different between idiosyncracy vs propensity

English

idiosyncracy (plural idiosyncracies)

This spelling is commonly considered a misspelling; etymologically the preferred spelling is idiosyncrasy, which is also the only spelling given by most dictionaries, such as the Dictionary.com Unabridged and the American Heritage Dictionary. This spelling is mentioned in the Oxford English Dictionary's entry for idiosyncrasy, but is marked as erroneous. Webster's Third New International Dictionary gives both spellings, but under the entry name idiosyncrasy.

From propense (“inclined, disposed”) +? -ity, the former from Latin pr?pensus, perfect passive participle of pr?pende?.

propensity (countable and uncountable, plural propensities)

An inclination, disposition, tendency, preference, or attraction.
- 1902, William James, The Varieties of Religious Experience, Lecture I:
  To the psychologist the religious propensities of man must be at least as interesting as any other of the facts pertaining to his mental constitution. It would seem, therefore, that, as a psychologist, the natural thing for me would be to invite you to a descriptive survey of those religious propensities.

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