Aaron Sorkin thinks women's on-screen roles have a lower ''degree of difficulty'' than men's.

A leaked email, published by The Daily Beast, shows the 53-year-old producer arguing that Meryl Streep and Dame Helen Mirren are two of the only female stars ''who can play with the boys''.

In the email, written in response to New York Times columnist Maureen Dowd, Aaron discusses the 2011 romantic comedy 'Bridesmaids'.

He observed: ''That was a great and very interesting column today. I'd only take issue with one thing and that's the idea that something like Bridesmaids is seen as a fluke and that's why we don't see more movies like Bridesmaids.

''There's an implication that studio heads have a stack of Bridesmaids-quality scripts on their desk that they're not making and it's just not true. The scripts aren't there.

''That's why year in and year out, the guy who wins the Oscar for Best Actor has a much higher bar to clear than the woman who wins Best Actress. Cate [Blanchett] gave a terrific performance in Blue Jasmine but nothing close to the degree of difficulty for any of the five Best Actor nominees.''

Later, he added: ''Helen Mirren and Meryl Streep can play with the boys but there just aren't that many tour-de-force roles out there for women.''