Great Britain is old. My country, Italy, is young. Both pride themselves on their past. Both are said to have marked national characters. There are old Roman stones in the basement of my building in Verona, and the heroes in my school were Leonardo and Michelangelo. And yet this national layer is a thin one. Dante marked my education, as did Shakespeare and Dostoevsky. I was born in bigoted Verona, and experienced a culture shock studying in libertine Bologna.

My country’s well-intentioned founding fathers invented an Italian identity that later gave rise to fascism

I grew up in a social class and generation that shared its habits and concerns with people around the planet more than with its fellow Italian citizens. My identity comes from my own family (unique, as any family is); my friends from childhood; the cultural tribe of my youth; my scattered friends in adult life. It comes from the constellation of values, ideas, books, political dreams, cultural concerns and common purposes that were shared, nurtured and fought for; that were passed along within communities and across national boundaries. This is what makes us all: a combination of layers and intersections in a network of threads that weave humanity together in an multiform and ever-changing culture.

But if this is the variegated identity of each of us, why do we organise our collective and political behaviour around the nation and its sense of belonging? Why Italy? Why Britain? The answer is easy: it’s not the power structure that’s built on identity, it’s the other way around. National identities are created by power structures. Seen from my young and still rather dysfunctional country, this is perhaps easier to spot than from the UK. But it is the same.

As soon as it has emerged, generally with fire and fury, the first concern of any centre of power – whether an ancient king or a 19th-century liberal bourgeoisie – is to promote a robust sense of self-identity. “We have made Italy, now we must make Italians,” Massimo d’Azeglio, a pioneer of Italian unification, famously said in 1861.

It is astonishing how differently history is taught in different countries. For a French person, world history is centred on the French revolution. For an Italian, it’s the Renaissance and the Roman empire. For an American, the key event that gave rise to the modern world, liberty and democracy, is the war of independence waged against Britain. For an Indian, the roots of civilisation are found in the era of the Vedas. We live among discordant narratives, ones that we share with our fellow citizens. They are designed to create a sense of belonging to fictitious families called nations. Less than two centuries ago, people in Calabria called themselves Greek, and today not everybody in Scotland or Wales would support England in the World Cup. National identities are political theatre.

My point is not to suggest that there is anything wrong with this. On the contrary, unifying people for the common good – whether Venetians, Sicilians or distinct Anglo-Saxon tribes – is wise politics. If we fight each other we are obviously worse off than if we work together. We benefit from cooperating rather than competing. Civilisation is the result of collaboration. Whatever the difference between Naples and Verona, things are better without borders.

The exchange of ideas and goods, of looks and smiles, the stuff that forms our complex reality, only makes us richer – in wealth, intelligence and soul. Bringing people together in a common political space is to everyone’s advantage. Reinforcing this process by means of ideology and political theatre to keep irrational conflicts at bay, and telling people that national identity is sacred, works rather well – even if it’s constructed.

Yet that’s precisely when national identity becomes a poison. It may foster solidarity at one level, but it can become a serious impediment to cooperation on a larger scale. My country’s well-intentioned founding fathers invented an Italian identity which, decades later, gave rise to fascism, the extreme glorification of national identity. Fascism went on to inspire Hitlers’s nazism. The intense emotional identification of Germans with a single Volk ended up devastating Germany and much of Europe. When we value conflict over cooperation, and stop searching for compromise and agreed rules, national identity becomes toxic.

Nationalist politics are spreading across the world, increasing tensions, sowing conflict, threatening each and every one of us. My own country has just fallen prey to this again. I think we should respond by saying loud and clear that national identity is fake. Focusing on the national dimension is good if it helps overcome narrow local interests, but it is despicable if it means promoting the interests of a single group (“our nation”) above the wider common good.

Today’s surge of localism and nationalism does not just result from mistaken hopes of political gain; it draws on the emotional appeal of a potential identity. Politics plays with our insatiable desire to belong. Foxes have earths and birds have nests, but a human being has no place to lay their head. Offering the fictitious home of “the nation” is cheap and politically rewarding. Glorifying local or national identity by placing it above cooperation is not just counterproductive, it is also miserable, ugly and morally reprehensible.

It is not that we do not have national identities – we do. Rather, it is that each of us is a crossroads of multiple and often stronger identities. By putting nation first, we betray others. Not because we are equal, but because we are different within nations. It is not that we don’t need a home, it is that we deserve, and have, better homes than the grotesque theatre of nation: our family, our friends, the community of those sharing our values around the world. We are many, and we have a wonderful place to care about, the Earth, and a marvellous, variegated tribe of brothers and sisters with whom to identify, and with whom to feel at home here.

• Carlo Rovelli is an Italian physicist and the author of The Order of Time