We often make split second decisions about people we meet in various situations. Those are character evaluations.  For example, “The person on the talent show is clearly deluded about their lack of singing ability.” “That politician has delusions of grandeur.” “She's deluded if she expects to be promoted.” "You have to be deluded to believe anything salesmen say.”

A delusion is a belief held by an individual or group that is demonstrably false, patently untrue, impossible, fanciful, or self-deceptive. A person with delusions, however, often has complete certainty and conviction about their delusory beliefs. They resist arguments and evidence that they are wrong.

People have illusions about smells (olfactory), taste (gustatory), temperature (thermoceptive), and touch (tactile). They may experience highly disgusting or very pleasant or unusual smells when meeting a particular person. They may find ordinary foods (oranges, chocolate, milk) have different tastes than others experience. They may find cool objects burning hot or warm objects frozen; traditionally smooth objects (like a balloon or cat's fur) may feel rough or uneven.

The most written about of all delusions, paranoia, has been shown to follow various stages: general suspiciousness; selective perception of others; hostility; paranoid “illumination” in which all things fall into place; and, finally, paradoxical delusions of influence and persecution. Delusions often totally preoccupy people and cause them considerable distress because they do not doubt their beliefs are correct.

Delusions differ from illusions. We have visionary and auditory illusions; for instance, that the sun goes around the earth or that ventriloquists’ dummies actually speak. We have selective memories /illusions of happy childhoods. These are things that seem true to the senses or memory, but are known to be false or have no basis in reality.

There are some caveats: Some religious  delusions are impossible to verify and hence falsify. Other delusions have a self-fulfilling prophecy, such as a jealous  person accusing and attacking an innocent partner, who then leaves them for another. In that sense, these people cause their delusions to come true.

 An emotional charge, like jealous, can tap a person's brain process and imagination to create false presumptions and assumptions that feed fear and paranoia. If a girlfriend does not return a message, an insecure boyfriend could begin the dark road of self-esteem hits to his own ego: maybe she no longer likes me?, who is she with?, is she out with another man?, where is she? is she having a good time without me? This self doubt has a cascade effect which could lead to the irrational boyfriend to lash out at his innocent girlfriend who was merely too busy at work to respond to his messages. So his fear about losing her is a self-induced reality caused by his delusions about an event which he lacks sufficient information to make a rational decision. The idea of "self-fulfilling prophecy" comes to mind.

Psychiatrists may diagnose someone as having a delusion disorder under a number of very specific situations:
  1. A person must manifest one or more non-bizarre delusions for at least a month.
  1. The person has not met other behavioral criteria to be classified as someone with schizophrenia.
  1. Audio and visual hallucinations are not prominent, though tactile and olfactory hallucinations may be.
  1. Despite the person's delusions or their behavioral consequences, their psychosocial functioning is not essentially impaired enough to be considered particularly odd or bizarre.
  1. If the specific delusions impact a person’s mood, these fluctuations do not last very long.
  1. The disturbance is not the result of physiological or medical conditions, like the medication or drugs a person is taking.
Sometimes psychiatrists say it is difficult to distinguish from other disorders like hypochondriasis (particularly among those with little self-awareness); body dysmorphic disorder (preoccupation with imagined bodily defects); Obsessive Compulsive as well as Paranoid Personality Disorder. 

The delusions of people with schizophrenia are often clearly bizarre, utterly implausible, not at all understandable; one might believe the brain  has been replaced by that of another person or that one has shrunk to be three feet tall. On the other hand, non-bizarre delusions could be possible. For instance, people may feel they are being followed, photographed or recorded, that somebody is slowly poisoning them, that their partner is always cheating onthem, or that their boss or neighbor is in love with them. A person can easily project negatives onto any situation because their minds are free to make blind speculations about the world around them. A person with low esteem or is self-centered can channel these negatives into a web of "the world is against me" personality traits. Or that "I am cursed because I am unlucky at everything."

