“One of the painful things about our time is that those who feel certainty are stupid, and those with any imagination and understanding are filled with doubt and indecision.” Bertrand Russell
Eights, Nines and Ones, when in a defense mode, have the habit of doing black and white, either/or, toggle switch thinking. This gives them a kind of security — a One told me that “I go right to the heart of things” — but it is the security achieved by having excluded any information that would cloud the issue. Remember– it is better to be vaguely right than clearly wrong.
This is slightly different from the general principle of the Dunning-Kruger effect. The Dunning-Kruger effect is that “incompetent people don’t know they are incompetent,” that explains the certitude of the members of the Tea Party. These can be any Enneagram style. Santorum and Bachmann are Sixes, for example.