It's subconscious.

I work with people from a variety of countries, and I naturally gravitate toward people from my own. We share the same cultural background, have the same accent, and have similar skin tones. So I go out of my way to correct that.

I'm a manager, so when I look at promotions, I have to take a minute to write down the pros/cons to justify a promotion to help counter that bias. I also try to spend more time (lunches, water-cooler type discussions, etc) with those who are different from me as well, otherwise I'd naturally cluster with those who are more similar to me. I notice it in the people from other cultures as well, they tend to cluster with those that are similar to them as well, so hopefully me being proactive encourages them to reciprocate.

At least that's how I see it, and it seems to be working out alright.