Which illness frightens you most? Cancer? Stroke? Dementia? To judge from tabloid coverage, the condition we should really fear isn't physical at all. "Scared of mum's schizophrenic attacks", "Knife-wielding schizophrenic woman in court", "Schizo stranger killed dad", "Rachel murder: schizo accused", and "My schizophrenic son says he'll kill… but he's escaped from secure hospitals 7 times" are just a few of dozens of similar headlines we found in a cursory internet search. Mental illness, these stories imply, is dangerous. And schizophrenia is the most dangerous of all.

Such reporting is unhelpful, misleading and manipulative. But it may be even more inaccurate than it first appears. This is because scientists are increasingly doubtful whether schizophrenia – a term invented more than a century ago by the psychiatric pioneer Eugen Bleuler – is a distinct illness at all. This isn't to say that individuals diagnosed with the condition don't have genuine and serious mental health problems. But how well the label "schizophrenia" fits those problems is now a very real question.

What's wrong with the concept of schizophrenia? For one thing, research indicates the term may simply be functioning as a catch-all for a variety of separate problems. Six main conditions are typically caught under the umbrella of schizophrenia: paranoia; grandiosity (delusional beliefs that one has special powers or is famous); hallucinations (hearing voices, for example); thought disorder (being unable to think straight); anhedonia or the inability to experience pleasure; and diminished emotional expression (essentially an emotional "numbness"). But how many of these problems a person experiences, and how severely, varies enormously. Having one doesn't mean you'll necessarily develop any of the others.

Why hasn't this been noticed by clinicians? Mental health professionals, inevitably, tend only to see the most unwell individuals. These patients tend to suffer from lots of the problems we've mentioned – the more difficulties you're experiencing, the more likely it is that you'll end up being seen by a specialist – prompting psychiatrists like Bleuler to assume these problems are symptoms of a single underlying condition. But defining an illness by looking only at the minority who end up in hospital can be a big mistake.

The traditional view has been that schizophrenia occurs in approximately 1% of people. But it's now clear that the sort of experiences captured under the label are common in the general population – frequently far less distressing and disruptive, for sure, but essentially the same thing. Take paranoia, for instance. Almost 20% of UK adults report feeling as though others were against them in the previous 12 months, with 1.8% fearing plots to cause them serious harm. We tested the level of paranoia among the general public by asking volunteers to take a virtual reality tube train ride, during which they shared a carriage with a number of computer-generated "avatars". These avatars were programmed to behave in a strictly neutral fashion, yet over 40% of participants reported that the avatars showed hostility towards them.

Moreover, triggering the odd sensations associated with schizophrenia is remarkably easy. Go without sleep for a night or two and you're likely to experience some very peculiar thoughts and feelings (as demonstrated by a recent study of sailors in solo races). Consume a lot of cannabis and the effects can be similar. Meanwhile, a classic study by the psychiatrist Stuart Grassian showed that prisoners placed in solitary confinement were soon prey to hallucinations and delusions.

What all this suggests is that schizophrenia isn't a specific, relatively rare, and rigorously defined illness. Instead, it covers a wide range of often unrelated conditions, all of which are also seen in people who are not mentally ill, and all of which exist on a continuum from the comparatively mild to the very severe. People with conditions like schizophrenia are simply those who happen to fall at the extreme end of a number of these continua.

What causes psychotic experiences? Research has pointed a decisive finger at living in cities, drug use, poverty, migration, traumatic experiences in childhood and later negative events such as being the victim of an assault. Experiences like paranoia are also linked with a number of psychological traits, such as a tendency to worry, feel depressed, sleep poorly, or jump to conclusions. These factors seem to work in what scientists call a "dose-response" manner: the more of them you experience, the more likely it is that your mental health will suffer.

Genetic factors also play a part, though there's no evidence for a single "schizophrenia" gene. Instead, a multitude of genes are likely to be involved – with their effect, crucially, conditioned by environmental factors. So the people who end up being treated for schizophrenia aren't the unlucky few who happen to have inherited a rogue gene. Genetic susceptibility exists on a spectrum too. The more of the relevant genes you possess, the further you are to the extreme end of the spectrum and the less of a push you'll need from life events to become ill. It's worth remembering, however, that genetic research into schizophrenia has focused on the people who present for treatment: the severest end of the continua. What it hasn't done is look at the various types of psychotic experiences across the general population.

Not everyone agrees with these new ways of thinking about schizophrenia. An editorial in the British Journal of Psychiatry, for example, lambasted the approach as "scientifically unproven and clinically impractical". But one thing is certain: deepening our understanding of psychotic problems must be a priority. Diagnostic criteria for mental illnesses change over time, and the same will happen with schizophrenia. Rather than getting sidetracked by day-to-day debates about the symptoms required for a diagnosis, it will be more productive to focus on the individual psychotic experiences, remembering that they don't only occur in those who come into contact with mental health services but exist on spectra in the general population. This isn't merely a theoretical issue: if we target specific problems, rather than a loosely defined illness, we're likely to improve treatment outcomes for the many people struggling with these debilitating experiences.

• Daniel Freeman and Jason Freeman are the authors of Paranoia: the 21st Century Fear, published by Oxford University Press. Daniel is a professor of clinical psychology and a Medical Research Council senior clinical fellow at the University of Oxford, and a fellow of University College, Oxford. Twitter: @ProfDFreeman. Jason is a psychology writer. Twitter: @JasonFreeman100