Expectation is often the cause of believing in the experience of seeing a ghost. When you expect a haunted place your brain will scan actively to see any threat. Such normalization of stimuli as shadows or sounds makes them appear paranormal. Your mind utilizes gaps by relying on pre-existing fears or hopes.
One of the most important factors is called pareidolia. It is the perception of faces among other things by the brain in random stimuli. This may happen where shapes or noises are wrongly misinterpreted in poor lighting as apparitions or voices. This internal drive towards finding patterns generates meaning where there is no meaning.
Sleep disturbances play a huge role. Experience of sleep paralysis is accompanied by extreme fear and lucid hallucinations in a conscious yet immobilised state. The semi-conscious conditions such as falling asleep (hypnagogic) and waking up (hypnopompic) distort reality. The feelings of hallucinations in such conditions are so real and can be referred simply as a ghost.
Interpretation depends so much on suggestibility and context. If one has heard ghost stories prior to the events, the probability to explain the occurrences by the supernatural is higher. Fear is amplified by being alone, when it is dark or one is in what is considered a haunted place. This anxious behavior renders ordinary events as being paranormal. Shared misinterpretations can be supported by group dynamics.
Strong feelings also impair the judgement. Fear increases the awareness, as well as makes some errors. Deep mourning may develop a dejected desire to reach out to the dead. This emotional need makes individuals to misinterpret random things as contact thus feeling a haunting presence.
Conclusion
An experience of ghosts is usually attributable to something in the mind leading to people experiencing something as a ghost. The factors of expectation, pareidolia, sleep problems, being more suggestible and experiencing powerful emotions come to play in a particular context to produce amazingly convincing illusions. The realization of these aspects gives a logical reason on why such experiences occur and proves that it is not a resultant effect of abnormal brain functioning.
