It’s one of prosperity’s side effects. When couples feel they need two incomes to survive, more moms lean toward full-time paying jobs. When they can manage on just one income (or one and a half), they lean toward part-time work or staying home full time. A few dads do, too, but it’s predominantly moms.

Many mothers, of course, choose to keep working, to maintain their incomes or careers. Of those 36 to 40, more than 40 percent work all year in full-time paying jobs–double the number 30 years ago. Other mothers have always stayed home or chosen flexible “mommy track” jobs. But there’s a palpable shift–led, as usual, by the boomers. In the 1960s, when women first muscled into the work force, at-home moms all but apologized for what they did. But once those same boomer women started families (often late in their 30s), staying home with the kids became the preferred thing to do.

Like their mothers, these women are adopting traditional roles–investing in their husbands’ careers rather than their own. Unlike their mothers (and thanks to the feminist achievement), they don’t feel trapped. “A lot of women my age don’t feel a big need to work because they know they can if they want to,” says Kate Francisco, 32, of Langhorne, Pa., a mother of two.

About choice: As feminists would say, it’s all about choice, and choice is influenced by circumstance. The stagnant ’70s and downsized ’80s sent mothers to work even against their will. The more prosperous ’90s freed many of them to reconsider.

I asked economics professor Diane Macunovich of Barnard College in New York City to look at the data on women and jobs. It’s too early to reach definitive conclusions, she says, but the changes all run in the same direction:

Young men’s real wages are going up. In particular, they’re going up in relation to what their parents earn. This gives them more confidence that they’ll reach or exceed their parents’ standard of living, Macunovich says. It also gives couples more confidence that they can rely on the husband’s earning power. (In only a small percentage of couples does the woman’s career take the lead.)

Young women’s fertility rates have tended to follow the changes in young men’s relative wages–an interesting factoid if ever there was one. And, yes, for twentysomethings fertility is up. Macunovich isn’t suggesting the kind of sexy frolic that instantly leaps to mind. She thinks that higher male earning power leads to earlier marriage and then to babies (whew).

Among mothers 36 to 40, work schedules are changing. More are opting for part-time jobs. During those years, which often coincide with the birth of a second child, more are leaving the labor force altogether.

The women most likely to go part time are those who earn the highest hourly pay. “Many economists thought that higher female earning power would kill off the family,” Macunovich says. “Instead, women are using their earnings to buy back personal time.” To keep them even part time, employers have to offer flexible schedules, telecommuting or shorter hours. Joanne Brundage, a former postal worker and founder of a support group, Mothers & More, in Elmhurst, Ill., calls it “sequencing”–switching in and out of the work force depending on your time of life.

Still, homemaking remains a luxury purchase. Among mothers with lower hourly pay, rising numbers are taking full-time paying jobs. They can’t afford to stay home.

A higher portion of women are choosing “women’s work,” such as nursing and teaching. It’s no coincidence that these jobs offer many options for part-timers.

When mothers first think of leaving paying work, they often hesitate. “I like the challenges of my job,” says Michele Smallidge, 34, of South Salem, N.Y., a hospital exercise physiologist who’s expecting her second child. Business reporter Angela Geiser, 31, of Temecula, Calif., worries about losing her identity. “I will probably continue to say that I’m a writer, even if I stay home,” she says.

A mom: That sounds familiar to New Yorker Cynthia Ryan, 38, who abandoned her home-based jewelry business after her daughter was born and now hosts a weekly mothers’ group. “We all found that the transition from work to home takes six months psychologically,” she says. “After that, you feel ‘I’m a mom’.”

Jocelyn Ravelo-Conde, 35, of San Diego, says it wasn’t easy to downsize to her husband’s $50,000 salary, but “it was hard to think who I’d leave my kids with and what values they’d pick up” if she kept her job. In Naperville, Ill., Kimberly Stemm, 27, misses her $35,000 pay but doesn’t want to miss her daughter’s youth.

Pediatrician Allison Seckler, 34, of Boca Raton, Fla., who now works two nights a week in the emergency room, says what’s really hard is minding kids. She loves it, but “it’s exhausting and you don’t get a break.” Smallidge thinks at-home moms are getting more respect. “People see the value in doing things properly, decorating, child care, the whole thing. It makes you feel important.”

None of these moms worries about dependency. Besides earning power, they have more financial savvy than older generations did. Many tend IRAs or 401(k)s from a former job. Joint bank accounts are the norm, as are joint decisions about savings and insurance.

These women generally plan to return to paying jobs. Employers will welcome them because of the labor shortage expected as older boomers retire, says Cornell University sociology professor Phyllis Moen. The moms may not get career jobs, but the work will be attractive enough. Maybe this is the generation that has it all.