Another factor that facilitates liking is similarity. If the audience feels as though they can identify with the characters in a film, they are more likely to see and enjoy it. Casting directors take this into account when actors are chosen, but just as importantly, screenwriters and producers take this into account when the script is being developed. If you look back to our slideshow of 15 popular actors, only 3 were minorities (4 if you include Cameron Diaz, but she is typically perceived as "white"), and only 4 were women (only one of which, Cameron Diaz, makes the list of top ten highest-grossing actors). Typically, the primary leads in blockbuster films are white males, with one or two minorities and women filling out secondary roles. Even the most wholesome of studios and production companies tend follow this formula. For example, all twelve of Pixar's films have featured male protagonists and no minority protagonists. Brave (2012) will be its first film with a female protagonist. There are two possible explanations for this major discrepancy. Either audience members aren't interested in movies with minority or female protagonists, or these individuals are not getting cast in high profile roles. I would argue that the latter problem is the greatest, and that these types of protagonists can and do appeal to a large audience.
The characters of the highest grossing film of 2012. Notice a pattern?
Gender If we operate under the assumption that people like to see characters they identify with, then the argument that audience interest is low makes little to no sense in the case of gender. About fifty percent of the world's population is female. If women are interested in movies as much as men, then there is no reason why films with female protagonist shouldn't garner as much of an audience as those with males. Obviously, this argument is oversimplified, but it is safe to say that there are many women who would be interested in more female leads. One potential argument in favor of "low audience interest" is that women have become so accustomed to films with male protagonists, that male oriented movies are not a deal-breaker for them. By contrast, men are so used to an abundance of male oriented films that female oriented films are automatically less appealing. For certain, there are a lot more women who go willingly to alpha-male action movies than there are men who go to chick flicks (albeit being dragged there by a date). Thus, producers alienate a smaller portion of potential viewers by using male leads.
Race This argument, in terms of race, is more complicated. Historically, the majority of Americans have been white. Although this is a quickly shrinking majority, "mainstream American culture" is still synonymous with "white American culture." Movies can do very well marketing toward a minority audience (for example, Tyler Perry has dominated black cinema of the 2000s, not necessarily through quality or originality). However, if a film wants to reach the widest possible audience, filmmakers must appeal to all demographics. Unfortunately, the attempts to do this have not been very effective. Movies tend to include an all-white cast, or to include a couple of minorities, as an attempt to have a character that others can identify with. Similarly to the case with women, minorities are so accustomed to all-white casts, that it is not a deal breaker. This is the default of film. Few question this casting. On the other hand, if a cast is all-black, all-asian, or all-Hispanic, people will take notice, and this casting will factor into their decision to see the film, implicitly or explicitly. This does not make viewers "racist." It is just a matter of what is familiar to them.
Consequently, it is almost a rule of thumb to at least cast white (or seemingly white) protagonists, even if the rest of the cast is not, in order to cast a wide net. An example of this rule in action is the Step Up movie franchise. The films revolve around hip hop dancing (a form of dance that was born in African-American communities), features a verydiverse cast of secondary characters, and a white male and female lead in--all four films.