I'm right behind Baz on this 1. It all depends on the spy ofc and how smart and motivated he or she is but if their intention is to destroy a tribe from within then it is more than just possible and I would go as far as likely to.
If self preservation is the motive behind the spy's actions which is what drives 99% of them, then you are far more likely to find them. This type of spy typically destroy's morale when the members find out and usually a number of members become either too focused on finding the spy or disillusioned that not everybody else loves the tribe the way they do.
How he affects morale in the short term comes down to the leadership and the short term actions that partially counter the spy's access to info.
Eventually though, the information filters down to everybody 1 way or another and as often as not a spy will learn more from others who are willing to talk about the tribe in general than from what is on the forums.
How the enemy use a spy is also crucial to the effects he has on your members ... most are revealed unintentionally. Then most handler's kick themselves for the mistake. Others though can intentionally leak what they have as PnP and the fallout from this when done well it is usually devastating.
Lastly, when a is a spy most useful. If you have 1 on UK19 then the chances are you are not in a position to act on more than 1/10th of the information you receive and the information regarding another tribe is changing so rapidly that last weeks news is usually just that. Basically about as helpful as the Scientist! :icon_redface: Imo, the later the game and the longer the spy has been in the enemy tribe the more harm a spy can do.
If ever there was a "Handling a spy guide", to be written then asking Nitrogen to do it, would be my first choice. Do some checking on the effects of well run spies on both worlds that he has won as duke and then come back and say they can't do much ... if you dare.