system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is the Altaic language family now considered "widely discredited?"

I know how to speak both Korean and Japanese, and can't help but notice how similar the grammatical structure is between the two language. However, on Wikipedia, both the Korean and Japanese language families are considered isolated languages. Why is this so?
gpt
Just because two languages appear superficially similar doesn't mean they're related. In particular, they may have come from completely different roots, but then later they influenced each other.

If you want to put, for example, Japanese and Korean in the same language family, you have to be able to demonstrate that they must have descended from the same protolanguage -- so it should be possible to partly reconstruct a hypothetical Proto-Altaic language. Then you should be able to detect regular sound correspondances -- that is, you should be able to show that a particular sound from Proto-Altaic changed in one way to make Japanese, and changed in a different way to make Korean.

To pick an example from our own language family, Indo-European: the (hypothetical) Proto-Indo-European language had a consonant that was pronounced like a "k", but with rounded lips. This changed to "qu" in Latin-based languages, but to "w" or "wh" (originally "hw") in Germanic languages, and "k" in Slavic languages. That explains why interrogatives nearly all begin with "qu" in French ("qui", "quel", "quand"), "w" in German ("wer", "welcher", "wann"), "wh" in English ("who", "which", "when") and "k" in Russian ("kto", "kotory", "kogda").

The problem is that this isn't very easy to do with Proto-Altaic, one of the reasons so many linguists now think there never was such a protolanguage.

Most damaging of all, though, is the question of morphology -- how words change depending on context (plural markers, case markers, that kind of thing). But many of the proposed relationships between the different languages have very little actual evidence to support them, and in some cases are even demonstrably false (for example, there is supposedly a relationship between the Turkish instrumental and the Japanese locative, but that almost certainly can't be because the instrumental and the locative mean completely different things).

It's important to remember that "widely discredited" doesn't mean "nobody believes it any more". It means "there is an argument about it": basically, the jury is still very much out. There is simply so little evidence to draw on, you can argue almost anything you like.