We all have irrational fears – flying is plainly scarier than getting in a car – and we all have odd rituals that we use to manage them. But what if we believed our own hype about these rituals and became cocksure, perhaps even harming ourselves?

Here is a concrete example. In the study of risk perception, people talk about "the licensing effect": when you take a vitamin pill, for example, you think you've done something healthy and wholesome, so you permit yourself to eat more chips and have a cigarette. It sounds like a nice idea, but a bit vague.

Two new experiments put flesh on these bones. Firstly, researchers took 74 undergraduates who were daily smokers, and divided them into two groups at random. The first group were given a dummy pill, a placebo, and were told just that: you're in the control group, taking a dummy pill, with no active ingredient. The other participants were in the vitamin pill group: you've been given a vitamin pill, they were told.

But in fact, the researchers had lied. Everyone in the study got the same dummy pill, with no active ingredient. Half of them thought they'd had a health-giving vitamin pill, because the intention was to see whether people's health behaviours change if they think they've had a nice, healthy vitamin pill.

After the pills, they were given a survey to fill out. Because it was Taiwan, where lots of people smoke, they were told: "This survey will take you about one hour to finish … you're allowed to smoke if you want."

The results were startling. Firstly, people who thought they'd had a vitamin pill gave different answers on the survey. These featured questions from the excellently titled Adolescent Invulnerability Scale (which has been reasonably well validated elsewhere), such as "Special problems, getting an illness or disease, are not likely to happen to me", "I'm unlikely to be injured in an accident", and so on. People who thought they'd had a vitamin pill rated themselves as generally more invulnerable.

The results for smoking were more worrying. There's no doubt smoking is bad for you. There's also no doubt the motives and justifications for smoking are complex. But people who thought they'd had a vitamin pill were 50% more likely to have a cigarette – 89% compared with 62% – and that result was highly statistically significant.

This might be a good moment to pause and remember that the Cochrane review on antioxidant vitamin pills – the pills that glossy magazines most like to recommend – found around 200,000 patients' worth of good randomised trial data, and overall, these pills do nothing to prolong life: if anything, it turns out, they actively increase your risk of dying.

So back to our study on risk compensation behaviour. They broadened the design in case students are somehow an exceptional case, and repeated the experiment with 80 new participants, aged 19 to 58, from the wider community: once again, the people who thought they'd had vitamin pills smoked more cigarettes, and once again they believed themselves to be more invulnerable to harm.

So they expanded the project even further, into two longer studies, broader in remit, and this time people who thought they'd had a vitamin pill were less likely to exercise and less likely to choose healthier food.

People often ask what the harm is from quackery. I don't think there needs to be one: quackery, overall, is more interesting than it is dangerous. But the message from these trials is clear. Believing, incorrectly, that you've done something healthy by taking a vitamin pill makes you more likely to take genuine, concrete, real-world risks with your health. It's a chilling thought, but ideas aren't without impact, and every time we humour a harmless myth – that vitamin pills are healthy, that some fashionable berry prevents cancer – we might be doing more harm than we think.