“Will these hands ne'er be clean?” In Shakespeare's play Macbeth, Lady Macbeth helps to plot the brutal murder of King Duncan. Afterwards she feels tainted by Duncan's blood and insists that “all the perfumes of Arabia” could not sweeten her polluted hands. Baffled by her compulsive washing, her doctor is forced to admit: “This disease is beyond my practise.”

In the 400 years since Macbeth was first performed, other doctors, psychiatrists, neuroscientists and clinical psychologists — myself included — have also found the problem beyond the reach of their own expertise. We see compulsive washing a lot, mostly as a symptom of obsessive–compulsive disorder (OCD), but also in people who have suffered a physical or emotional trauma, for example in women who have suffered sexual assault. The events trigger a deep-seated psychological, and ultimately biological, response.

We know that the driving force of compulsive washing is a fear of contamination by dirt and germs. An obsessive fear of contact with sexual fluids, for example, can drive compulsive washing in OCD and force people to restrict sexual activity to a specific room in the house. Compulsive washing fails to relieve the anxiety. Most patients with OCD continue to feel contaminated despite vigorous attempts to clean themselves. Why does repeated washing fail?

There is much debate at present about the direction that psychiatric medicine and research should take. We should not underestimate what we can continue to learn from the careful observation of patients. Such observations have led my colleagues and me to diagnose a new cause of OCD and other types of compulsive washing: mental contamination.

Lady Macbeth could be an example of someone suffering from this psychological problem. Mental contamination can be evoked without contact with a tangible contaminant. It is a feeling of internal dirtiness caused by a psychological or physical violation. The source of the pollution is not an external contaminant such as blood or dirt, but human interaction. The affected person develops strong feelings of contamination that are evoked by direct contact with the violator or indirect contacts such as memories, images or reminders of the violation.

The source of their problematic contamination is not physical but mental.

Commonly, these patients are unwilling or unable even to speak the name of the violator. Milder forms of this mechanism are prevalent in society — in the course of a bitter divorce, for example. The emotional violations that can cause mental contamination include degradation, humiliation, painful criticism and betrayal. There is a moral element in most cases of mental contamination. Separate psychometric research has confirmed mental contamination as a coherent and measurable concept.

This discovery has large and immediate implications for clinical treatment. A common technique to treat compulsive washing — and one that I helped to develop — aims to reduce the fear, in the expectation that this will also reduce the need to wash. Called exposure and response prevention, it asks the patient to repeatedly touch the contaminating object or substance — rubbish, for example — and encourages them not to wash or clean as they then want to. The therapy aims to untangle the psychological 'conditioned' association between fear and the source of the fear.

Exposure and response prevention is a feature of cognitive behavioural therapy for OCD and other mental disorders across the world. Yet it is a demanding treatment. Up to one-quarter of patients drop out or refuse it, and up to three in ten of those who remain fail to improve. Over the past decade or so, it has become clear that one reason that these people are not helped by exposure therapy is that the source of their problematic contamination is not physical but mental. Feelings of mental contamination are diffuse, mainly internal and not accessible. In these cases, therefore, repeated hand-washing is misdirected. And so, crucially, is exposure therapy based on physical contact.

In an early study of 50 young women who had experienced a sexual assault, we found abundant evidence of contact contamination and also mental contamination. After the assault they felt polluted, and understandably engaged in vigorous washing. However, a substantial minority of the women continued to feel polluted for many months after the event.

Mental contamination is essentially a cognitive disorder. The patient's memories, thoughts and images are the root of the problem. They need a cognitive approach, and my colleagues and I have developed an effective cognitive treatment for mental contamination. We unravel the circumstances of the violation and why the patients believe that they remain under threat. After working with many patients, we carried out a recent (non-blind) study with 12 people with severe OCD, most of whom had not been helped by traditional cognitive behavioural therapy (A. E. Coughtrey et al. Cogn. Behav. Prac. 20, 221–231; 2013 ). Using the notion of mental contamination, and treating it accordingly, we managed to reduce the symptoms significantly for nine of them. We now plan to conduct a full-scale randomized controlled trial. If the effects of the therapy are confirmed, it would have a major impact. We would at last be able to treat the many patients who are currently, like Lady Macbeth, beyond our help.