Most social science research about beliefs excludes atheist participant samples . . .

Exactly, believe means to accept something as true, or false, without sufficient evidence. So "belief" is stupid by definition. That wording does not allow for deliberately thinking in terms of probability instead of true or false. psik
Not quite:
http://dictionary.reference.com/browse/believe?s=t be·lieve [bih-leev] Show IPA verb, be·lieved, be·liev·ing. verb (used without object) 1. to have confidence in the truth, the existence, or the reliability of something, although without absolute proof that one is right in doing so: Only if one believes in something can one act purposefully. verb (used with object) 2. to have confidence or faith in the truth of (a positive assertion, story, etc.); give credence to. 3. to have confidence in the assertions of (a person). 4. to have a conviction that (a person or thing) is, has been, or will be engaged in a given action or involved in a given situation: The fugitive is believed to be headed for the Mexican border. 5. to suppose or assume; understand (usually followed by a noun clause): I believe that he has left town.
'without sufficient evidence' is not the same as 'without absolute proof'. Sufficient evidence is allowed.