When Kathy Warden takes over as CEO of Northrop Grumman in January, women will run three of the primes, as the largest American defense firms are known.

Warden joins Marillyn Hewson at Lockheed Martin and Phebe Novakovic at General Dynamics as CEOs, while executive vice president Leanne Caret heads Boeing’s defense business.

Their rise coincides with a sharp uptick in women in influential security-related positions on Capitol Hill.

Texas Republican Kay Granger holds the gavel of the powerful House Defense Appropriations Subcommittee, joining an elite club that has until now been open only to men. Meanwhile, women from both parties on the Senate Armed Services Committee have pushed into law dozens of new policies and regulations aimed at changing the military’s male-dominated culture.

However, the ascension to the top ranks of the boardroom and plum security committee assignments on Capitol Hill has not been matched in other parts of the defense apparatus. There has never been a female Defense secretary. There has never been a female member of the joint chiefs of staff.

What accounts for this discrepancy? While there are various theories, no single explanation seems to suffice.

Government vs. private sector

As undersecretary of Defense for policy during the Obama administration, Michèle Flournoy was the third most senior civilian in the Pentagon. She credits the Obama administration with making diversity — of gender, background, thought — a priority.

“There was a focus from the top,” she said. “We worked the issue to make the leadership cadre look more like America, because that’s what you want in national security.”

That does not appear to be a priority during the Trump administration, she said.

The data bears this out. For President Donald Trump’s political appointees, men outnumber women at every department. At the Defense Department, 46 out of 52 positions confirmed or awaiting confirmation went to men. Overall, 33 percent of Trump’s appointees have been women, whereas 47 percent of the workforce and 43 percent of the government workforce are women.

If not for Trump’s choices, there might be more women in senior roles at the Pentagon, said Mieke Eoyang, vice president of the left-of-center think tank Third Way’s national security program.

“This president values combat service,” she said, pointing to the prominent roles given to past ground combat generals like Defense Secretary James Mattis and White House chief of staff John Kelly. “Something happens when you have a reassertion that men are better at national security, which stems from the assumption that only men are willing to risk their lives for national security.”

Since most combat positions only became open to women in the 1990s, the cohort of women with combat experience is still a few years away from having the résumés needed to land top roles — like combatant commanders and service chiefs of staff — for which combat leadership experience is viewed as a prerequisite, she said. This means that women are likely a few years away from being more fully represented at the top military ranks.

On the Pentagon’s civilian side, the Defense Department has struggled to build a gender-balanced workforce, said Loren DeJonge Schulman, a former DoD and National Security Council official who is now a senior fellow at the Center for a New American Security.

In recent years, federal agencies have faced spending cuts, hiring freezes and budget turmoil that have caused human resource headaches and workforce management distractions, making it difficult to put a premium on recruiting women at a proportionate rate. Additionally, military veterans receive hiring preference — half of the Pentagon’s civilian hires are veterans and veterans are predominantly men, she said.

Retention also becomes a significant issue, Schulman said. Midcareer parents face challenges in many industries, but in national security, where lives are actually on the line and handling classified information can be part of the job, family-friendly benefits like flex time and telecommuting can be hard to accommodate, she said.

“From College to Cabinet,” a study on women in national security published last year by the Center for a New American Security, found that between 1992 and 2012, the percentage of women in the federal government’s general schedule, or GS, program rose from 33 percent to 45 percent. But women held only 37.8 percent of supervisory roles.

“This indicates that while women’s representation throughout the GS workforce has increased, they are either leaving government service earlier than their male counterparts, or not being promoted at the same rates,” the report concluded.

Case study after case study

Both Flournoy and Schulman noted that the private sector has embraced the conclusion — found in case study after case study produced by Harvard Business School and other academic institutions — that more diverse teams are more efficient and productive.

“This is kind of a known and accepted fact in the private sector,” Flournoy said. “You actually get better performance with more diversity. It’s not just the right thing to do, it’s essential to do for better performance.”

But it has yet to become engrained in the Pentagon’s culture.

Schulman thinks this is in part because it can be hard to quantify results in national security, where a good outcome may be that a crisis was averted. Without data backing up the benefits of a diverse workforce, it’s difficult to convince national security leaders in government of the value.

There are comparable findings about diversity and government, said Heather Hurlburt, a former White House and State Department staffer who is now director of the new models for policy change program at New America, a think tank with an emphasis on technology. But they are much less well known and not seen as part of the mainstream in the United States.

“It hasn’t been the same kind of imperative” for the federal government, she said. “It’s been a nice-to-have as opposed to must-have.”

The importance of mentors

At defense companies, where the lines of succession are mapped out in boardrooms years ahead of time, candidates are carefully groomed for top roles.

These corner-office opportunities would not be available to women if previous CEOs had not individually placed value on ensuring diversity in the boardroom, said Anita Antenucci, a senior managing partner at Houlihan Lokey who leads the firm’s aerospace, defense and government practice.

In that way, a few motivated people like former General Dynamics CEO Nick Chabraja and Northrop Grumman’s outgoing CEO Wes Bush can change corporate culture by nurturing candidates they want to succeed.

“The defense sector is one that looks very carefully at talent development,” said Antenucci, who was quick to credit Chabraja and Bush for their roles in helping advance the careers of Novakovic and Warden.

That is not to say that the women running these companies didn’t rise to the top on their own merits.

“It’s a performance-based job,” said Third Way’s Eoyang. “They are doing well for the companies, and they got those jobs because they could hit their performance marks.”

Flournoy also took note of industry’s leadership development, mentoring programs and focus on diversifying leadership ranks.

“Industry gets a consistency of focus,” she said. “Most CEOs will be there for a decade.”

Sometimes, mentorship in the military falls victim to the “mini me” phenomenon, where leaders take a shine to proteges who remind them of their younger selves, said Flournoy. In the male-dominated military, this can mean that fast-tracking mentorship relationships go largely to men.

If Congress wants to help, it can remove barriers that prevent women from serving, fund leadership development programs and focus on sexual harassment, said Flournoy.

But Flournoy stopped short of suggesting quotas for female hires. Quotas can create the assumption that the woman in any given role is underqualified and was only hired because of that special consideration, she said.