Most of us help out people we favour (e.g. friends and relatives). We like people like ourselves e.g. individuals who went to the same college, work with us or are the same race. We tend to identify with people who are a lot like us. We are more comfortable doing favours for those within whom we identify than for those noticeably different. Individuals tilt towards helping people who share our nationality, religion, race, gender and alma matter.
However, there are some problems with this tendency. Human beings tend to exclude underrepresented minorities. When majority of people favour people similar to them, they allocate scarce resources (e.g. jobs and college admissions) and thus discriminate those different from them.