Copied from: “Why I am not a Christian” by Keith M. Parsons
The actual occurrence of the Resurrection is not the best explanation for the fact it was believed. We have by now a very thorough understanding of how belief in paranormal phenomena can become firmly entrenched even though no such phenomena occurred. The article “Hallucinations” in the Encyclopedia of Psychology, 2nd ed., says that 1/8 to 2/3 of the normal population experiences waking hallucinations (Hall, 1994, pp. 101-102). Causes of hallucinations in normal persons include social isolation, rejection, and severe reactive depression. The disciples were very likely to be experiencing a strong sense of rejection, isolation, and depression after the execution of Jesus. It is not at all unlikely that more than one of them experienced vivid hallucinations of Jesus.
People greatly underestimate the influence of hallucinations on history:
Recent studies of perception and memory and the psychology of anomalous experience show that miracle reports are very likely in many circumstances where no miracle has occurred. Human perception is not passive; we are not biological camcorders. Rather, perception is complex, much prone to error, and easily influenced by background beliefs and expectations. In other words, we often “see” what we expect to see, want to see, or fervently hope to see rather than what is actually there. Predispositions bias our observations—this is one of the best-established principles of psychology. We all know that second or third hand accounts are unreliable—mere “hearsay evidence.” But any good trial lawyer or stage magician will tell you just how fallible eyewitnesses can be.
Likewise, psychologists have shown that memory is not a passive recorder but highly susceptible to influence by circumstance, bias, and suggestion. We also know that sane, rational people sometimes suffer extraordinary delusions that convince them that they have been abducted by aliens, seen ghosts, or witnessed miracles. It follows that even an honest, educated, and rational person who claims to have seen a miracle can easily have misperceived or misremembered.