A recent paper attempts to link Japanese and Korean using reconstructed words common to both languages. Similar attempts have been made in the past, always with some objections. This is by Alexander Francis-Ratte (now at Furman University):
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[https://etd.ohiolink.edu/apexprod/rws\_olink/r/1501/10?clear=10&p10\_accession\_num=osu1460644060](https://etd.ohiolink.edu/apexprod/rws_olink/r/1501/10?clear=10&p10_accession_num=osu1460644060)
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[https://etd.ohiolink.edu/apexprod/rws\_etd/send\_file/send?accession=osu1460644060&disposition=inline](https://etd.ohiolink.edu/apexprod/rws_etd/send_file/send?accession=osu1460644060&disposition=inline)
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Historical connections might provide evidence if Japanese and the extinct Goguryeo language were also related. It was spoken in the area including modern Korea, and many cognates seem to exist. An overview in:
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[https://en.wikipedia.org/wiki/Goguryeo\_language](https://en.wikipedia.org/wiki/Goguryeo_language)
3 comments
Even if this is true, The Japanese and Korean governments would never admit to it, and would likely actively suppress such a correlation. The similarity between the two languages is undeniable however.
You’d probably be better off asking about this on r/linguistics
Just skimming the publication, its very clear that Francis-Ratte still uses many of the pitfalls in Koreo-Japonic comparisons, namely, heavily relying on “cognates” only found in Western Old Japanese that are not found in especially Ryukyuan (see how many cognates only have Western Old Japanese comparada) and plenty of ad-hoc segmentation without language-internal justification such as arbitrarily segmentinh Korean *noc-* and *nwoph-* and *noli-* based on “similar words in Japanese”
One main problem is that we can find many Korean-Japanese similarities and many Japanese-Ryukyuan similarities but not much Korean-Ryukyuan similarities where we see Korean/Ryukyuan retention that Japanese innovated
Moreover, we can clearly see that Japanese shows strong convergence with Korean as the centuries moved on with many supposed similarities being post Old Japanese innovations
Regarding Goguryeo evidence, it is important to note that all the supposed Japonic place names come from Centra Korea where it is believed many Japonic speakers once lived. If we look towards more Northern area, ie the core Goguryeo area, we find much more actually Koreanic featuresn