"I thought I'd be married by 30," says the Boston therapist. "This is a great divergence from what I imagined my life would be."

There's a new biological clock out there--the one ticking inside bachelors. After decades in which men statistically had the upper hand in the dating world, the demographics have reversed: For a big chunk of the dating pool--people ages 30 to 44--the number of single men and women are now about even, or in some cases, slightly tipped in women's favor. The odds are especially dismal for men looking for younger mates: By 2010, according to the United States Census Bureau, men in their late 30s and early 40s will outnumber women five to 10 years younger by two to one.

`The tables have turned'

What's happening here is a subtle but significant change in the birth rate. While the numbers of boys and girls born every year are roughly the same, the overall birthrate dropped 40 percent from 1955 to 1973. Because more than half of all men marry younger women, that means their pool of prospects shrinks a bit every year. "The tables have turned," says Sherry Cooper, an economist who has written about demographic shifts. "Guys in that 35-year-old range are going to have a harder time."

Matchmakers and dating companies are already seeing the impact. Social Circles, a New York singles group, has seen membership among 35- to 44-year-old men soar 25-fold since it started in 1997, while women in the same age group grew at about half that rate.

At It's Just Lunch, which pairs professionals, the percentage of female membership dropped 9 percent in the last three years. And online firm Match.com is so eager to recruit women, it started a new ad campaign to find more.

"We're all chasing after the same women," says Jim Hague, a 33- year-old Web designer from California who says he got only a handful of daily e-mails from some online services. His female friends, however, got 200 e-mails a day. Indeed, 40-year-old Suzanne Mulroy got so many e-mails from her service that she put it on hold. "I thought I'd get a response," Mulroy says, "but I didn't think I was going to get this deluge."

All of which, of course, is a significant shift from the 1980s; at the start of that decade, for example, there were about 1.3 women for every eligible man from 35 to 44. The odds were even better for the narrower group of men in their late 30s dating women in their early 30s: Almost two women for every single man. Many people still remember the 1986 Newsweek article that famously, if controversially, declared that a single, college-educated, 40-year-old woman had a better chance of being killed by a terrorist than of ever tying the knot.

Odds worse for men

But in the years since, the odds have gotten worse for the one- time supermale. Far from an abundance of bachelorettes, today there's a small shortage -- for every million 30-something women, there is a surplus of 80,000 men of the same age. Men looking for younger women will find even more competition: Within nine years, there will be one woman that's 30 to 34 for every two men 35 to 44, according to one set of projections by the United States Census.

How did this shift occur? For starters, with more women than men on the dating scene, men played the field and postponed marriage-- sometimes until their 40s, much later than previous generations.

Thinking they had tons of options, especially as divorce rates grew, some men got pickier, too, demanding not only good looks but also good jobs from their mates. That narrowed the field even more: By one estimate, men in their early 30s making $75,000 or more outnumber women of the same earning power two to one.

Pop culture's part

All the while, pop culture only perpetuated the belief that men had the advantage, with shows like "Sex and the City" and novels like "Bridget Jones's Diary" harping on themes of the desperate, single woman.

And lots of people still believe it. "Men feel they have the upper hand," says Lisa Doherty, a 40-year-old public-relations executive.

When she has gone on dates, Doherty says men have told her they want a younger woman.

But slowly, evidence of the shift is cropping up.

Take personal ads, the quintessential dating device of the '80s and '90s. While the ratio of men to women placing ads varies from city to city, many towns are seeing notable jumps in male advertisers. At Chicago Magazine, for example, the percent of personals placed by women skidded 38 percent in just two years.

So when will things get better for members of the lonely hearts men's club?

Not until the generation born in the late 1970s and early 1980s comes of marriageable age, according to demographers. By then, higher birthrates will tip the scales back in men's favor. About 2005, experts say it'll be rich pickings for men who now are in their early 20s.