Some delusions cause people to make dramatic changes in their life: leave their job or partner, move from their house (or even leave the country), or dress very differently. The person with delusional disorder, however, appears normal when their delusional ideas are not being discussed.
People with delusions can become very moody, often causing their relationships and work to suffer. Interestingly, some cultures and groups have particular beliefs that may in other cultures be seen as clinically delusional.

It is a relatively rare disorder usually occurring later in life, particularly among people with relatives who have other disorders. Most appear argumentative and hypersensitive.  Many do not seek treatment and become, over the years, more and more isolated.

 Psychiatrists have noticed five clear types of delusions:
  1. Erotomanic. These individuals believe someone is seriously in love with them, more in the Hollywood romance, even spiritual way, rather than in the sexual sense. It is often a famous person—a film star or famous athlete—but also can be powerful superiors at work. Whilst someone with this delusion can keep it a secret and do very little, others may expend a great deal of energy trying to contact their delusional lover via emails, visits, or stalking. Most are women, but men with the delusions tend to act more boldly and get in more trouble with the law, particularly if they believe their ‘lover’ is in trouble or imminent danger.
  1. Grandiose. These are sometimes called delusions of grandeur and manifest when a person believes (with no evidence) that they are special: they have amazing abilities or have made a vitally important discovery. Often the delusions are religious for those with the disorder, often believing that they have a unique and privileged relationship with the "The Almighty." Sometimes they feel they are a prominent person and have special relationships with other prominent people.
  1. Jealous. This is clearly manifested in the strong, but unfounded belief that a partner is unfaithful and cheating on them: Odd bits of "evidence" are put forward for these claims. They may hire a private detective, attempt to imprison their partner as well as physically and verbally attack them.
  1. Persecutory. This is the belief that someone or some group is conspiring against them. They could be cheating, spying on, harassing, or gossiping about them, or even attempting to poison or drug them. They are often angry and resentful with deep feelings of injustice. Many attempt to quell the persecution by legal means or appealing to authorities. It is the most common type of all the delusionary disorders. Some even get violent and aggressive towards those they believe are deliberately targeting them.
  1. Somatic. This is the delusion that one’s body is somehow strange or not functioning properly. It may be the belief that one smells odd, or that particular parts (nose, breasts, feet) are particularly odd, misshapen or ugly. Often people with these delusions believe they may have some internal bug or parasite that is destroying or affecting some very specific part of their body.
The causes of delusions are unknown. Current interests in neuropsychology have lead some to speculate that malfunctioning biological features may cause or exacerbate the problem. Some have implicated basal ganglia, others the limbic system and still others the neocortex. Investigations continue.

For others, genetic explanations are best because so many with delusional disorders have first-degree relatives with these and related disorders.Other researchers point out that many with the disorder have had difficult childhoods characterized by instability and turbulence, callousness and coldness. They consider delusions to be an impairment in the ego defense system aimed to protect and bolster the self. They see the paranoid or persecutory delusions as an attempt to project onto others things they do not like to admit in themselves.

People often lie, fake, or deceive, even to themselves.  Psychologists call this dissimulation, but have recently distinguished between two very different types of dissimulation:
  • Impression management. This is all about presenting oneself in a positive light, perhaps forgetting certain things and sharing small “white lies” about another.
  •  Self-deception. Strictly speaking, this is not lying but is more like a delusion. If someone says they have a sense of humor, but everyone who knows them says they do not, they are deceiving themselves. Similarly, when someone feels ugly or plain whilst everyone else (friend, acquaintance, stranger) believes they are not, it implies a negative self-deception. At interviews, some forms of self-deception begin to get close to delusions.
There entire main cast of LOST had clear indications of delusional characteristics. From "white lies" about one's self or accomplishments, to bold face grandeur Napoleon complexes, LOST is a clinical study of how various diverse egos still have a common thread of mental imbalance, even in seemingly normal, happy people.