In a cafe in south London, two construction workers are engaged in cheerful banter, tossing words back and forth. Their cutlery dances during more emphatic gesticulations and they occasionally break off into loud guffaws. They are discussing a woman, that much is clear, but the details are lost on me. It’s a shame, because their conversation sounds fun and interesting, especially to a nosy person like me. But I don’t speak their language.

Out of curiosity, I interrupt them to ask what language they are speaking. They both switch easily to English, explaining that they are South Africans and had been speaking Xhosa. In Johannesburg, where they are from, most people speak at least five languages, says one of them, Theo Morris. For example, Morris’s mother’s tongue is Sotho, his father’s is Zulu; he learned Xhosa and Ndebele from his friends and neighbours and English and Afrikaans at school. “I went to Germany before I came here, so I also speak German,” he adds.

Was it easy to learn so many languages? “Yes, it’s normal,” he laughs.

To communicate for trade, travel and so on, it would have been necessary for early humans to speak other tongues

He’s right. Around the world, more than half of people – estimates vary from 60-75% – speak at least two languages. Many countries have more than one official national language – South Africa has 11. People are increasingly expected to speak, read and write at least one of a handful of “super” languages, such as English, Chinese, Hindi, Spanish or Arabic, as well. So to be monolingual, as many native English speakers are, is to be in the minority and perhaps to be missing out.

Multilingualism has been shown to have many social, psychological and lifestyle advantages. Moreover, researchers are finding a swath of health benefits from speaking more than one language, including faster stroke recovery and delayed onset of dementia.

Could it be that the human brain evolved to be multilingual, that those who speak only one language are not exploiting their full potential? And in a world that is losing languages faster than ever – at the current rate of one a fortnight, half our languages will be extinct by the end of the century – what will happen if the current rich diversity of languages disappears and most of us end up speaking only one?

I am sitting in a laboratory, headphones on, looking at pictures of snowflakes on a computer. As each pair of snowflakes appears, I hear a description of one of them through the headphones. All I have to do is decide which snowflake is being described. The only catch is that the descriptions are in a completely invented language called Syntaflake.

It’s part of an experiment by Panos Athanasopoulos, an ebullient Greek with a passion for languages. Professor of psycholinguistics and bilingual cognition at Lancaster University, he’s at the forefront of a new wave of research into the bilingual mind. As you might expect, his lab is a Babel of different nationalities and languages, but no one here grew up speaking Syntaflake.

The task is profoundly strange and incredibly difficult. Usually, when interacting in a foreign language, there are clues to help you decipher the meaning. The speaker might point to the snowflake as they speak, use their hands to demonstrate shapes or their fingers to count out numbers, for example. Here, I have no such clues and, it being a made-up language, I can’t even rely on picking up similarities to languages I already know. By the end of the session, I have to admit defeat.

Studies suggest bilingual people think differently according to the language they are using. Photograph: Rex

I join Athanasopoulos and glumly recount my difficulty in learning the language, despite my best efforts. But it appears that was where I went wrong: “The people who perform best on this task are the ones who don’t care at all about the task and just want to get it over as soon as possible. Students and teaching staff who try to work it out and find a pattern always do worst,” he says.

“It’s impossible in the time given to decipher the rules of the language and make sense of what’s being said to you. But your brain is primed to work it out subconsciously. That’s why, if you don’t think about it, you’ll do OK in the test. Children do the best.”

The first words ever uttered might have been as far back as 250,000 years ago, once our ancestors stood up on two legs and freed the ribcage from weight-bearing tasks, allowing fine nerve control of breathing and pitch to develop. And when humans had one language, it wouldn’t have been long before we had many.

Language evolution can be compared to biological evolution, but whereas genetic change is driven by environmental pressures, languages change and develop through social pressures. Over time, different groups of early humans would have found themselves speaking different languages. Then, in order to communicate with other groups – for trade, travel and so on – it would have been necessary for some members of a family or band to speak other tongues.

We can get some sense of how prevalent multilingualism might have been from the few hunter-gatherer peoples who survive today. “If you look at modern hunter-gatherers, they are almost all multilingual,” says Thomas Bak, a cognitive neurologist who studies the science of languages at the University of Edinburgh. In Australia, where more than 130 indigenous languages are still spoken, he says, multilingualism is part of the landscape.

“You will be walking and talking with someone and then you might cross a small river and suddenly your companion will switch to another language. People speak the language of the Earth,” says Bak. This is true elsewhere, too. “Consider Belgium: you take a train in Liège, the announcements are in French first. Then, pass through Leuven, where the announcements will be in Dutch first, and then in Brussels it reverts to French.”

