WASHINGTON – Oh, boy. Or maybe I should say: Oh, boys!

Because here we go again! As a female columnist at Foreign Policy, I apparently have the solemn duty to point out that President Obama has populated the top ranks of the national security and foreign-policy establishment exclusively with fellas. Where are those binders full of women when you need them?

In his cabinet choices so far, Obama has even managed to take a step backward from his first term, replacing Secretary of State Hillary Clinton, the sole woman allowed into the clubhouse, with Senator John Kerry. Obama let Susan Rice, a smart, tough, accomplished woman and an obvious choice to replace Clinton, be driven out by rock-throwing little boys from the Hill.

Then, for defense secretary, he inexplicably selected former Republican Sen. Chuck Hagel over Michèle Flournoy, the universally respected former undersecretary of defense for policy (and a Democrat to boot, not that I’m counting or anything).

So here’s what Obama’s second-term national security and foreign-policy team looks like, so far: Secretary of defense? White guy. Secretary of state? White guy. CIA director? White guy. Director of national intelligence? White guy. Chairman of the Joint Chiefs of Staff? White guy. White House chief of staff? White guy. National security adviser? White guy.

To use one of Obama’s favorite phrases, “Let me be clear”: I have nothing against white guys. I have a white guy for a father, two white guys as brothers, and a white-guy husband. I love them all.

But all the same, it sure would be nice to see a few more girls in the club. In particular, Obama missed a historic opportunity to be the president who appoints the first female secretary of defense.

It’s fine to say that such critical foreign-policy and national security positions ought to go to the best guy for the job, but sometimes, the best guy is a woman.

Pick your favorite realm of action. Investing? Studies show that female investors are less prone to risky investment decisions than their male counterparts – and over time, they consistently earn higher returns.

Preventing civil conflict? High rates of female inclusion in national governments seem to be correlated with lower rates of civil conflict.

Development? In developing countries, investing in education for girls is more strongly correlated with economic growth than anything else. Investing in men, on the other hand, turns out not to be such a great idea much of the time.

Women who participate in microfinance programs are more likely to make timely repayment, comply with program rules and use credit to benefit their families and communities. Men, not so much.

Similarly, in refugee camps, aid agencies have found that when you distribute food aid to women, they use it for the benefit of their families – men, on the other hand, are more apt to sell the food to buy something for themselves.

I’m not arguing that if only women ruled the world, we’d all live in peace and harmony because women are just naturally nicer and kinder and more nurturing than men. (There’s a two-word refutation to that little fantasy: Margaret Thatcher. In fact, history’s chock full of bloodthirsty gals.)

But the numbers don’t lie – and though causation is far harder to determine than correlation, the correlations are pretty suggestive. Virtually across the board, increasing female participation in an enterprise appears to be correlated with better outcomes.

Perhaps that’s because women are “different” in some inherent way; perhaps it has nothing to do with biology and everything to do with the roles women are currently socialized into playing; perhaps it’s simply that diverse groups tend to be more resistant to the perils of “groupthink” and better at generating creative solutions than homogeneous groups.

(This last would suggest that populations in which women are over-represented might end up prone to some of the same problems as populations in which women are under-represented).

Regardless, the near-term implications are pretty straightforward: In many arenas, the average woman seems to outperform the average man, and diverse groups that include both men and women outperform homogeneous groups made up solely of men. If this is true in education, development, investing and conflict prevention, it’s a pretty good bet that it’s also true in the national security and foreign-policy domains.

Putting together an all-male national security team is the equivalent of hanging a sign on the clubhouse door: Girls keep out!

The presence (or absence) of senior women in America’s foreign-policy leadership sends a message abroad as well as at home.

In Afghanistan and the Middle East in particular, the United States is spending billions trying to protect women from violence and discrimination, increase girls’ access to education, and increase female participation in traditionally male-dominated fields such as law, business, government, the police and the military.

When women are visible and vital parts of our own leadership teams, we show our Afghan or Saudi or Pakistani partners that we practice what we preach – and this has concrete benefits for our ability to achieve our foreign-policy and national security goals.

So here’s a thought: It’s not too late to increase the gender diversity on your national security team. How about appointing Susan Rice or Michèle Flournoy to replace Tom Donilon as national security adviser?