"It is easy for three people to keep a secret, as long as two of them are dead." B. Franklin

A secret can be kept successfully (for a time) if all of the knowlegdeable parties feel their is a serious need to keep it.

The Manhattan Project, the Normandy landings... these secrets were kept because those who new felt there was so much at stake that the secret had to be kept. Watergate, Monicagate... these secrets were "outed" becuase someone who knew did not feel there was enough at stake to warrant keeping them secret; indeed, it was felt in the case of Watergate that what was being done was wrong and needed to be told and in the case of Monicagate that "he has it coming." Some secrets are lost because some people love to brag and show off how "in" they are. And, of course, there are secrets that are not kept because someone sees a profit in the telling (Aldrich Ames comes to mind).

Unless The Powers That Be (TPTB^TM) could convice everyone involved that the earth was doomed if it got out, keeping knowledge of ET secret would be impossible. Someone wouldn't be convinced, someone would think it wrong to withhold, someone would have an agenda, someone would brag, someone would want money...

Franklin was right.

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Never attribute to malice what can be adequately explained by ignorance or stupidity.
Isaac Asimov