NEW YORK, (Reuters Life!) – For more than a decade the number of women rappers signed to major American record labels has been in steady decline, leading hip-hop industry experts to ask, where have all the female rappers gone?

The 1980s and 1990s saw the rise of women rap stars such as Queen Latifah, Salt-n-Pepa, Foxy Brown, Lil' Kim and Missy Elliot, but in recent years few acts have broken into the big time. The decline has been so pronounced that in 2005, the Grammys eliminated the best female rap categories from their annual music industry awards.

That drop is explored in the documentary, 'My Mic Sounds Nice: A Truth About Women in Hip-Hop'.

The hour-long film, to be aired on Black Entertainment Television (BET) on August 30, traces the history of women MCs from The Sequence's 1979 hit 'Funk You Up' to Nicki Minaj, one of the few breakout successes of the 2000s and arguably the most visible woman in rap today.

Along the way, documentary explores several reasons why the decline has occurred.

One theory is that the emergence of sexually provocative women like Foxy Brown and Lil' Kim in the late 1990s brought pressure on other female MCs to follow suit – something that did not always mesh with the tough, streetwise images those other rappers had already cultivated.

"You look at people like Da Brat and how their images changed and you really see the conundrum these women were in, now that this sexual thing took precedent," the documentary's producer and director Ava DuVernay, told Reuters.

Another explanation was offered by Sylvia Rhone, president of Universal Motown Records, who signed Missy Elliot. She said women rappers have failed to adapt to changing tastes in hip hop, which has seen the recent rise of singers who blend genres and whose images do not conform to hip hop stereotypes.

"Our fresh voices are B.o.B., Kid Cudi and Drake. Where are the female artists influenced by that? Where's that left-of-centre person," Rhone said.

Others in the documentary note that with just a handful of female rappers reaching superstardom, labels see signing women as less financially lucrative than men. Nikki D, the first female rapper signed to Def Jam, says she has heard record executives say directly that they would not sign female rappers "because they don't sell."

Ironically, the debate comes as Minaj's 'Your Love', tops Billboard's rap chart. But Minaj is an exception, experts say.

It has been seven years since a woman held the number one spot on a rap chart – Lil Kim's 2003 hit 'Magic Stick' featuring 50 Cent. Missy Elliot's 2002 single'Work It' was the last song by a woman with no featured guest to hold that position.

The success of international women stars in hip-hop also has been inconsistent.

French rapper Diam's 'Dans Ma Bulle', was the top-selling album in France in 2006. British rapper and singer M.I.A. has enjoyed success on both sides of the Atlantic. But in the United States, much-hyped British rapper Lady Sovereign has failed to replicate the success of her debut 'Public Warning'.

Duvernay said she jumped at the chance to make the documentary because she was a rapper signed to a major label development deal in the early 1990s. "I was very passionate about this and very protective of the story because in a lot of ways it was mine," she said.