FORTUNE — Call it the New Girls’ Network. At professional services firms in fields like law, accounting, and advertising, the partners who control the biggest share of revenues wield the most influence. For those on their way up, getting there has traditionally meant working closely with senior partners on key client accounts, with the aim of taking them over when the partner retires.

For women, that happens less often. So female fast-trackers have found a different strategy: rainmaking, or tapping their networks outside the firm to recruit new clients — many of whom are women, too.

That’s the finding of a new study co-authored by Forrest Briscoe, who teaches management and sociology at Penn State’s Smeal College of Business, and published in the current issue of the Journal of Professions and Organization. Firms that want more diversity at the senior partner level, the study says, can increase revenues at the same time by encouraging even more of the rainmaking that many female junior partners already do.

MORE: The BIG risks in the Nasdaq

The researchers analyzed detailed data from inside a major corporate law firm from 1993 to 2007. They found that both the inheritance and the rainmaking paths to senior partnership are risky. Hoping to inherit clients from a higher-up usually takes years of work on those accounts, with no guarantee of a payoff if the senior partner then decides to pass the baton to someone else.

Rainmaking is even riskier, the study says, in part because it takes just as much time and effort but isn’t billable unless and until a new client signs on. So a junior partner who invests in building and cultivating a strong outside network “will risk appearing less productive and profitable” in the short run.

Even so, for women, rainmaking is often the only option, for reasons that are familiar by now. Most senior partners, and their clients, are still white males who, left to their own devices, usually take other white males under their wing and groom them as successors.

“It’s not a conscious decision based on discrimination,” Briscoe says. Rather, he says, it’s a question of what sociologists call homophily, or “a matter of people’s ‘comfort zones,’ where we tend to gravitate toward people who are more like us, or who remind us of our younger selves.”

Luckily for women, homophily’s effects operate outside their firms as well as inside. “There are many more women now who are in-house counsel at big companies, or who are in other roles where they’re choosing professional services firms for their companies,” Briscoe notes, and that often works in female rainmakers’ favor.

MORE: Mideast airlines up the ante in the luxury arms race

For firms that want to shake things up and change, for example, how clients get passed down through the ranks, Briscoe has two suggestions. First, he says, “big partnerships don’t always look at how clients are inherited. So taking a close look at exactly how those decisions are made would be a good first step.”

Second, he says, firms might want to support the rainmaking path to senior partnership by allowing more time for networking outside the office. That may well mean recognizing that fewer billable hours now can mean higher revenues, and more women senior partners, down the road.