A good rule to remember is that the title of a paper should not ask a question but rather provide the answer. There should preferably be no question marks, exclamation marks, citations, percentages or other special letters in the title. It shall be easy to read and easy to understand. There should also not be names of geographical places unless it is very important for the message. Finally the title should generally include a verb to make the message active.
Of course there can be exceptions to these rules and for instance in letters to editors and editorials it may be appropriate to raise a question in the title rather than give the answer immediately.