After Arabella Mansfield took and passed the Iowa bar exam in 1869, she challenged the state law excluding her from practicing law, in the process becoming the first female lawyer in the U.S. Nearly 150 years later, Mansfield is lending her name to a new rule aimed at closing the gaps between women and minorities in leadership roles in Big Law.

Derived from the National Football League's so-called Rooney Rule, which requires that NFL teams interview at least one minority candidate for head coach or general manager positions, the Mansfield Rule states that at least 30 percent of a law firms' candidate pools for any leadership or governance position—including equity partnership promotions and lateral positions—must be comprised of women or minorities. Earlier this month, 30 large law firms signed on to pilot this new approach within the next year.

“These law firms have signed on [to] help us form the idea, put it into fruition, see what works, see what doesn't work,” said Caren Ulrich Stacy, CEO of the Diversity Lab, which is working with the firms to develop the Mansfield Rule. “We're going to stick with the firms and we're going to help them measure and track and then [see] where the needle has moved over the course of the year so that its second iteration next year could be even better.”