Which is more affirmative: "I think" or "I guess"?

Solution 1:

The difference is a qualitative one, not quantitative describing probability.

It varies by context, but typically, "I think" indicates that you're basing what you're about to say based primarily on knowledge, thought or experiences you had before the current situation, whereas "I guess" indicates that you're doing on-the-spot speculation at that moment. The presence of either word doesn't indicate a level of certainty (this is given by context, including the tone in which it is spoken.)

Example:

A: I need to go home soon. It's 11 PM now, when does the last train leave? B: I think it's at 10 PM. Let me check. Yeah, 10 PM. I guess you have to call a cab.

B uses "I think" because he's recalling information he had before, and uses "I guess" when he's doing an on-the-spot appraisal of the current situation.

Solution 2:

In my experience, North American usage is closer how you describe South Asian usage. I think is much more affirmative than I guess.

More than the difference in affirmativeness, though, is the intent. I think X connotes that the speaker has given X conscious thought and consideration, and according to their best judgment, calculation, or recollection, X is true. It is important to note that someone's best judgment may not be close to 100% confident. 60% or so would be sufficient for I think (so in this way is different than the description of South Asian usage).

I guess is often used in phrases like I guess so or simply I guess, in response to an inquiry. When used this way, it means the speaker doesn't care much or hasn't really given it much thought, but doesn't have any strong reason to disagree.

So, I think stocks are a better investment than bonds indicates that the speaker honestly believes stocks are better than bonds. He may not be sure, but he has analyzed them and he would bet on stocks. I guess stocks are a better investment than bonds indicates that the speaker has either already observed that stocks are better than bonds (so he is quite certain that they are in fact better, but he didn't have to really think about it) or that he has been told that stocks are better, and that is good enough for him.

Finally, phrases like I would guess X or my guess is that X indicate conjecture. The speaker is saying "well, I don't really have sufficient information, but if I have to make a guess, my guess is X".