The connection with culture and geography is why Athanasopoulos invented a new language for the snowflake test. Part of his research lies in trying to tease out the language from the culture it is threaded within, he explains.

Ask me in English what my favourite food is and I will picture myself in London choosing from the options I enjoy there. But ask me in French and I transport myself to Paris, where the options I’ll choose from are different. So the same deeply personal question gets a different answer depending on the language in which you’re asking me. This idea that you gain a new personality with every language you speak is a profound one.

Athanasopoulos and his colleagues have been studying the capacity for language to change people’s world views. In one experiment, English and German speakers were shown videos of people moving. English speakers focus on the action and say: “A woman is walking” or: “A man is cycling”. German speakers, on the other hand, have a more holistic view and will include the goal of the action: they might say (in German): “A woman walks towards her car” or: “A man cycles to the supermarket”.

Part of this is due to the grammatical toolkit available. Unlike German, English has the -ing ending to describe actions that are ongoing. This makes English speakers much less likely to assign a goal to an action when describing an ambiguous scene. When he tested English-German bilinguals, however, whether they were action- or goal-focused depended on the country in which they were tested. If the bilinguals were tested in Germany, they were goal-focused; in England, they were action-focused, no matter which language was used.

Thomas Bak, who believes multilingualism might once have been much more common than it is today. Photograph: Murdo MacLeod/The Guardian

In the 1960s, one of the pioneers of psycholinguistics, Susan Ervin-Tripp, tested Japanese-English bilingual women, asking them to finish sentences in each language. She found that the women ended the sentences very differently depending on which language was used. For example, “when my wishes conflict with my family... ” was completed in Japanese as “it is a time of great unhappiness”; in English, as “I do what I want”. Another example was “real friends should… ”, which was completed as “help each other” in Japanese and “be frank” in English.

From this, Ervin-Tripp concluded that human thought takes place within language mindsets and that bilinguals have different mindsets for each language, an extraordinary idea but one that has been borne out in subsequent studies; many bilinguals say they feel like a different person when they speak their other language.

These different mindsets are continually in conflict, however, as bilingual brains sort out which language to use.

In a recent study of 600 stroke survivors, cognitive recovery was twice as likely for bilinguals as for monolinguals

In a revealing experiment with his English-German bilingual group, Athanasopoulos got them to recite strings of numbers out loud in either German or English. This effectively “blocked” the other language altogether, and when they were shown the videos of movement, the bilinguals’ descriptions were more action- or goal-focused, depending on which language had been blocked. So, if they recited numbers in German, their responses to the videos were more typically German and goal-focused. When the number recitation was switched to the other language midway, their video responses also switched.

So are there really two separate minds in a bilingual brain? That’s what the snowflake experiment was designed to find out. I’m a little nervous of what my fumbling performance will reveal about me, but Athanasopoulos assures me I’m similar to others who have been tested and, so far, we seem to be validating his theory.

In order to assess the effect that trying to understand the Syntaflake language had on my brain, I took another test before and after the snowflake task. In these so-called flanker tasks, patterns of arrows appeared on the screen and I had to press the left or right button according to the direction of the arrow in the centre. Sometimes, the surrounding pattern of arrows was confusing, so by the end of the first session my shoulders had been hunched somewhere near my ears and I was exhausted from concentrating. It’s not a task in which practice improves performance (most people actually do worse second time around), but when I did the same test again after completing the snowflake task, I was significantly better at it, just as Athanasopoulos had predicted.

“Learning the new language improved your performance second time around,” he explains. Relieved as I am to fit into the normal range, it’s a curious result. How can that be?

The flanker tasks were exercises in cognitive conflict resolution – if most of the arrows were pointing to the left, my immediate impulse was to push the left button, but this wasn’t the correct response if the central arrow was pointing right. I had to block out my impulse and heed the rule instead. The part of the brain that manages this supreme effort is known as the anterior cingulate cortex (ACC), part of the “executive system”. Located on the frontal lobe, it is a toolbox of mental attention skills that enables the brain to concentrate on one task while blocking out competing information and to switch focus between different tasks without becoming confused.

Many countries have more than one official language. Photograph: Jeff Morgan/Alamy

The snowflake test primed my ACC for the second flanker task, just as speaking more than one language seems to train the executive system more generally. A steady stream of studies over the past decade has shown that bilinguals outperform monolinguals in a range of cognitive and social tasks, from verbal and non-verbal tests to how well they can “read” other people.

“Bilinguals perform these tasks much better than monolinguals – they are faster and more accurate,” says Athanasopoulos. “And that suggests their executive systems are different from monolinguals.”

