It's not clear without further scientifically designed testing whether there is more "cultural bias" in grouping people by saying "All of them look very Nigerian to my eyes" or by saying that it is easy to see which of the varied individuals grow up in the West and which grew up in Nigeria. (It's an empirical question, first of all, whether observer identifications of that kind can be verified by controlled experiments with unfamiliar individuals in either case.) There are definitely different cultural biases among human beings about whether environment of development matters most or ancestral lineage group matters most in making distinct human individuals more or less alike for the social purpose of grouping them by some commonality. One refreshingly interesting fact I learned from the book The History of White People by Nell Irvin Painter

http://www.amazon.com/History-White-People-Irvin-Painter/dp/...

is that the ancient Greek authors were overwhelmingly of the opinion that what made different people groups different from one another was different living environments (climate and topography), with the Greeks of course thinking that their living environment tended to produce superior people.