principle of certainty

Thouless (1935) published a paper demonstrating that religious beliefs followed his “principle of certainty,” that is, people tended either to be believers or atheists, few acknowledged that where there are extreme beliefs, pulling in opposite directions, a middle (agnostic) position may be more reasonable. - The Psychology of Politics by Hans J. Eysenk

Is there evidence in the book to contradict the widespread belief that authoritarianism was always and exclusively on the right (conservatism) side, or was there an equally strong left-wing authoritarianism? Were communists and fascists essentially united by a common personality type, and a common belief in an essentially authoritarian ethos, rejecting democratic methods and aims?

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