Doublethink is a trait of only the citizens of Oceania in that their very existences depend upon their ability to believe two contradictory or even opposite ideas to be true simultaneously.
The Party demands complete cooperation and obedience from its Outer Party members. Announcements which change the enemy of war from Eurasia to Eastasia at the drop of a hat are to be accepted instantaneously, without pause and without questions. The Party even notes through these broadcasts that upon this shift, the country had NEVER been at war with the first nation.
Any shift in facial expression or curious glance at another makes a person subject to vaporization. As a result, the citizens have learned that in order to survive, they must master doublethink. Those that do not, perish.