What is the difference between 「とは限らない」and 「とも限らない」

Question

When looking it up on the internet I find that とは限らない means 「イメージと違って、100％～と言えないという意味」. But the problem is I also get similar meanings for とも限らない.

I've asked a Japanese speaker about it and they gave the answer below:

「Xとは限らない」は、Xが成立しないことがある、と言う意味。
「Xとも限らない」は、Xが成立しないことがあるし、同類の事柄があることを暗示している。

たとえば、
「お金持ちになれたらいいのに。」
A:「お金持ちが幸せだとは限らないよ。」
B:「お金持ちが幸せだとも限らないよ。（お金があるとトラブルになることもあるから。）」

「早く日本に行って日本語を勉強したい。」
A:「早く日本に行くことが良いとは限らないよ。」
B:「早く日本に行くことが良いとも限らないよ。（いまここでしっかり基礎を学ぶことも大切かもしれないし、 日本で生活するための準備をきちんとすることも重要だよ。）」

Based on the answer he gave me, I still don't get the difference. They both seem the same but I know there has to be a difference.

Answer 1

とはいえない and ともいえない are interchangeable, but the latter sounds a little milder and indirect. This usage of も is described on 明鏡国語辞典 as the twelfth definition:

(12) さりげなくとりたてて、文意をやわらげる。
(ア) 《多く、文末表現に使って》一歩引く気持ちで、その判断をやわらげる。

「自由に発言することも可能だ」「もう帰ってもいいんだぜ」「彼だって素直になることもある」「その件なら知らなくもない」「そんな感じがしないでもない」「乞われれば応じもしよう」「やむをえないと考えもするが…」

一歩引く気持ち (literally "take-a-step-back feeling") is a confusing explanation, but anyway this も adds an extra nuance of admitting, concession, euphemism, etc. Perhaps an English word that has a similar role is an interjection 'well' as in "Well, you can say that, but..." or "They were, well, not quite good." They are perfectly grammatical without 'well', but they sound a little milder, "nuanced" or "deep" with it.

I think the native speaker you asked explained like that because they wanted to say this も has such a "nuance". But this role of も may be difficult to grasp if you try to analyze it as an extension of 'also' too much. In this case, I think this も in とも限らない can be translated as "you know" or "well" (if it needs to be translated).

By the way, this usage of も is explained in the dictionary next to this も. And this question is also loosely related.

Answer 2

From that person's answer, it seems that とも implies other things as well.

I want to hurry and go to Japan to study. The とも version that rushing to japan is not necessarily a good thing, and implies other things like, you should focus on studying Japanese here and now, or prepare to live in Japan"?

It adds implications.