Kiwis who imbibe the caffeine equivalent of one and a half cups of espresso coffee – or seven cups of instant – are more likely to hallucinate according to a new study, which may explain the ascendant popularity of coffee culture in New Zealand.

Researchers at Durham University in the UK found that people with a caffeine intake of that level, whether from coffee, tea, caffeinated energy drinks, pills or chocolate; are three times more likely to hear voices and see things that aren’t there than those who consume a single short black, reports LiveScience.

The results were released in the journal Personality and Individual Differences.

While most coffee addicts aren’t known for their hallucinations, when it happens enough to compromise their daily functioning, they are considered psychotic.

There is about 315 milligrams of caffeine in seven cups of instant coffee, or nine colas, six cups of strong tea, four Red Bulls, or roughly one and a half cups of espresso coffee at a good cafe.

Caffeine can increase the capacity for physical or mental labour when used in moderation, but when used in excess the drug can cause nervousness, irritability, muscle twitching, intoxication, headaches, heart palpitations and insomnia.

Non-smoking university students numbering 200 were asked about their caffeine intake from various forms for the study.

Their propensity for hallucinatory experiences along with their stress levels were assessed, with some participants reporting hearing voices, seeing things that weren’t there, and sensing the presence of dead people.

One explanation is that caffeine can exacerbate the physiological effects of stress, where the body releases a hormone called cortisol. The body releases more cortisol in response to stress after people have ingested caffeine.

The extra cortisol is theorised to link caffeine intake with an increased likelihood of hallucinating according to study leader Simon Jones, a graduate student at Durham's Psychology Department.

"Hallucinations are not necessarily a sign of mental illness. Most people will have had brief experiences of hearing voices when there is no one there, and around three percent of people regularly hear such voices. Many of these people cope well with this and live normal lives", said Mr Jones to LiveScience.