The only people who can speake athunticaly for a group are people who belong to that group. Only a black woman who has had to endure the daily injuries of dealing with casual raceism and sexism can truly speake of the effect theses things have on black women. This is true of all groups, it is also true that only members of that group can decide what issues are important to them. The spectacle of conservative white male pundits telling black people in the wake of the murder of Trayvon Martin that they were focusing on the wrong problem is obscene.
This does not mean that only blacks can speake on black issues or trans people on trans issues, it does mean that if your not a member of the group you're studying you have to be damn sure and double check your biases and privilage or risk creating another monument to bad scholarship like Bell Curve: Intelligence and Class Structure in American Life (A Free Press Paperbacks Book)
or The Man Who Would Be Queen: The Science of Gender-Bending and Transsexualism. Of course if you're a stright white cis male and produce such idiocy you will be defended for just criticism, if you're any one else you will be ignored.
The glory of the internet is that it has given various groups a chance to speak for themselves. A few of us have even risen to some prominence. Now the rest of us need to listen to the voices of people from groups not our own. It can be difficult to listen because sometimes it seems like what they are saying is a direct personal criticism of us but even if it is we need to listen and learn.