by K. Hansen, Feb 29, 2022 in WUWT

What exactly is the decline effect?  Is it the fact that certain scientifically discovered effects decline over time the more they are studied and researched? Almost, but not really.  The Wiki has this definition for us:

“The decline effect may occur when scientific claims receive decreasing support over time. The term was first described by parapsychologist Joseph Banks Rhine in the 1930s to describe the disappearing of extrasensory perception (ESP) of psychic experiments conducted by Rhine over the course of study or time. In its more general term, Cronbach, in his review article of science “Beyond the two disciplines of scientific psychology” [ also .pdf here ] referred to the phenomenon as “generalizations decay.”^[1] The term was once again used in a 2010 article by Jonah Lehrer published in The New Yorker.”

Some hold that the decline effect is not just a decrease of support over time but rather that it refers to a decrease in effect size over time – or, according to some, both because of one or the other.  That is, the support decreases because the effect sizes found decrease, or, because of decreasing support, reported effect sizes decrease.  The oft cited cause of the decline effect are: publication bias, citation bias, methodological bias, and investigator effects.  Part 1 of this series was an example of investigator effects.

Let’s be perfectly clear:  In no case does the decline effect refer to an actual decline in real world effects of some physical phenomena, but only to effect sizes found and/or reported in research reports over time.

One of the best discussions of the decline effect was published in The New Yorker over a decade ago.  In an article titled: “The Truth Wears Off — Is there something wrong with the scientific method?” by Jonah Lehrer.  At 2100 words, it is about a 10 minute read – and worth every minute.

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