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This is an excerpt from the English-language Mainichi Shimbun:

About 70 percent of Japanese working women say they're suffering from a lack of sleep, with around 80 percent saying insufficient shuteye affects their looks, a survey by skincare company Nagase and Co. has showed.

When I was a student, I learned that "show" becomes "shown" after has/had. A Google search showed that quite a few English-language Web pages use this--not only by private individuals but by organizations who should be proofreading text that they publish.

It is usage that I don't know? Or, it is just incorrect?

Thank you in advance for any information on this. Emotion: smile
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Language changes. Over the centuries, all language is subject to a levelling [leveling] process. The Americans made sweeping changes to BrE in spelling, vocabulary and even in some structural aspects. {BrE is now being hevily influenced by some of these structural changes}

The long and the short of this, Epigene, is that 'showed' is an acceptable past participle alternative to 'shown'. But don't just take my word for it.

M-W ONLINE:
Main Entry: show
Function: verb
Inflected Form(s): showed /'shOd/; shown /'shOn/; or showed; show·ing

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Compact Oxford English Dictionary

show

• verb (past part. shown or showed)

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AHD:

show: Inflected forms: showed, shown or showed, show·ing, shows
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I think it's quite a long debate. Emotion: smile In the end, it all boils down to how frequently
something is used by speakers. If many people start using new forms, then after a decade or so, expressions that didn't sound natural at first now do sound natural. Dictionaries will adopt it and then
it will be correct usage; and that's how a new grammar rule is born.

It's quite interesting though because this topic borders on the discussion about whether we should let grammar determine our language, or that we should let our language be subject to changes without any restrictions and let common speech completely determine our language. Too much of either can never be good I suppose. Language needs change, but it needs rules as well.

(just for the record, I think 'shown' sounds better) Emotion: smile (but that may simply be because I learnt/ed it like that)Emotion: wink
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Comments  
It just sounds better to say, has shown, not has showed.
One American expert says that "shown" is better than "showed." Furthermore, he points out that you have to use "shown" in the passive: a good movie was shown.
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Any look at a concordance of Shakespeare shows numerous instances of "showed", and also that this is in origin British, and originally as acceptable as "shown."
 dokterjokkebrok's reply was promoted to an answer.