© Image credit: Elena KalisAlice-in-Wonderland syndrome, thought insertion, clinical lycanthropy, Paris syndrome and more…
But they all have one thing in common: being detached from reality. Delusions do not listen to reason and they do not bow to facts.
Here are twelve of the strangest delusional beliefs...
1. Alice-in-Wonderland syndrome
Named after the novel by Lewis Carroll, this delusion affects perceptions of both space and time.
The sufferer may see some objects as smaller than they really are and others as bigger than they really are. They may also find it difficult to judge time.
A relatively common delusion, not necessarily associated with mental illness, people sometimes report experiencing this as children or even just before falling asleep.
Often, though, the delusion is caused by migraines, which Lewis Carroll suffered from and may have used as inspiration for the story.
2. Cotard's syndrome
Also known as 'Walking Corpse Syndrome', Cotard's is when the sufferer believes he or she is dead, does not exist or has lost their internal organs.
Typically those suffering from Cotard's will deny they exist; naturally, this means they find it very difficult to make sense of reality.
People with Cotard's become very withdrawn from others and tend not to look after themselves properly.
The delusion is often found amongst people suffering from schizophrenia.
3. Capgras delusion
Thought to be neurologically similar to Cotard's, the Capgras delusion is the belief that a person who is close to the sufferer has been replaced by an imposter who appears identical, but isn't the same person.
The delusion is named after French physician Joseph Capgras who first described it.
Capgras is often associated with schizophrenia but the delusion may also result from brain damage and dementia.
4. Folie à deux
Winner of my special award for most attractively named delusion, Folie à deux literally means 'madness shared by two'.
So much better than the technical name of 'shared psychotic disorder'.
It's when two (or more) people who (usually) live in close proximity come to share the same delusion(s).
5. Thought insertion
Thought insertion is the delusion that the sufferer's thoughts are not their own.
The person will sometimes think they are coming from another specific person and sometimes they won't know where they are coming from.
The delusion of thought insertion is often a symptom of schizophrenia.
6. Paris syndrome
Paris syndrome is a transient experience that affects tourists to Paris who find that the City of Light does not live up to their expectations.
They may experience hallucinations, delusions of persecution, anxiety and other somatic symptoms.
Paris syndrome may sound like a joke, but around twenty Japanese tourists a year are thought to be hospitalised with it.
Some think it is brought on by culture shock, as the Japanese have a particularly idealised view of Paris.
The usual treatment for Paris syndrome is to go home.
7. Jerusalem syndrome
Paris does not have the monopoly on causing visitors to be struck down by mental illness.
Some visitors to Jerusalem can become obsessed with the city after arriving.
Those experiencing the syndrome may suffer from anxiety, start wearing a toga, begin singing hymns or shouting out verses from the Bible.
Some even begin giving poorly practised sermons in public.
Estimates place the number of people who require hospital admission from this syndrome at around 40 per year.
As for Paris syndrome, the normal treatment is to go home.
8. Othello syndrome
This is the belief that the sufferer's partner is cheating on them, despite there being no evidence whatsoever.
It's much more than just common-or-garden jealousy, though, with sufferers experiencing strong obsessive thoughts.
They may continuously check up on their partner, stalk them, interrogate them about where they've been and, in extreme cases, it can lead to violence.
9. Ekbom's syndrome
Ekbom's syndrome is the stuff of nightmares.
In this particularly nasty form of hypochondria, the sufferer thinks their body has been infested by parasites.
It is not uncommon for sufferers to contact pest control specialists or dermatologists rather than psychologists or psychiatrists.
It is named after Swedish neurologist Karl Axel Ekbom, who wrote about it in the 1930s.
10. Clinical lycanthropy
Clinical lycanthropy is the belief that the person has, or is in the process of, turning into an animal.
And it's not just wolves that people believe they are turning into: published cases of clinical lycanthropy include people believing they were turning into frogs, cats, horses, birds, hyenas and even bees.
The condition is very rare.
11. Reduplicative paramnesia
This condition is usually caused by brain damage and results in the person believing a place or location has been duplicated and/or moved somewhere else.
Soldiers with head injuries have been reported to believe that the hospital in which they are recuperating is actually in their home town, when in fact it is nowhere near.
It shares some features with Cotard's and Capgras in that people, places or things have been replaced or are somehow transformed.
12. Subjective doubles syndrome
This is where someone genuinely believes that they have a doppelgänger who looks like them, but has a different personality and a different life.
Sometimes the doppelgänger can be a stranger, sometimes it can be a family member.
In some cases sufferers can become enraged by someone they perceive has having stolen their appearance, and this can lead to psychological or physical attacks.
The syndrome is most commonly seen in people with bipolar disorder or schizophrenia, although it is very rare.
They missed out true dopplegangerism (which is related to true lycanthropy) so I guess we're not deluded......