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 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 or Twitter or Ford or Coca-Cola . “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.