Increasingly it makes sense to me that the workforce is segregated by gender.

There are, in fact, jobs where mostly women belong, and there are jobs where mostly men belong, and that'€™s fine. It'€™s outdated to think there are no differences between men and women. And once we accept there are differences, we need to study them instead of downplay them.

One of the most difficult parts of coming of age today is that there are no clear paths in the new topography of work. The terms quarterlife crisis and emerging adulthood have come to us as a result of the new scramble to figure out where to go in adult life.  In order to create safe, compassionate, growth-oriented paths through adult life, we need to understand where women and men fit best.

I have taken a lot of shots at this topic before. Most notably, I'€™ve pointed out that women want to be with kids more than men do. That explains Pew's findings that most women want part-time jobs rather than full-time jobs after they have kids, but men do not.

But what about gender differences before there are kids? Where do men belong? Where do women belong? Here are three places women do not generally fit:

1. Highly competitive sales jobs are not for most women.

Most women are not happy when they are competing, rather than collaborating with their co-workers, whereas men love competition, according to a study from the University of Chicago. And  research from the National Bureau of Economic Research shows that even in the case of men who are poor competitors or women who are strong competitors, the gender-based preferences for competition persist.

Women who achieve high levels of success played sports as kids and experienced huge satisfaction with winning. However women are less likely than men to choose competitive environments due to millions of years of evolution encouraging women to mitigate risks to protect themselves for their children. (Tons of studies support this, but my current favorite is from Anne Campbell, psychologist at Durham University in England.)

So who is a good fit for a career in sales? Richard Goldman, COO of Birkman International, a company that helps businesses make intelligent hires by using the Birkman Method for personality assessments, explains that collaborative, team-player types simply do not make good salespeople. When it comes to sales, though, the people who are the best fit for the high-level jobs are those who have an eat-what-you-kill mentality.

2. Men are better at very high-level math, science and engineering.

A 2008 survey of US universities by the National Science Foundation revealed that less than 30% of PhDs in the physical sciences were awarded to women. Higher up the ranks, women make up only about 10% of full professorships in physics-related disciplines. Yet a study from psychologists Stephen Ceci and Wendy Williams of Cornell University found no evidence of gender bias during the interview and hiring process for science positions.

Now that we have a few decades of data coming from girls who were encouraged to do math, we can say, with a decent amount of certainty, that the average girl is as good at math as the average boy. But in the world of hotshot math, women are outclassed.

One fundamental difference between the male and female brain is gray matter. And University of California at Irvine released solid data to explain why men are good at math.

"Evolution has created two different types of brains designed for equally intelligent behavior," said Richard Haier, professor of psychology who led the study.

"In general, men have approximately 6.5 times the amount of gray matter related to general intelligence than women, and women have nearly 10 times the amount of white matter related to intelligence than men. Gray matter represents information processing centers in the brain, and white matter represents the networking of—or connections between—these processing centers."

This chart, from Gene Expression, shows the difference in brain makeup.

Also, now that we have a slew of data about Asperger'€™s syndrome, we can say that the people who are incredibly terrible with language (white matter) or incredibly gifted with mathematical thinking (gray matter) are usually boys. Boys, rather than girls, populate the two extreme ends of the bell curve.

So it should not be surprising or controversial that studies repeatedly find that there are large gender differences among extremely gifted math students. More boys are gifted.

Now the world starts making sense. This is why there are more men in math and science positions in universities. This is why the hot-shot companies in Silicon Valley are full of male engineers and not women.  And this is why we need to stop complaining that science departments are boys clubs. It'€™s not just the department—high end scientific thinking is a boys club.

3. Men are best suited for the insanely fast-paced startup arena.

More than 95% of venture capital goes to male-run startups. (To be clear, we are talking here about companies that plan to grow to more than $100 million in sales over five years and these companies court investors to achieve that.)

These startups are largely male, and the reason is that women are not interested in the crazy life that startup founders live. Women prefer lifestyle-focused companies where they can have better control over the intersection of their work life and personal life.

I laid out the argument in a post on TechCrunch, but, as you can imagine, it comes down to kids. Women want to spend more time with kids, and they have less desire to give up everything for their company. It would be great if you could do both, but when it comes to startups, you can'€™t.

If you poll women who are CEOs of venture-backed startups as well as mothers, you will find that most women have either lost their marriage or their sanity while trying to run a startup and a family. Of course, no woman (besides me) will say this publicly because it will kill her career as an entrepreneur. One woman, (who has been lauded on TechCrunch for her startup), told me confidentially that she is getting a divorce and her husband thinks her drive is pathological. Which, frankly, is probably true, because much has been written about how most successful entrepreneurs are almost-but-not-quite crazy.

And Sara Lacy has explained the process whereby the boys-club startup founders perpetuate the boys club: if you get through it once, you can keep going, and helping other people to live out the crazy, family-unfriendly pipe-dreams of startup founders.

Many of you will want to tell me stories of the exceptions. I know. I'€™m one of them. I'€™ve driven myself and my ex-husband completely insane with my startup dramas, and I keep doing them. Each time I tell myself I will stay small. I will do a lifestyle business, and then I don'€™t. I think too big and I take in outside funding.

But going into a field where you are the gender minority is very difficult. For one thing, people judge you more harshly, and you are more quickly to be deemed a failure. And it feels uncomfortable. Sure, it'€™s fun to be the only woman in the room because you get a lot of attention, but on balance, Live Science reports that women are uncomfortable in a male-dominated setting.

One of the worst adages of feminism was "you can be anything." Because you can'€™t. You can'€™t be an iconoclast if you'€™re not. And most of us are not. It'€™s a lonely life. Why encourage it? Look at the paths that have a more sure footing. Look at the lives they create. If you like that life, give it a try.

When we look to create new paths for new careers, look at what real lives look like along the way. If you are 20 and you want to be the rare woman getting a PhD in math, ask yourself, are you really that different from all other people? And if you are 30 and you want to get funding for your startup, ask yourself if you truly are crazy enough to give up sanity for a company. Most of us look for more stable, surefooted ways to go through life. That'€™s rational behavior.