Atticus was brought up to be racist. He was surrounded by racism from an early age and was taught that black people are somehow inferior to white people. Despite this, he "goes against his raising" by treating African Americans the same as he treats white people; with respect and dignity. This is uncommon for Maycomb, a small town in the South where racism runs rampant. Mrs. Dubose disapproves of Atticus's decision to defend Tom Robinson, claiming he is going against his upbringing.
