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Illusion vs Delusion: What’s the Difference? Forensic Psychiatry

Illusion and delusions are some of the psychiatry terms that are often used in defining the mental state of a mentally ill patient. In forensic psychiatry, illusion and delusion are defined as:

Illusion is defined as the false interpretation of the external object or stimulus that exists in reality. On the other hand, delusion is the false film belief in something that is not factful and persists even the belief is proven to falsify.

Illusion Vs Delusion (Table)

DefinitionFalse firm belief in something that is not a fact or existFalse interpretation by senses
StimulusNon-Existent StimulusExternal and explicit  
SituationHappens when a person in the abnormal conditionHappens even in a normal state of mind
Nature of ExperiencePersonalUniversal, experienced by all
ExperienceExperience can’t be identical (differs with people)Experience may be identical
TreatmentRequired proper psychologist treatmentSelf-corrected by the person

Related Definition:

  • Psychiatry is the subject that deals with the study of mental illness, particularly the diagnosis and treatment of mental disorders.
  • Forensic Psychiatry is the application of psychiatry in the administration of justice. It is very much important to understand the mental state of the criminal (with mental illness) to know the cause and motive of the crime since sometimes mental illness can lead to the crime.
  • Mental illness is defined as a disorder of mind or personality in which there is impairment of mental or emotional components. It’s sometimes referred to as “insanity” or “lunacy”.


It is a disturbance of thought and a false belief in something that is not a fact, and exists even if its falsity is clearly demonstrated or proved.

Moreover, a person having delusion is used to prove himself/herself right by reasoning or arguments.

Delusion is seen in the case of schizophrenia. However, not seen in neurotic illnesses like anxiety neurosis or obsessive-compulsive disorder (OCD)

Types of Delusion

There are many types of delusions, some of them are as follows,

1. Delusion of Grandeur: The patients with this image that they are rich and famous wherein he/she is actually poor.

2. Delusion of persecution: Beliefs that other people cause harm to them.

3. Delusion of influence: Beliefs that their actions and thoughts are controlled by other people.

4. Delusion if Infidelity: Beliefs that their husband/wife is unfaithful.

5. Delusion of Self accusation: Keeps blaming themselves for the trivial incidents that happened in the past. Sometimes called to be Delusion of self-reproach or self-criticism

6. Nihilistic delusion: Imagines that nothing exists around him in the real sense of the time.

7. Erotomania: This is usually seen in women, with a belief that an employer or superior is in love with her.

8. Ekbom syndrome (delusions of parasitosis): Person believes that he/she is infested by parasites sick as bugs or insects crawling on or under the skin.

9. Delusion of Poverty:– The person convinced himself that he is or will be bereft of all material possessions.

10. Delusion of reference:– The person believes that he is referred by all agencies, media, and people around him usually for his negative nature.

11. Hypochondriacal delusion: Persistent belief of having one or more serious diseases such as cancer, based on unrealistic interpretation of physical signs and symptoms.

12. Delusion of doubles (doppelganger): Belief that some other person has been physically transformed into themselves.

13. Cotard delusion/syndrome: A person holds a delusional belief that he is dead or, lost his internal organs.

14. Delusion of replacement of significant others (Capgras syndrome): A belief that someone close to him has is replaced by an exact double.

15. Delusion of disguise (Fregoli’s phenomenon): Person identified some strangers as familiar people in his/her life.

16. Folie á Deux: Mental illness shared by two persons, usually their family members. Example: the same delusional thoughts occurs in the family members.


It is a disturbance of perception and a misinterpretation of real external stimulus, e.g., perceiving a rope as a snake.

In other words, it is a false interpretation of a real object.

Types of Illusion

A. Types of Illusion based on Universality

1. Universal Illusion: Also called permanent illusion. It is seen in all individuals and doesn’t change with experience or practice.

For example:

  • Railway tracks appear to be converging to all of us.
  • Sky seems to meet the earth’s surface at the horizon.

2. Personal Illusion: It differs from person to person. For example, a person interprets a rope as a snake.

B. Types of Illusion Based on Senses

The three main types of illusions include:

  1. Optical illusion: In the case of optical illusion, person has a visual world in his/her own mind using either old memory stored in it or the current sight of the material.
  2. Auditory illusion: The auditory illusion is the misinterpretation or misleading of the sound of hearing. It may be possible that the observed sound may be or may not be the physical stimulus. In either case, a person perceives the sound as a relation to the stimulus.
  3. Tactile illusion: Mostly experienced by patients who recently had a surgically cutting off a limb. In these cases, the person actually feels pains in the parts that are cut off from the body.

However, all these illusions are common manifestations of several psychological disorders such as schizophrenia and psychosis, which affect the mental state of the person thus it’s important to understand such mental disorders of the person. 

Delusion Vs Illusion Example of Rope And Snake

A. Illusion: false interpretation of one thing for another thing that exists in reality. For example: a person interprets a rope as a snake, but rope exists in reality. When that person mistakes a rope for a snake, he is filled with fear but as soon as he understands that it was rope, he feels relaxed. So, that person has an illusion.

B. Delusion: False interpretation but a firm belief that persists even after proven false. For example: if the same person accepts rope as a snake, but even after being proven wrong by another person that it is a rope, he persists in his belief and didn’t admit he is wrong. That person is suffering from delusion.

Note: If the same person interprets a snake without any object, it can be said to be hallucinating.


  • Online Articles:
  • Books:
    • Anil Aggrawal’s Forensic Medicine
    • Biswas’s Forensic Medicine
    • Reddy’s Forensic Medicine
Cheguri Preethi- the guest author

A post-graduate student, currently pursuing MSc. Forensic Science from Garden City University, Bengaluru. In her free time, she either invests in browsing the internet or enriches her brain by reading books. [Know More]

Cheguri Preethi

Guest Author

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