Ideological impediments to marriage may be legal (USSR citizens may not marry foreigners) or institutionalized within religious doctrine (Muslim women may only marry Muslim men, Jewish marriage traditionally must be between Jew and Jewess although either may have been converted), or simply the result of prejudice and practice. Ideological impediments to marriage may also be constituted by nationalism, tribalism or other group philosophies if these take an ideological rather than a traditional form. Such impediments are caused by and perpetuate indoctrination, cultural and social division and conflict.