Brain imaging studies show that when a bilingual person is speaking in one language, their ACC is continually suppressing the urge to use words and grammar from their other language. Not only that, but their mind is always making a judgment about when and how to use the target language. For bilinguals, with their exceptionally buff executive control, the flanker test is just a conscious version of what their brains do subconsciously all day long – it’s no wonder they are good at it.

A superior ability to concentrate, solve problems and focus are, of course, valuable in everyday life. But perhaps the most exciting benefit of bilingualism occurs in ageing, when executive function typically declines: bilingualism seems to protect against dementia.

Psycholinguist Ellen Bialystok made the surprising discovery at York University in Toronto while she was comparing an ageing population of monolinguals and bilinguals. “The bilinguals showed symptoms of Alzheimer’s some four to five years after monolinguals with the same disease pathology,” she says.

Bialystok thinks this is because bilingualism rewires the brain and improves the executive system, boosting people’s “cognitive reserve”. It means that as parts of the brain succumb to damage, bilinguals can compensate more easily.

Bilingualism can also offer protection after brain injury. In a recent study of 600 stroke survivors in India, Bak discovered that cognitive recovery was twice as likely for bilinguals as for monolinguals.

Such results suggest bilingualism helps keep us mentally fit. It may even be an advantage that evolution has positively selected in our brains, an idea supported by the ease with which we learn new languages and flip between them and by the pervasiveness of bilingualism throughout world history. Just as we need to do physical exercise to maintain the health of bodies that evolved for a physically active hunter-gatherer lifestyle, perhaps we ought to start doing more cognitive exercises to maintain our mental health, especially if we only speak one language.

In recent years, there has been a backlash against studies showing the benefits of bilingualism. Some researchers tried and failed to replicate some of the results; others questioned the benefits of improved executive function in everyday life. Bak wrote a rejoinder to the published criticisms and says there is now overwhelming evidence from psychological experiments backed by imaging studies that bilingual and monolingual brains function differently. He says the detractors have made errors in their experimental methods.

English and Spanish: two of the global giants of language. Photograph: John Moore/Getty Images

Bialystok agrees, adding that it is impossible to examine whether bilingualism improves a child’s school exam results, for example, because there are so many factors. “But,” she says, “given that at the very least it makes no difference – and no study has ever shown it harms performance – considering the very many social and cultural benefits to knowing another language, bilingualism should be encouraged.”

To maintain the benefits of bilingualism you need to keep using your languages and that can be tricky, especially for older people who may not have opportunities to practise. Perhaps we need clubs where people can meet to speak other languages. Bak has done a small study with elderly people learning Gaelic in Scotland and seen significant benefits after just one week. Now he aims to carry out a much larger trial. In the meantime, it makes sense to talk, hablar, parler, sprechen, beszél, berbicara – in as many languages as possible.

Creating new languages

Every year, humanity loses 30-50 languages. Of roughly 7,000 we still have, just 10 are used by half the world’s speakers. It seems inevitable that eventually the world will use just one – Spanish, perhaps, or Mandarin or English.

While the number of natural languages has fallen, people have tried to create entirely new ones. Volapük, invented by a German priest in 1880, was one of the first attempts at an artificial universal language. Conferences were held in Volapük and periodicals and books were published in the language, which, at its peak, claimed a million speakers.

It was usurped at the end of the 19th century by Esperanto, which was made up by a Polish Jewish opthalmologist and claims 2 million speakers today. Various attempts were made to stamp out Esperanto, surely the most reliable sign of a “real” language. The most aggressive attack came from the Nazis, who hated it because it had been invented by a Jew. It was taught illicitly in concentration camps, by prisoners who told guards it was Italian. In the end, however, Esperanto’s failure to become universal came down to the same pressures that threaten the rest: the handful of languages that are truly global.

Even as tongues succumb to extinction, new pidgin dialects – word and grammar hybrids of pre-existing languages – emerge. Kiezdeutsch originated in Turkish migrant communities in Germany, but has now become a common way of speaking for young people who otherwise have perfect German, including those with no Turkish origins. Like British teens using Jafaican (or multicultural London English) – a melange of Jamaican patois, Los Angeles rap-speak and south London slang – Kiezdeutsch is strongly tied to identity and how the speakers see themselves in society.

Meanwhile, as channels such as MTV are broadcast internationally, English speakers across Europe are modifying their accents as well as their vocabulary, so even if English ends up the one global language, it won’t be the Queen’s English everyone is speaking.

• This is an edited version of an article that first appeared on Mosaic and is republished here under a Creative Commons licence.