The National Bureau of Economic Research in the US has published this working paper on detecting fake trading in cryptocurrency exchanges. It says that quantifying the wash trading on each unregulated exchange, it averaged over 70% of the reported volume. It also documents how these fabricated volumes (trillions of dollars annually) improve exchange ranking, temporarily distort prices, and relate to exchange characteristics (e.g., age and userbase), market conditions, and regulation.
https://www.nber.org/system/files/working_papers/w30783/w30783.pdf
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