by Leigh Gallagher, Fortune Senior Editor

Why do women lag men on Fortune’s 40 Under 40 list of the most influential rising stars in business? Yesterday we delved into this question and shared insights from two experts: Facebook COO Sheryl Sandberg and McKinsey director Joanna Barsh. Sandberg’s and Barsh’s opinions provoked some interesting comments, including one from David in New York City, who declared that women “have all the brains and tools but lack the drive,” followed by Michelle in Toronto, who argued that women “are not nurtured at a young age to be entrepreneurial or business-minded.”

A few more insights from Barsh, who carefully explored this territory to research her new book, How Remarkable Women Lead. Barsh believes that women are more interested than men in finding meaning in their careers than, say, wealth or power. We buy into that–and we’ve been struck over the years by how many stars of Fortune’s Most Powerful Women list, such as former Procter & Gamble (PG) president Susan Arnold, have thrown in the towel for a break or a better life outside of business.

But Barsh also thinks that women may be wired with a lower tolerance for risk, and therefore are less likely to risk it all to try to create the next, say, Google (GOOG) or Twitter or Ford (F) or Coca-Cola (KO). “Women are so busy pleasing everyone that they’re in the get-an-A-mode,” she says. “That causes you not to take risk.”

Such theorizing gets really interesting when you delve into the question of why women don’t take as big risks as men do. Theories abound, but one of Barsh’s favorites comes from Florida State University social psychologist Roy Baumeister, who focuses on the theory that in ancient times, many more women reproduced than men. Baumeister suggests that the ratio could be as much as two times (say, 80% of women and only 40% of men were fortunate enough to have progeny).

Starting with the premise that our biological necessity is to reproduce, a woman’s job was to protect her eggs, whereas men’s job was to risk it all to win a mate. As Baumeister theorizes, over time that risk-taking requirement became hard-wired for men at the same time the nurturing instinct wired itself in women.

The fact is, we are descendants of women who played it safe–and men who, to win a woman’s hand (and the chance to procreate), had to compete. “In order to fulfill their biological goal, men had to take huge risk,” Barsh says. “They had to go to war and come back with money.” Not all men did, which could explain why fewer of them reproduced than the women, who were the ones being pursued.

Such anomalies of risk-taking aside, men have greater variability in general, Barsh notes. This variability enhances the chances that more men than women hit the heights in the corporate world; meanwhile, more men than women also fall in the gutter. “If you take the smartest people and the dumbest people, there are more men in both buckets,” Barsh explains, adding, “Look at people in jail. There are mostly men in that group, too.”

Come to think of it, a few of those jailbirds were once in business, and probably took risk too far.

Just look at Fortune’s annual Most Powerful Women list to see the strides of women in business: Today an executive, to make the cut, generally needs to oversee some $6 billion in revenue, vs. about $1 billion 11 years ago when we launched the list. Still, the debate rages about why women aren’t moving up the ranks faster. My theory: Women tend to view power horizontally and live their lives in chapters–and so, even if all the glass ceilings across the world shatter, women will not catch up with men in terms of power. Still, we were surprised to discover how few women made Fortune’s just-released 40 Under 40 list of the most influential young people in business. So we asked Senior Editor Leigh Gallagher, who commandeered the project, to share her insights–which are fresh, smart and eye-opening. Here’s Leigh on the dearth of female rising stars:

In putting together Fortune’s 2009 list of the 40 Under 40, here’s the trend that stood out: Men outnumber women in our rankings by a ratio of 7 to 1.

Yes, there are only five women on the list. We can express indignation at this because it wasn’t our intention or our choice. In fact, we shook the trees pretty hard to make sure we weren’t missing anyone.

Make no mistake: The women who made our list–financial analyst Meredith Whitney, Google VP Marissa Mayer, Ning CEO Gina Bianchini, CNBC (GE) anchor Erin Burnett and Coca-Cola (KO) marketing’s Wendy Clark–have all zoomed past men in their respective fields. But there are no women listed who, say, founded a Google (GOOG) or a Facebook or a Twitter. None who are self-made hedge fund billionaires. None who made it to the top 10. So, while the accomplishments of those five women are tremendously impressive, they still, for the most part, trail those of most of the men on our list.

What gives? I asked Sheryl Sandberg, COO of Facebook and one of the most prominent leaders in Silicon Valley, to weigh in. She cited some interesting numbers: Just 15 of Fortune 500 CEOs are women. Yet at the college level, more women are getting degrees than men–and even some professional schools are graduating more women than men. “So the question is,” Sandberg says, “what happens between leaving school and age 40 to make this list predominantly male?”

Sandberg (who, by the way, would have made the 40 Under 40 list if she hadn’t turned 40 this year) says she suspects that “disequilibrium of household responsibilities,” as she calls it, is a key reason women slip behind. Even when both men and women in a household work fulltime, she notes, studies have shown that women do the majority of the childcare and housework. Perhaps that’s not so surprising, but Sandberg’s point is that advancements on the domestic front seriously lag the gains women have made in the workforce.

“Over the past 40 years, we have made more progress in the workplace than in the home,” Sandberg says. “I believe the No. 1 thing we could do to change the numbers in the professional world is to find a way to balance responsibilities in the home.”

McKinsey director Joanna Barsh, who pioneered the firm’s Centered Leadership Project to help develop women leaders, thinks along the same lines. She points to the practical limitations that child rearing can put on women’s careers. “Not everybody slows down,” Barsh says. “But an awful lot of women choose to slow down and enjoy those years.” And for many women, she notes, the slowdown comes at the precise time when career opportunities tend to surge. “Your hormones are causing you to want to take care of everyone in your family at the very moment when you’re building your goal lines.”

Many women “zig and zag” rather than pursue one goal in a straight line, as most men are programmed to do. Women can still accomplish a great deal career-wise, but it might happen when they’re well past 40. Barsh’s idea for fixing the gender imbalance on the 40 Under 40 list? “Give women a ten-year handicap!”

Well, we can’t do that. But we can hope to see more women in the 40 Under 40 rankings in coming years. To be sure, half the people on our “Ones to Watch” list of fast risers are women. I asked Sandberg how long she thinks it will be until a woman co-founds a $100 billion company. Her answer: “Who says she hasn’t already?” She makes a good point. And we’ll be watching.

P.S. Tune into Postcards tomorrow for further discussion. And for more of Sandberg’s insights about navigating a successful career, read her first-personer, “Don’t Leave Before You Leave.”