Why do so many people these days seem so stressed out and anxious? It’s a common question, among mental health professionals and laypeople alike, but there’s a case to be made that it’s exactly upside down. How come there’s anyone who isn’t paralysed by anxiety, every hour of every day? After all, anxiety thrives in conditions of uncertainty – and nowadays the world is full of potential threats we don’t fully understand and can’t control.

Most of us just have to take it on trust that planes won’t fall out of the sky, or that the milk in our fridge won’t give us listeria. Sudden, unpredictable movements in the global financial system threaten to ruin anyone’s livelihood at any moment; plus now we have all the many unknowns around Brexit, an unstable liar in charge of America’s nuclear codes, and the omnipresent spectre of climate change. And as if all that weren’t enough, we spend our days marinating in an online environment designed to stoke panic about any remaining threats we might have been managing to ignore.

Some definitions may help here. “Stress”, as psychologists tend to use it, means an immediate response to an external pressure, and a moderate amount can actually be a good thing: totally unstressed people never revise for exams, or meet work deadlines. In most everyday contexts, when the external pressure stops, so does the stress. Which means that by far the simplest way to deal with stress, whenever possible, is to deal with whatever’s bothering you head on – to tackle the difficult piece of work, to talk to the friend you’ve fallen out with – or, failing that, to distance or distract yourself from its source. (Persistent and chronic stress requires a different approach.)

But anxiety is a particular kind of internal response to stress, and it’s frequently far more fraught. As the Australian author Sarah Wilson points out in her combined memoir and anxiety self-help manual, First, We Make the Beast Beautiful, the problem isn’t simply that there are a lot of reasons to be anxious. It’s also that, perversely, society actually rewards certain anxious behaviours, such as being frenetically busy and driven – while the “reward” for managing to free yourself from anxiety might be a reputation for laziness, complacency, or showing insufficient concern about the state of the world. Then there’s the fact that anxiety is self-reinforcing: once you’re feeling anxious, you’re primed to seek further things to feel anxious about – including, as if that vicious circle weren’t frustrating enough, your anxiety itself.

“Anxiety”, of course, is also the name for a family of official psychological disorders. But it’s a condition that makes it unusually clear that the line between “mental illness” and “ordinary human distress” is a subjective one, dependent as much on cultural conventions as on science. The main reason “generalised anxiety disorder” is so much more prevalent now is that it was only defined as a disorder in psychiatry’s bible, the Diagnostic and Statistical Manual of Mental Disorders, in 1980. (If you feel “keyed up or on edge”, or have “difficulty concentrating”, it’s possible you qualify.) And one main reason it surged from 2001 onwards was a concerted media push by GlaxoSmithKline, after it received US approval to market its antidepressant Paxil (Seroxat in the UK) in the treatment of anxiety. “Local newscasts around [the United States] reported that as many as 10 million Americans suffered from an unrecognised disease,” the journalist Brendan Koerner has written. “Viewers were urged to watch for the symptoms: restlessness, fatigue, irritability, muscle tension, nausea, diarrhoea, and sweating …”

To be clear, none of this is to suggest that those with a diagnosed anxiety disorder don’t have a real illness, or that medication isn’t often part of the solution: “The basic premise for an anxiety disorder, or when anxiety becomes a clinical problem, is when anxiety controls our life, rather than us being able to control our anxiety,” says Robert Edelmann, emeritus professor of forensic and clinical psychology at the University of Roehampton. But it’s also a reminder that anxiety, at root, isn’t a bizarre psychological anomaly, but a fundamental aspect of human functioning. The problem, explains the psychology writer James Clear, author of Atomic Habits, is that it’s a response evolution bequeathed us for a setting radically different from today’s.

Prehistoric humans lived in an “immediate-return environment”, as other mammals still do: their moment-to-moment choices mattered because of the immediate difference they made. You saw a predator and felt a surge of anxiety, which motivated you to evade it. Or you felt dangerously hungry, and anxiety focused your attention on quickly finding food. Once the threat was resolved, the anxiety would evaporate. But modern humans live in a “delayed-return environment”. We get paid for our work at the end of the week or month; we study for educational qualifications that take years. When we save money – or don’t – the consequences might not be felt for decades. And so the anxiety has nowhere to go. Instead, it accumulates and curdles.

This helps explain why national and international news events, such as Brexit or the election of Donald Trump, are such a widespread source of personal anxiety, including of the clinical kind. Some people – undocumented immigrants in Trump’s America, for example – are affected in direct, unambiguous ways. But even if you ultimately won’t be, you’ll have no way of knowing that for some time. Moreover, it often feels like there’s nothing you can do in response – no equivalent to the prehistoric hunter-gatherer’s decision to start running away, or go looking for food. When no constructive action seems possible, we resort to worry and rumination, which feel somehow constructive, even though they aren’t. One reaction to the anxiety of being immersed in a 24-hour news cycle “is that people try to find out more information, because anxiety is about a lack of control and they believe that having more information will make them feel more in control”, says the American therapist Lori Gottlieb, author of the forthcoming book Maybe You Should Talk to Someone. “But it doesn’t – it just makes people feel more anxious.”

This is why most non-pharmaceutical solutions to anxiety, whatever its cause, involve the limited and realistic exertion of control: figuring out what constructive actions you can take, and taking them, while refraining from struggling to control things you can’t, which is a recipe for additional anxiety. (This is the “dichotomy of control”, a distinction dating to the Stoics of ancient Greece and Rome.) You can’t personally guarantee a comfortable retirement, or long-term physical health, let alone the optimal relationship between Britain and the rest of Europe. But you can calculate what you can afford to save, and track your saving. You can exercise a few times a week, and eat more leafy greens. You can take local, concrete political action. Whether or not you end up achieving your desired goal, your anxiety levels will almost certainly fall.

Finally, it’s worth recognising – as the Danish philosopher Søren Kierkegaard observed in The Concept of Anxiety back in 1844 – that all anxiety contains a kernel of good news: you wouldn’t feel anxious in the first place if you had no freedom, and if there weren’t at least the possibility of things turning out well. “One would have no anxiety if there were no possibility whatever,” wrote the psychologist Rollo May, paraphrasing Kierkegaard. If you knew with absolute certainty that life from now on would bring only failure and defeat, you might well be depressed, but you wouldn’t feel on edge. Anxiety is the experience of knowing that life might bring success, fulfilment and joy, combined with the fear that you don’t know how to ensure that’s what will happen. And while severe anxiety can certainly be a debilitating problem, demanding treatment, some sense of uncertainty about the future is surely part of what makes life worth living. If you ever actually managed to rule out the potential for any nasty surprises, you’d find that you’d ruled out the possibility of any good ones, too.

The Antidote: Happiness for People Who Can’t Stand Positive Thinking by Oliver Burkeman is published by Vintage (£9.99). To order a copy for £8.79 go to guardianbookshop.com.