Folie à deux, shared psychosis,^[2] or shared delusional disorder is a psychiatric syndrome in which symptoms of a delusional belief and sometimes hallucinations^[3][4] are transmitted from one individual to another.^[5] The same syndrome shared by more than two people may be called folie à trois, folie à quatre, folie en famille ("family madness"), or even folie à plusieurs ("madness of several").

Recent psychiatric classifications refer to the syndrome as shared psychotic disorder (DSM-IV – 297.3) and induced delusional disorder (F24) in the ICD-10, although the research literature largely uses the original name. This disorder is not in the current DSM (DSM-5). The disorder was first conceptualized in 19th-century French psychiatry by Charles Lasègue and Jean-Pierre Falret and is also known as Lasègue-Falret syndrome.^[3][6] The term is from French for "madness of two".

This syndrome is most commonly diagnosed when the two or more individuals concerned live in proximity and may be socially or physically isolated and have little interaction with other people. Various sub-classifications of folie à deux have been proposed to describe how the delusional belief comes to be held by more than one person:

Folie à deux and its more populous cousins are in many ways a psychiatric curiosity. The current Diagnostic and Statistical Manual of Mental Disorders states that a person cannot be diagnosed as being delusional if the belief in question is one "ordinarily accepted by other members of the person's culture or subculture" (see entry for delusion). It is not clear at what point a belief considered to be delusional escapes from the folie à... diagnostic category and becomes legitimate because of the number of people holding it. When a large number of people may come to believe obviously false and potentially distressing things based purely on hearsay, these beliefs are not considered to be clinical delusions by the psychiatric profession and are labelled instead as mass hysteria.

As with most psychological disorders, the extent and type of delusion varies, however it usually mimics the delusion of the inducer and is almost very similar to it.^[8] The inducer does not realize that they are making the other person sick but instead think they are helping by alerting the second person of what they deem to be "truth".

Psychology Today magazine defines delusions as "fixed beliefs that do not change, even when a person is presented with conflicting evidence."^[9] There are 4 main types of delusions that are passed on from an inducer to a secondary person: bizarre delusions, non-bizarre delusions, mood-congruent delusions and mood-neutral delusions.

As with most, if not all personality disorders, shared delusional disorder affects the psychological and social aspects of the person's life. Biologically, since most delusions are accompanied by fear and paranoia, they increase the amount of stress experienced. For example, if Lisa believes that radiation from the sun is poisoning her food she is going to be extremely stressed and fearful for her life. Increased stress will negatively affect her physical health because stress increases blood pressure, heart rate and breathing rates which put her at risk of developing cardiovascular disease. Additionally, she will be at an increased risk of developing diabetes, becoming obese and the functioning of her immune system will be lessened as a result of stress.^[12] These health risks increase with the severity of the disease, especially if the condition is left untreated and becomes chronic, leading to the development of anxiety and depression.

In fact, delusional disorder puts those affected with it at a heightened risk of developing other psychological disorder such as anxiety and depression. This is because 55% of people with shared delusional disorder are genetically predisposed to psychological disorders like bipolar disorder, schizophrenia, anxiety, and depression...etc. This predisposition is usually triggered by the heightened state of fear, worry, and hopelessness experienced by those with SDD through increased levels of cortisol and therefore dopamine levels.^[13] Since shared delusional disorder itself is a very frightening and stressful disorder to live with, adding anxiety which is characterized by nervousness, worry, fear and apprehension ^[14] and depression, a state of despondency and dejection makes their life impossibly difficult.

The most obvious effect of shared personality disorder is probably the isolation from society. If Chris believes that the FBI is stalking him or that there are people trying to hurt him, he is going to disassociate from society and stop talking to most people, especially if he senses any hostility or lack of belief in this delusion. Since shared delusional disorder usually occurs in those who are socially isolated, further isolation will only make the disorder worse as it will pull the secondary person closer to the inducer and away from anyone trying to help them.

No-one knows what causes SDD exactly but stress and social isolation are the main contributors.^[15] When we are socially isolated the few people we do talk to become very important to us, and therefore they are seen as more trustworthy, so when an inducer is sharing their delusions, the second person is more likely to believe them. Additionally, since they are socially isolated people developing shared delusional disorder do not have others reminding them that their delusions are either impossible or not likely and are therefore more likely to develop SDD. In fact, the treatment for shared delusional disorder is for the person to be removed for the inducer and seek additional treatment if necessary.^[16]

Stress is also a factor because it triggers mental illness. The majority of people that develop shared delusional disorder are genetically predisposed to mental illness; however this predisposition ( i.e. genes for schizophrenia that need to be activated) is not enough to develop a mental disorder. However, when that person becomes stressed their adrenal gland releases the stress hormone cortisol into the body which released increased levels of dopamine in their brain and changes in dopamine levels are linked to mental illness.^[13] As a result, stress puts one at a heightened risk of developing a psychological disorder such as shared delusional disorder.

