Evidence is demanded for communicating the change in preferred decision.

You mean status quo bias, like the argument against the Many-Worlds interpretation?

If a guy on the forum says, to my disbelief, that stopping eating the cookies in my particular situation will actually make me even more overweight, I won't be able to change my mind as a result of hearing his assertion.

It's funny that you mention this, because I actually know of an author that says something just similar enough to that idea that you could maybe confuse what she says as meaning you should eat the cookies.

Specifically, she posits a mechanism which causes some people to eat compulsively when they believe they will not have enough food in the future, regardless of whether they're hungry now. She actually encourages these people to keep stores of indulgence foods available in all places at all times, in order to produce a feeling of security that negates their compulsion to eat now -- in effect, they can literally procrastinate on overeating, because they could now do it "any time". There's no particular moment at which they need to eat up because they're about to be out of reach of food.

I bring this up because, if you heard this theory, and then misinterpreted it as meaning you should eat the cookies, then it would be quite logical for you to be quite skeptical, since it doesn't match your experience.

However, if you simply observed your past experience of overeating and found a correlation between times when you ate cookies and a pending separation from food (e.g. when being about to go into a long meeting), I would be very disappointed for your rationality if you then chose NOT to try bringing the cookies into the meeting with you, or hiding a stash in the bathroom that you could excuse yourself for a moment to get, or even just focusing on having some right there when you get out of the meeting.

And yes, this metaphor is saying that if you think you need studies to validate things that you can observe first in your own past experience, and then test in your present, then you've definitely misunderstood something I've said.

(Btw, in case anyone asks, the author is Dr. Martha Beck and the book I'm referring to above is called The Four-Day Win.)