Postdiction [pohst-dik-shuhn], also known as retroactive clairvoyance, is an effect of hindsight bias (the tendency to perceive events that have already occurred as having been more predictable than they actually were) that explains claimed predictions of significant events, such as plane crashes and natural disasters. In religious contexts it is frequently referred to by the Latin term ‘vaticinium ex eventu,’ or ‘foretelling after the event.’

Through this term, skeptics postulate that many biblical prophecies (and similar prophecies in other religions) appearing to have come true may have been written after the events supposedly predicted, or that their text or interpretation may have been modified after the event to fit the facts as they occurred.

Skeptics of premonition use these terms in response to claims made by psychics, astrologers and other paranormalists to have predicted an event, when the original prediction was vague, catch-all, or otherwise non-obvious. Most predictions from such figures as Nostradamus and James Van Praagh are written with such seemingly deliberate vagueness and ambiguity as to make interpretation nearly impossible before the event, rendering them useless as predictive tools. After the event has occurred, however, details are shoehorned into the prediction by the psychics or their supporters using selective thinking — emphasize the ‘hits,’ ignore the ‘misses’ — in order to lend credence to the prophecy and give the impression of an accurate ‘prediction.’ Inaccurate predictions are omitted.

Accusations of postdiction might be applicable if the prediction was: Vague (e.g. a disaster is predicted without details as to type or location), Open ended (with a very long cut-off date or none at all and therefore runs indefinitely), Recycled (reused again and again in order to match the most recent event), a Catch-all (covers more than one possible outcome; e.g. ‘If you attack you will destroy a mighty empire’), Shotgunned (one of a number of similar predictions all made concurrently to claim credit if even one of them happens), Statistically likely, Unfalsifiable, Unavailable until after the fact (e.g. psychic Tamara Rand, who ‘predicted’ that Ronald Reagan was in danger of someone with the initials ‘J.H.’ …the day after the assassination attempt), or Cherry picked (counting the hits and not the misses).

Postdiction often resorts to tenuous allegorical explanations to turn literal misses into hits. For example the postdiction might explain that a famous person has suffered a ‘spiritual’ death to explain why they are still walking around despite a prediction that says otherwise. Another problem is ‘moving the goalposts,’ where an event is shoehorned to fit the prediction because it differs in some significant way. For example, the prediction predicts an earthquake on one day when in fact it happens on a different day.

In the field of cognitive science, postdiction is defined as a the justification process that allows a reader to make sense of a concept in a given context. The term was coined by psychologist Walter Kintsch in 1980 and refined by cognitive scientist Afzal Upal in 2005. ‘Heath & Heath’ used Upal’s definition without explicitly citing him in their 2007 book ‘Made to Stick.’ Concepts that can be justified in a given context are called postdictable. In the field of neuroscience, postdiction has a slightly different meaning: here it indicates that the brain collects up information after an event before it retrospectively decides what happened at the time of the event. Some perceptual illusions in which the brain mistakenly perceives the location of moving stimuli may involve postdiction.

Postdiction is sometimes confused with ‘retrodiction,’ the act of making a scientific ‘prediction’ about the past, such is in the field of archeology.

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