It's based on talking to people and watching them change their opinion though time.

It may be completely off due to sampling bias, but I don't think there is any source out there with better confidence, as I have never seen anybody doing a serious poll of the subject.

EDIT: Well, in reply to your edit, how did you search for that research? All I can see is trash news about some huge investment coming from China to Brazil in the future or how the next Brazilian president is a xenophobe that will reject money from anywhere. I still haven't seen any serious handle of the subject, by researchers or the press.