Reducing Biases in Diagnosis

I need help explaining how the client's cultural identity in the case below could have contributed toward personal biases and diagnosis and how a clinician might mitigate, or reduce, the appearance of personal biases in diagnosis.

The case:

Marvin, a 45 year-old African American male, has come to your office because of feelings of anger about his recent firing as a mid-level corporate executive. He held this position for fifteen years, but was ostensibly let go as part of a larger organizational restructuring. Marvin shared with you; however, that he is skeptical of this explanation because he has had several incidents where co-workers were overheard making various racially-charged epithets toward various employees. He noted that the "biggest racist of them all" became his direct supervisor two years ago, and shortly after he received the first of two disciplinary reports. As he mentioned, he had never experienced problems with work prior to these reports; as a result, these reports were "bogus complaints" and upset him.

Marvin is quite animated and communicates his anger about what has happened very clearly to you. He notes that this is not the kind of thing that would bring him to a therapist's office - "I'm not crazy!" Rather, he came in to placate his wife. She complains that Marvin is not only angry and unkind toward her and the children, but that he sits around all day and has no interest in much of anything except video games. Marvin's wife, who attended the first part of this session, noted that they have not had sex in three months, he has stopped going to the gym (something he did daily), and he seems "very pessimistic" about this poor mark on his record. She noted that while Marvin has always had some of these tendencies, they have gotten significantly worse since he was let go from his job.