You are probably as fatigued as I am about this topic: why are women still lagging behind men at law firms?

For many years, I've looked for explanations: is it simple sexism? The dearth of role models and mentors? The clash of work and family? Are women too complacent? Too bitchy? Are they wearing the wrong shoes?

My assumption has been women want the brass ring of partnership, but hit hurdles along the way. What I haven't explored as much is this: maybe women just aren't interested. I've seen this first-hand; not only from women on the ascent, but also those who've made partner and want out.

Simply put, women aren't enamoured with Big Law, and they're bolting - leaving in greater numbers as they move up in seniority. According to the American Bar Association, women over 40 represent 41% of lawyers at law firms, and only 27% are women over 50.

Are women dropping out because law firms are still de facto boys' clubs? Or is it something else – such as a different conception of a satisfying career?

The distinction can be fuzzy. "It's difficult to say, but I think the two go hand in hand," says Kate Hooker, a former Cadwalader Wickersham & Taft associate who's now senior counsel at startup Greenhouse Software. "There were so few women in leadership that I didn't see what my own path would be."

It seems that women are keeping their eyes open for other gigs. "There are several pivotal moments," says career coach Elena Deutsch (pictured), who runs Women Interested in Leaving Law. She says women tend to explore their options after the third year and at the seventh or eighth year, "when they're seasoned but still young enough to try something new." These moments continue, she says: "Women in their 50s see 20 years ahead of them and want to do something with more meaning."

The hypocrisy of saying we'll support you, then turning around and saying you didn't reach 80% – that was the last straw

It doesn't help that firms sometimes turn young women off, even when they think they're doing the right thing. Alix Devendra says she had every intention of returning to work at Nixon Peabody in San Francisco after her pregnancy. The firm encouraged her to go on an 80% schedule, but put her on notice a month later that she wasn't reaching her mark. She quit the next day. "I didn't like the tone, subtext, culture," she says. "The hypocrisy of saying we'll support you, then turning around and saying you didn't reach 80% – that was the last straw."

Stacie Collier, an employment law partner in Devendra's group at Nixon Peabody says, "We were incredibly disappointed to see her leave," She adds, "We tried to help her, and I'm sorry that she didn't feel supported. She was someone we wanted to retain - high potential and really smart." Collier says the firm now has a parental ramp-up programme whereby associates are paid 100% of their salary for 80% work.

Despite getting all the signals she was likely to make partner, one former associate at a big Los Angeles firm quit in her eighth year. "I worked all the time," she says. "If I had seen a path to a reasonable lifestyle, I might have stayed." She says she had burned out on firm life: "I'm good at my job, but I didn't find meaning in it."

Indeed, "meaning" is what women often say they want - and what's lacking at big law firms. "I've been doing this for 20 years, and I'm doing it at a very high level, but is this what I want to do for the next 20?" asks one female partner at an top 100 US law firm. "What's so rewarding about slaving away for clients who think they own you?"

It seems women want more – and not just money - than law firms are offering them. Consultant Melissa McClenaghan Martin, a former Fried Frank associate, says women want meaning and they find it "through business development, and deep client or sponsor relationships."

But it's tougher for women to develop those relationships than it is for men. Somewhere along the line, Martin says, women are taking themselves off the partnership track. "It happens because there are more reasons for women to opt out than there are to opt in."