could the group have achieved such accuracy in unified collective behaviour. To watching friends and foes alike, this signals dedication and within-group cohesion, but also strength and potency.

Coalition signalling
Hagen and Bryant (2003) claim that music and dance have, at least in part, always served as a ‘coalition signalling system’. If a group wants to attract new members and form new alliances, or deter an enemy, the quality of the group or coalition needs to be assessable. While the size of the group may be an important attribute that hints at its level of appeal, there are two other important features that can provide information about the quality of a coalition. The first one is the motivation of the group to act collectively to achieve a common goal, which can be derived from the internal stability or the levels of cohesion among the group members. The second feature of coalition quality is the ability of group members to act together. All parties have an interest that information on coalition quality is communicated quickly, and groups need to adopt strategies to signal and detect it at the same time.

According to Hagen and Bryant, music and dance may be particularly useful signals because they have two important universal features – synchrony and variation – which both require time and practice if they are to be carried out in a complex and sophisticated manner. This means that only long-established and well-functioning coalitions are able to perform complex music and dance pieces, signalling high levels of cohesion and capability to other groups. Displays of coordination skills can be used to demonstrate strength, frighten another group and discourage them from attack, but they can also be used to demonstrate collective interest and the intent to form an alliance and to cooperate. The ritualistic Maori dance, the haka, is a perfect example of this dual function. While the haka is traditionally referred to as a war dance, it is also frequently practised to greet important visitors and to honour exceptional individuals or groups of people.

The dual function of synchrony
In order to preserve themselves, social groups always have to engage in two social processes at the same time. They need to maintain ingroup cohesion, and they need to translate their internal cohesion into an external signal that, depending on the social context, either attracts new members (and even whole groups) or deters enemies.

Research has shown that perceivers have intuitive theories about the type of group they are confronted with and the relational properties of the group. The entitativity, essentialism or ‘groupness’ of a group, referring to the extent to which a group is perceived as a coherent and an agentic unit, has been identified as a particularly prevalent concept, which human beings use to form intuitive judgements about social groups (Lickel et al., 2001). From the synchrony literature we know that observers, when asked to rate interacting individuals in terms of their social closeness, report that they perceive those who are in synchrony with each other as one entity and as having stronger social bonds than those who are not synchronised. One 2016 study by Fessler and Holbrook even showed that human beings draw inferences about the cohesion and strength of coalitions from synchronous behaviour. In their study the researchers tested how participants would estimate the fighting capacity of either soldiers or terrorists in relation to the observed synchronicity of their footsteps. They found that participants rated synchronised targets as more muscular and larger.

Synchrony as a social signal
Synchronous behavior, then, seems to have a dual function. It not only creates and maintains cohesion within groups, it also sends a compelling social signal to those who observe it: ‘We are a functional and potent social entity characterised by high levels of cohesion. We have internalised a shared social identity. We are close and strong.’

Many questions remain unanswered, however, about the role of observers and the signalling power of group synchrony, especially in relation to politics. Does a display of large-scale synchronous activities primarily cause feelings of awe and admiration or does it signal potency, intimidation and animosity? The political topicality is hard to miss. Donald Trump has announced his intentions this year in November to hold the first military parade since the end of the Gulf War in 1991, with costs estimated at up to $50 million dollars. His parade to display military power seems like a dangerous idea to critics, who draw attention to threats of war and oppose any attempts to demonstrate and reinforce America’s waning global hegemony. Trump, on the other hand, believes that a military parade would be great for his country’s spirit. It seems likely that both sides are right. While a military parade would send a message of dominance to adversaries, it may simultaneously inspire the nation, boost its self-esteem and increase feelings of solidarity, connectedness and national identity.

One thing is certain: we shouldn’t underestimate the effects that the synchronous behaviour can have, for actors and observers alike. Our bodies are powerful instruments in any social context and we sometimes – intentionally or unintentionally – align our behaviours with those around us. This affects us as individuals while at the same time we are affecting others. Synchrony is a powerful social signal.

Jorina von Zimmermann is a Research Associate at UCL
[email protected]

Daniel C. Richardson is in the Department of Experimental Psychology at UCL
[email protected]


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