Mental illness is a strange and puzzling thing. The causes of most of the major mental disorders are largely unexplained, but by and large one doesn't think of them as diseases that are transmitted from one person to another. However, on very rare occasions, just such a thing can occur - this Entry will look at two distinct ways that odd and erroneous beliefs can pass between two or more people.

Folie à Deux

Folie à deux¹, also known as Shared Psychotic Disorder² or Induced Psychotic Disorder³, is a disorder in which two (or, in rare cases, more) people who have a close relationship share strange and untrue beliefs.

Incidence and Signs of the Disorder

Folie à deux is very uncommon. Since it was first described in 1877, the number of cases diagnosed is only in the hundreds, but new cases are reported in medical journals from time to time.

In order for the diagnosis to be made, there must be two people who share the same delusions. A delusion is a false belief (or set of false beliefs) that the individual holds abnormally strongly. Common delusions include a belief that people are 'out to get' the individual, that he or she has extraordinary powers, or that people can read his or her mind. In most situations, while there are common types of delusions, the details of these delusions vary markedly from person to person. In folie à deux, these details are very similar in two different people.

The two individuals usually have a very close relationship - they are often members of the same family. Mother-son, husband-wife and twin-twin cases have been reported. Usually one individual is dominant in the relationship (the 'primary'), while the other is more impressionable and open to the other's ideas (the 'secondary')⁴. In most cases, the pair are geographically or socially isolated from other people: this means that the secondary individual has difficulty in getting a 'reality check' from others.

• Folie imposée: The delusions are transferred from a psychotic individual to a mentally sound person. The separation of the two individuals is often all that is required to cure the secondary individual.

• Folie simultané: Identical delusions arising in two individuals at the same time.

• Folie communiquée: Similar to folie imposée, this differs in that the delusions are adopted by the secondary individual only after a long period of resistance. In this case, separation alone isn't enough to cure the secondary individual.

• Folie induite: New delusions are adopted by a mentally ill individual under the influence of another mentally ill individual. This was first described by Oliver Sacks after it occurred in a psychiatric hospital.

Folie à deux and Crime

When horrendous crimes are committed by two people who have a close relationship, often folie à deux is suggested as an explanation. The Parker-Hulme murder in New Zealand in the 1950s⁵ and the Moors Murderers have been held up as examples of the disorder - although neither of the pairs have been formally diagnosed with it.

Treatment

The first and most important thing to do is to separate the individuals. This breaks the chain of the pair reinforcing each other's beliefs.

The primary individual is treated in a similar way to anybody with a psychotic illness - this will usually involve antipsychotic medication.

Some controversy exists around whether doctors also need to prescribe antipsychotics for the secondary individual. Traditionally, people thought that separation of the pair was all that was needed, but as more cases have been described, it's become clear that this doesn't always work. Because of this, often both members of the pair will get antipsychotics as well as being separated.

Mass Psychogenic Illness

By contrast, Mass Psychogenic Illness (MPI)⁶ isn't considered to be a mental disorder at all - rather, it is an uncommon occurrence, presumably related to acute anxiety, which relies on the dynamics of large groups of people for it to spread.

It can be best be illustrated by an example⁷. A teacher at a high school noticed a strange, petrol-like smell upon arriving at work that morning. Shortly after smelling this, she developed headaches, became nauseated, dizzy and short of breath. Several students in the same classroom experienced the same symptoms, the room was evacuated and emergency services were called.

Shortly afterwards, the whole school was evacuated and gathered on the playing fields, where they watched the emergency services go about their work. The teacher who noticed the smell, and several of her students, were loaded into ambulances while the school watched. Later that evening, one hundred people from the school (including a parent who had come to pick up their child) had visited local emergency rooms with similar symptoms.

Thorough investigations by the fire service and the gas company didn't turn up a problem, but when the school reopened on the following Monday several more students had symptoms, ambulances were called, and the school was evacuated again. This time, 71 people ended up in emergency rooms. A large-scale investigation by Government infection and toxin control agencies yielded nothing.

Eventually, the assessment of MPI was made. MPI usually consists of a striking set of non-specific symptoms which are transferred from an 'index case' (in this case, the teacher) to many other people in quick succession. The symptoms (which are also consistent with anxiety) are transmitted via line of sight rather than proximity - people who had little or no theoretical exposure to the 'toxin' developed symptoms after seeing other affected individuals across an open field. It is almost invariably linked with a difficult-to-trace source, like gas, a virus or (in earlier centuries) demonic possession.

MPI is more common in teenagers and young adults, but has occurred in other workplaces as well. The affected people are not mentally ill, or any more prone to mental illness than anybody else. It usually passes in a few hours or days, but some people (particularly those with outstanding litigation or compensation claims) continue to have symptoms.

As with a lot of psychological problems, a clear approach to treating and managing MPI is poorly understood. The symptoms are clearly real - it is just the source of them that is difficult to grasp for affected individuals, and it is seen as demeaning when investigators suggest a psychological diagnosis. What is clear is that the mind works in mysterious ways, and they only become more mysterious when that mind interacts with others.