What is the essential dividing line between human beings around the world? The one between the haves and the have-nots? East and west, rural and urban, secular and religious? Or maybe globalists and nationalists – a split purported to explain Putin, Brexit and the rise of Trump? These divisions are all significant, but none provide a consistent way of understanding differences observed from antiquity to the present day, in everything from international relations to relations in our homes.

My research across hundreds of communities suggests that the fundamental driver of difference is not ideological, financial or geographical – it’s cultural. Behaviour, it turns out, depends a lot on whether the culture in which we live is a “tight” or “loose” one.

This is simply a way of describingthe strength of social norms and the strictness with which those norms are enforced. All cultures have norms – rules for acceptable behaviour – that we take for granted. As children, we learn hundreds of them: to not grab things out of other people’s hands, to put on clothes each day. We continue to absorb new norms throughout our lives: what to wear to a funeral; how to behave at a rock concert versus a symphony; and the proper way to perform ritualsfrom weddings to worship. Social norms are the glue that holds groups together; they give us our identity, and they help us coordinate in unprecedented ways. Yet cultures vary in the strength of their social glue, with profound consequences for our worldviews, our environments and our brains.

Tight cultures have strong norms and little tolerance for deviance, while loose cultures are the opposite. In the US, a relatively loose culture, a person can’t get far down their street without witnessing a slew of casual norm violations, from littering to jaywalking to arguing loudly on the street. By contrast, in Singapore, gum is banned, streets are pristine and jaywalkers are rare. Or consider Brazil, a relatively loose culture, where arriving late for business meetings is more the rule than the exception. In fact, if you want to be sure someone will arrive on time in Brazil, you say com pontualidade britânica, which means “with British punctuality”. Meanwhile, in Japan, a tight country, there’s a huge emphasis on punctuality – trains almost never arrive late. On the rare days that delays do occur, some train companies will hand out cards to passengers that they can submit to their bosses to excuse a tardy arrival.

A discovery I and my team published in Science is that the strength of a culture’s norms isn’t random. Though they were separated by miles, and in some cases centuries, tight cultures as diverse as Sparta and Singapore have something in common: each faced (or faces) a high degree of threat, whether from Mother Nature – disasters, diseases, and food scarcity – or human nature – the chaos caused by invasions and internal conflicts. Strong norms are needed in these contexts to help groups survive. And when we look at loose cultures, from classical Athens to modern New Zealand, we see the opposite pattern: they enjoy the luxury of facing far fewer threats. This safety is used to explore new ideas, accept newcomers, and tolerate a wide range of behaviour. In contexts where there are fewer threats and thus less of a need for coordination, strong norms don’t materialise.

Analysing hundreds of hunter-gatherer groups, as well as nation-states including the Aztecs and Incas, we found that cultures that experienced existential threats, such as famine and warfare, favoured strong norms and autocratic leaders. Our computer models show a similar effect: threat leads to the evolution of tightness.

This tight-loose logic also applies to regional differences within countries. We’ve shown that US states with histories punctuated by high threat, including more natural disasters, higher pathogen prevalence and food instability, are much tighter than those that enjoyed relative safety. Similarly, communities that face financial danger – hunger, poverty, bankruptcy – and higher occupational hazards, are substantially tighter. This helps explain why those on low incomes have consistently told us they desire strong rules and leaders. In fact, when we ask respondents to free-associate from the word “rules”, low-income subjects consistently write positive words such as “good”, “safe” and “structure”, while wealthier ones write down words such as “bad”, “frustrating”, and “constricting”. These preferences arise early: in our lab, three-year-olds from low-income families were more visibly upset than peers from wealthier homes when they saw puppets violate clear rules.

Is tight better, then, or loose? The answer is, neither are. Both confer different advantages and liabilities, depending on your vantage point. Tight groups have cornered the market in social order: they have lower crime and tend to be cleaner and more coordinated. They also exhibit higher self-control: they tend to have fewer problems with obesity and debt, and lower rates of alcoholism and drug abuse. Loose groups are comparatively more disorganised and experience a host of self-regulation failures; yet they excel at openness. They’re much more tolerant, creative and flexible. Tight groups, by contrast, are far less innovative, more ethnocentric, and more resistant to new ideas. This is what I call the tight-loose trade-off; advantages in one realm coexist with drawbacks in another.

Tight-loose differences can explain global patterns of conflict, revolution, terrorism and populism. They operate as a universal faultline, causing cultural cohesion to buckle and rifts to open up. As threats arrive, groups tighten. As they subside, groups loosen. Threats don’t even need to be real. Our experiments show that, as long as people perceive a threat, the perception can be as powerful as objective reality.

Threats lead to a desire for stronger rules – and obedience to leaders who promise to deliver a tight social order. Our research confirms that the strongest Trump supporters, as well as the supporters of Marine Le Pen in France, believe their country is threatened, whether by terrorism, illegal immigration, natural disasters or disease. They felt their countries were too loose, and they wanted tighter rules and stricter leaders. Fearful voters also drove the UK’s Brexit decision and the candidacies of far-right or autocratic politicians in Poland, Russia, the Philippines, Austria, Hungary, the Netherlands and Italy.

Tight-loose is the key to anticipating our divides – mild clashes, in the case of a construction worker rolling his eyes at a gold-cufflinked Wall Streeter, or more lethal ones, such as when those who live by the tenets of a sacred text come into contact with those who dismiss guiding texts altogether. With greater awareness of the cultural code of tightness-looseness, we can better understand why we act the way we do – from revolution to war, from Brexit to Trump.

• Michele Gelfand is a professor of psychology at the University of Maryland and the author of Rule Makers, Rule Breakers: How Tight and Loose Cultures Wire Our World