Those helming the studios' top-grossing 100 movies in each year from 2007 and 2016 were predominantly male, middle aged and white, a new academic survey has shown, raising the issue of bias among the highest echelons of the American film industry.

Of 1000 films analysed, less than one in 24 directors over the decade were female, and across those 45 directing jobs only three females were black and three Asian.

"When industry leaders think director they think male," the report of the School of Journalism at the University of Southern California found, throwing a spotlight on the Oscars in the category of best director where all five contenders are male including Mel Gibson for Hacksaw Ridge and Damien Chazelle for La La Land.

Over the past two or three years a powerful social-media campaign has grown around activists who are seeking to embarrass Hollywood about the awards' lack of diversity.

The list of acting nominees this year includes a black actor and an Asian actor - Mahershala Ali (Moonlight), who is African American, and Dev Patel (Lion), who is British of Indian descent - both nominated in the best supporting actor category.

Three of the best supporting actress nominees are black: Viola Davis (Fences), Naomie Harris (Moonlight) and Octavia Spencer (Hidden Figures).

Then Denzel Washington is vying for best lead actor for Fences and Ruth Negga was nominated for best actress for Loving.

The study found that women rarely sat in the director's chair to among the lucrative genres of science fiction/fantasy, action or thrillers and were more likely to work in drama and comedy, skewing "closely to those films that match stereotypically feminine genres".

Only one woman had directed a horror film. Only one Asian director of either gender helmed a top-grossing thriller. And only one black director led an animated film.

The opportunities for women were also more restricted. Male directors worked acrosss seven decades from their twenties to the eighties while the work of women ranged from their thirties to their sixties.

Women were also far more likely than their male colleagues to get one shot at directing a single lucrative movie money spinner.

Eighty per cent of women only made one movie in the ten years studied compared to the 54.8 per cent for males.

"In Hollywood, female directors are usually one and done," the study notes, adding: "The span of a females' careers is limited whereas for males it appears limitless."

Interestingly, the top performing male director of the period was the black actor and filmmaker Tyler Perry, specialising in gospel films, who worked on three times more films than that of Anne Fletcher best known for The Proposal (2009), and Hot Pursuit (2015).

"Female directors and their work seem to hold little value", the report states, "while in contrast male directors, stories and audiences are seen as profit centres. . .  . Very few women directors - if any - have been attached to large scale action or tent-pole films in the last decade."

Only last month the annual Celluloid Ceiling Report from San Diego State University's Centre for the Study of Women in Television and Film, noted a drop for female representation in a range of production roles.

Women made up only 17 per cent of all behind-the-scenes roles on the 250 highest-grossing films at the US box office in 2016, a two per cent drop from the previous year.

One possible fix, the latest study suggests, is encouraging buyers, agents, and executives to make sure short lists for upcoming projects are at least 30 percent female.

The relationship, too, between lead character and the director's gender or race should be uncoupled so that women can tell the stories that centre on male characters and ensure black directors tell stories that focus on white protagonists.

A-list directors should use their clout to bolster female directors, and mentor them in the same way as male directors and financiers should use their clout to invest in movies which give under represented woman and ethnic groups early career opportunities.

Otherwise neither storytelling or storytellers would reflect the world audiences live in.

"We can sit by as this continues for another decade or can act to ensure that equality and inclusion are the hallmark of entertainment in the years to come," the USC report ends.