More from DOD's own regs regarding who does and doesn't get a security clearance:
C2.1. STANDARDS FOR ACCESS TO CLASSIFIED INFORMATION OR ASSIGNMENT TO SENSITIVE DUTIESMore on security clearance guidelines from About.com
C2.1.2. Clearance and Sensitive Position Standard. The personnel security standard that must be applied to determine whether a person is eligible for access to classified information or assignment to sensitive duties is whether, based on all available information, the person's loyalty, reliability, and trustworthiness are such that entrusting the person with classified information or assigning the person to sensitive duties is clearly consistent with the interests of national security.
C2.2. CRITERIA FOR APPLICATION OF SECURITY STANDARDS
C2.2.1. Criteria for Application of Security Standards. The ultimate decision in applying either of the security standards set forth in paragraph C2.1.2. and C2.1.3., above, must be an overall common sense determination based upon all available facts.
The criteria for determining eligibility for a clearance under the security standard shall include, but not be limited to the following:
C2.2.1.1. Commission of any act of sabotage, espionage, treason....
C2.2.1.5. Unauthorized disclosure to any person of classified information, or
of other information, disclosure of which is prohibited by Statute, Executive Order or Regulation.
C2.2.1.7. Disregard of public law, Statutes, Executive Orders or Regulations including violation of security regulations or practices.
C2.2.1.8. Criminal or dishonest conduct.
C2.2.1.9. Acts of omission or commission that indicate poor judgment, unreliability or untrustworthiness.
C2.2.1.11.2. Any other circumstances that could cause the applicant to be vulnerable.
C2.2.1.15. Any knowing and willful falsification, cover up, concealment,
misrepresentation, or omission of a material fact from any written or oral statement, document, form or other representation or device used by the Department of Defense or any other Federal Agency.
C2.2.1.16. Failing or refusing to answer or-to-authorize others to answer questions or provide information required by a congressional committee, court, or agency in the course of an official inquiry whenever such answers or information concern relevant and material matters pertinent to an evaluation of the individual's trustworthiness, reliability, and judgment.
"C3.7.9..2. Generally, the Deputy Under Secretary of Defense for Policy will only approve periodic reinvestigations of persons having access to Secret information if the unauthorized disclosure of the information in question could reasonably be expected to: C3.7.9.2.1. Jeopardize human life or safety. C3.7.9.2.2. Result in the loss of unique or uniquely productive intelligence sources or methods vital to the United States security."
