Have you ever noticed that it’s impossible to tickle yourself? Yes, it’s one of those strange scientific facts that’s generally accepted, yet poorly understood.However, there are some exceptions to this rule. If you find that you can tickle yourself, you might want to get checked out by a doctor. Chances are that you’re in for some serious mental health issues later in life.

The first study that found some people can tickle themselves was published back in 2000. The results showed that neurotypical people could not tickle themselves into laughter, but those suffering from schizophrenia could.

The lead researcher, Sarah-Jayne Blakemore, theorized that their brains could not cancel out the consequences of their own actions, which is why they were able to tickle themselves. Researchers gathered two groups of students — those who had never been diagnosed with a mental illness, and those with identified schizotypic traits. People in the latter group were not necessarily schizophrenics, but were diagnosed by doctors as being at risk for developing the illness later in life. The results, as you can imagine, proved the previous theory correct.

Those with schizotypic traits rated self-tickling as being more ticklish than being tickled by another person. While further research is clearly needed, this might become a crude benchmark test for examining predisposition to schizophrenia. Of course, if you suspect that you might have schizophrenia, you should make an appointment to see a doctor immediately. Self-diagnosis is frowned upon, but the ease of conducting this test makes it tempting.