An omnipotent being presents itself before you, posing a simple dilemma: either George, an 18-year-old co-ethnic, or Michael, an 18-year-old foreigner will be killed. You are also told that (a) neither George or Michael are your close kin, and (b) that Michael is twice closer to you genetically than George. You can decide to spare either George or Michael. Who would you choose?If ethnocentric behavior arises from our desire to further our genetic interests, then we would choose to spare Michael. If it arises from our desire to support members of our ethnic group irrespective of genetic interests, then we would choose to spare George.
I would personally choose to spare George. I suspect that this would be the case for at least most of my countrymen, that they would choose to spare a fellow Greek rather than e.g., Turk or Bulgarian who happened to be closer to them genetically. Even though condition (b) is an ideal case for the "genetic ethnocentrism" thesis (you are told the exact degree of relatedness of the individuals affected by our actions), I believe that it does not create nearly the same motivation to save Michael than the simple notion that "George is a Greek" does to save him.
Condition (a) is very important to elicit the above choice. Imagine for example that you're an American of German ancestry fighting in WWII. If you're an ethnocentric German-American, then you will doubtlessly choose to kill a German to protect a fellow American, even though you're most likely closer kin to the German than to the American. But, if you knew that the German e.g., was your cousin, then you might be tempted to look the other way.