Are you baised?

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I have seen myself that when my friends call up home, they either speak only to their father or mother. Among children of their own, they would love some more than the other. At work place, I have seen some workers are alloted good quality work, whereas some arent. What makes people go biased? I may understand the benefit of being biased in the office or work environment, but how could it be possible within our own families?