(Reuters) - A gay marriage advocate, a physicist modeling brain activity, a psychologist studying racial bias in policing, and a cartoonist exploring family life were among 21 winners of $625,000 "genius" grants, the U.S. organization awarding them said.

Saxophonist Steve Coleman, renowned for "infusing iconic spontaneous music idioms" to forge a new sound, and physicist Danielle Bassett, who has applied mathematics to the modeling of brain connectivity, were also named on Tuesday as fellows by the John D. and Catherine T. MacArthur Foundation.

The group, which began its program in 1981 to provide money to help fund the specialized work of ambitious free-thinkers, uses anonymous nominators and selection committees to decide who gets the no-strings-attached grants, made to each recipient over a five-year period.

Recipients, who usually do not know they are being considered unless they win, join 897 other MacArthur fellows, the group said.

Among the 2014 winners are:

* Civil rights lawyer Mary Bonauto, 53, who spearheaded the legal battle in Massachusetts that led to the first U.S. court ruling to strike down a gay marriage ban in 2003.

* Cartoonist and graphic memoirist Alison Bechdel, 54, known for a long-running comic strip about lesbian friends and for her critically acclaimed 2006 memoir about growing up a lesbian with a closeted gay father in rural Pennsylvania.

* Stanford University social psychologist Jennifer Eberhardt, 49, for her studies of racial bias and policing and the criminal justice system. She has begun work to help local police agencies "build and maintain trust with the communities they serve."

* Documentary filmmaker Joshua Oppenheimer, 39, whose "The Act of Killing" shines a light on death squads after a failed communist-led coup attempt in Indonesia in the 1960s.

* Jonathan Rapping, a 48-year-old Atlanta lawyer and founder of Gideon's Promise, a nonprofit seeking to teach public defenders to help poor clients more effectively.

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* Mathematician Yitang Zhang, 59, of the University of New Hampshire, who "emerged from relative obscurity with a landmark achievement in analytic number theory: the so-called bounded prime gap, which essentially establishes that the difference in spacing between two consecutive prime numbers is, infinitely often, bounded by a fixed number," the organization said.

* Poet and University of Pittsburgh writing professor Terrance Hayes, 42, for his work reflecting race, gender and family. "Hayes conjoins fluid, often humorous wordplay with references to popular culture both past and present in his subversion of canonical poetic forms," the foundation said.

(Reporting by Eric M. Johnson; Editing by Lisa Von Ahn)