How did Chief become deaf and dumb?

It should be noted that the Chief is neither hearing
impaired or mentally challenged.  The Chief is merely pretending to be "deaf and dumb." 
This way, he is able to remain distinct from the world while interacting with it on the
terms that he finds acceptable.  The Chief understands fully that the desire to control
is something that is present throughout the American social culture of the 1950s, and
recognizes that the only way to be free of this is to remain as distinct and withdrawn
from it as possible.  Chief ends up changing his position on being so isolated from a
social world through his interactions with McMurphy.  It is through this that he
understands that his position of withdrawal is not a way to impact the world and create
change.  In the end, his fear of "the Combine" and social control is only enhanced by
his isolation.  Through interacting with the world and being an active agent in it, the
Chief understands his ability to change it, reason enough for his evolution throughout
the text.