Writers often assign a single culture to an entire race or sometimes even an entire planet. An entire race or planet will follow the exact same philosophy, lifestyle, clothing style, and even hairstyle.
A good example of this would be the elves and the drow: in order to create a group of 'evil' elves, they were split off into two distinct races - 'good' elves with fair skin who lived above ground, and 'evil' elves with black skin who lived in subterranean realms.
See also Planet of Hats @ TVTropes
Avoiding the ProblemEdit
To avoid this problem, one needs to keep in mind that unless a race functions as a collective, it will be a large collection of individuals. No society has ever been completely homologous; there will always be variations in ideas and fashions. Someone will eventually try a new hairstyle or propose a new policy; someone else will decide to try it. Someone may have opinions and ideas that the others find largely unacceptable, but this person may have other ideas that are very good.
You are unlikely to find a race of people who largely share the same ideals except for one radical; instead, you are more likely to find a race that largely shares the same ideals with part of the population expressing varying degrees of dissent. People will have different ideas about what should be done about what bothers them, some better than others.
- Original Writerium Article