Shared delusional disorder is hard to diagnose because usually, the afflicted person does not seek out treatment because they do not realize that their delusion is abnormal as it comes from someone in a dominant position who they trust. Furthermore, since their delusion comes on gradually and grows in strength over time, their doubt is slowly weakened during this time. Shared delusional disorder is diagnosed using the DSM-5 and according to this the person afflicted must meet three criteria:

Reports have stated that a phenomenon similar to folie à deux was induced by the military incapacitating agent BZ in the late 1960s.^[17][18]

After a person has been diagnosed, the next step is to determine the proper course of treatment. The first step is to separate the formerly healthy person from the inducer and see if the delusion goes away or lessens over time.^[16] If this is not enough to stop the delusions there are two possible courses of action: Medication or therapy which is then broken down into personal therapy and/or family therapy.

Family therapy is a technique in which the entire family comes into therapy together to work on their relationships and to find ways to eliminate the delusion within the family dynamic. For example, if someone's sister is the inducer the family will have to get involved to ensure the two stay apart and to sort out how the family dynamic will work around that. The more support a patient has the more likely they are to recover, especially since SDD usually occurs because of social isolation.^[21]

With treatment, the delusions and therefore the disease will eventually lessen so much so that it will practically disappear in most cases. However left untreated it can become chronic and lead to anxiety, depression, aggressive behavior and further social isolation. Additionally Unfortunately there are not many statistics about the prognosis of shared delusional disorder as it is a rare disease and it is expected that the majority of cases go unreported; however, with treatment, the prognosis is very good.

Shared delusional disorder is most commonly found in women with slightly above-average IQs who are isolated from their family, and are in relationships with a dominant person who has delusions. The majority of secondary cases (people who develop the shared delusion) also meet the criteria for Dependent Personality Disorder ^[48] which is characterized by a pervasive fear that leads them to need constant reassurance, support and guidance.^[22] Additionally, 55% of secondary cases had a relative with a psychological disorder that included delusions and, as a result, the secondary cases are usually susceptible to mental illness. The disorder can also occur in cults to a serious extent; one example is the case of the Heaven's Gate Cult, a UFO religious militarism cult led by Marshall Applewhite, who had delusions about extraterrestrial life. The members of the cult developed the same delusion, and went on to commit suicide with the intention of their spirits joining an extraterrestrial spacecraft heading towards a comet.^[23]

In May 2008, in the case of twin sisters Ursula and Sabina Eriksson,^[24] Ursula ran into the path of an oncoming articulated lorry, sustaining severe injuries. Sabina then immediately duplicated her twin's actions by stepping into the path of an oncoming car; both sisters survived the incident with severe but non-life-threatening injuries. It was later claimed that Sabina Eriksson was a 'secondary' sufferer of folie à deux, influenced by the presence or perceived presence of her twin sister, Ursula – the 'primary'. Sabina later told an officer at the police station, "We say in Sweden that an accident rarely comes alone. Usually at least one more follows – maybe two."^[25] However, upon her release from hospital, Sabina behaved erratically before stabbing a man to death.^[26][27][28]

Another case involved Margaret and her husband Michael, both aged 34 years, who were discovered to be suffering from folie à deux when they were both found to be sharing similar persecutory delusions. They believed that certain persons were entering their house, spreading dust and fluff and "wearing down their shoes". Both had, in addition, other symptoms supporting a diagnosis of emotional contagion, which could be made independently in either case.^[29]

The psychiatrist Dr Reginald Medlicott published an article about the Parker-Hulme murder case called “Paranoia of the Exalted Type in a Setting of Folie a Deux - A Study of Two Adolescent Homicides”, arguing that the intense relationship and shared fantasy world of the two teenaged friends reinforced and exacerbated the mental illness that led to the murder: “each acted on the other as a resonator increasing the pitch of their narcissism.”^[30]

In 2016, a case involving a family of five from Melbourne, Australia made headlines when they abruptly fled their home and travelled more than 1,600 km (1,000 mi) across the state of Victoria because some of the family had become convinced someone was out to kill and rob them. No such evidence was found by the police, and the symptoms of those involved resolved on their own once the family returned to their home.^[31]

The book Bad Blood: Secrets and Lies in a Silicon Valley Startup suggests that this ailment plagued the founder of Theranos, Elizabeth Holmes, and her boyfriend/business partner Ramesh Balwani.

It was suspected a family of eleven members from Burari, India suffered from this condition.^[32][33] On June 30, 2018, the family committed suicide due to the shared belief of one of its members.^[34]