Does "I'm not sure" always cast a sense of disapproval? [closed]

I've got a feeling that when I say "I'm not sure if X is Y", people often take that as if I were saying "X is (probably) not Y". Is it a reasonable interpretation?

Almost all times I use that phrase to express "I do not know", as the phrase "I'm not sure" appears to me neutral without any indication of posi/negativy (So to avoid the misunderstanding, I end up verbosely saying e.g. "I'm not sure, I simply don't know").

In the comment to https://english.stackexchange.com/a/533308/25295 @Isabel Archer said the following, which aligns with the way I use the phrase in question.:

I'd say that "I am not sure" can also be used in a broader context. For example, someone might ask, "Is today Monday or Tuesday?" and you might answer, "I am not sure." You're not saying you think something is not true; you're just saying that you don't know.

By the way my English experience is largely limited to that of north American.

It has been used as a euphemism for a negative or rejection with enough frequency that if one does not intend to imply unlikelihood, there are many contexts in which another word choice may be clearer. I am not sure, however, whether this is a linguistic or cultural phenomenon. The above "I'm not sure whether/if A or B" isn't one of these contexts because it suggests either A or B is right.

"I'm not sure," all by itself (just the words) does not infer disapproval of any kind, but instead is simply saying you don't know the answer. But it may also invite the follow-up question, "Well, which do you think it is?" and thus possibly putting you on the hook if you supply the incorrect answer.

But going beyond the words themselves, you must also consider body language and inflection (so important in understanding English). "I'm not sure," rarely conveys disapproval, especially when compared to "I don't know," which is highly subject to how the words are delivered. Furrowing your brow and shaking your head while saying "I don't know," without eye contact is dismissive at best.

My response when I don't know is "Good question!" That is a common response from speakers who are taking questions from an audience--it's polite, affirmative, and by itself (without any followup) conveys lack of knowledge.

I’m not sure is not the same as I don’t know, so when you say “I’m not sure, I simply don’t know.” You are making two statements: the first indicates uncertainty – you probably “feel” that one choice is better but that the decision is hard; the second amends the first statement and replaces it with a statement that says you have a complete lack of knowledge about the choice.”

It is better to say either “I’m not sure” or “I simply don’t know.” They carry different nuances:

An example of a physical problem:

A: “Do you think this rope is strong enough to pull the car out of the mud?”

B1: “I’m not sure.” – The nuance here is that B recognises that A has a problem and, by saying “I’m not sure”, B is saying that B has that problem also. (It does not help A, but it makes A feel as if his indecision is not caused by stupidity or an unusual lack of knowledge.)

B2: “I don’t know.” = the nuance here is that B is not being helpful or sympathetic. (In exactly the same way as B1, this does not help A, but it sounds to A as if you are not sympathetic to his problem and are not engaging with him or do not have the knowledge which he expected you to have.)

B3: “I don’t know.” [emphasising “I”] The nuance here is that B is suggesting that he is not the correct person to ask as he has no knowledge of the matter. It is the equivalent of “How should I know? I know nothing about ropes or cars!”

An example of an abstract problem:

A2: “Do you think this dress makes me look fat?”

B4: “I’m not sure.” [said in an uncertain tone] – The nuance here is the same as B1 above.

B5: “I’m not sure.” [said in an positive tone] – The nuance here is that B probably disagrees but wants to hear A’s reasons.

B6: “I don’t know.” [said in a sympathetic tone] The nuance here is that B seems to suddenly recognise that A has a problem, and by saying “I’m not sure”, B is saying that B has just noticed that problem also. It is neutral and indicates that B will engage with A in discussing the merits of the dress.

However, English is a very tonal language, emphasis, de-emphasis, rising and falling tones can all add subtlety to almost any statement.

In my opinion, "I'm not sure" implies that you've done some research on it, but it remains ambiguous, and you can not confirm with supportive or conclusive evidence one way or another. "I don't know," "I have no idea" implies that one does not have any previous knowledge on the subject nor has engaged in any research on the matter...or one just might not care to engage on the topic.