"A brilliant woman is a plague," lamented Jean-Jacques Rousseau. "A plague to her husband, her children, her friends, her valet, everyone." Rous seau would not be happy if he cast his eye over the think tanks of the centre left today, as they are experiencing an epidemic of femininity.

So complete is the feminisation of progressive think-tank leadership that when Jennifer Moses, former head of the Liberal Democrat-leaning think tank CentreForum, was scooped into the new Downing Street talent pool last month, interest was sparked in her nationality (American), her party allegiance (non-Labour) and her Goldman Sachs-generated wealth (gigantic) - but not her gender. Meanwhile, Demos is run by Catherine Fieschi; the Institute for Public Policy Research (IPPR) is co-directed by Lisa Harker and Carey Oppenheim on a job-share basis; and the Social Market Foundation is run by Ann Rossiter. At least on the non-Tory side, all the top wonks are women.

This feminisation is, in part, a simple reflection of the general rise of women in public life. "Part of it is purely statistical," says Fieschi. "There are simply more women in public positions." But Fieschi, and others, think there may be more to it than that. These organisations are important ideas factories for progressive politics, but are also independent organisations at some distance from the dysfunctional, tribal, macho culture of Westminster and Downing Street. As such, they provide perfect platforms for women who want to make an impact on politics without having to play the boys' games.

"The kinds of demands that being a special adviser makes on your life are ones that women in particular might reject," suggests Oppenheim. "A think-tank role gives you more control over your time." Journalism and research organi sations also provide perches for high-profile women such as Polly Toynbee at the Guardian and Julia Unwin, director of the Joseph Rowntree Foundation.

"It is a cause for celebration to see women in these jobs," says Katherine Rake, director of the Fawcett Society, which campaigns for women's rights. "But if you look at Downing Street or even the cabinet, where the number of women has actually fallen, you see that governmental power remains mostly male."

Access to key government jobs still appears to require a Y chromosome. Moses will be one of the very few women in Gordon Brown's No 10, alongside Oona King, the former MP with the tough task of making the government more female-friendly.

The rise of the wonky women can also be seen as part of the evolution of the think tanks themselves. The principal progressive think tanks have been through three stages, suggests Fieschi. The first stage was a "blue-sky" period, when idealistic directors were encouraged to think boldly about a progressive future. James Cornford, the first director of IPPR, was an academic by background and a marvellous iconoclast. He cared little for what ministers, or shadow ministers, thought. But this was at a point when Labour had been out of power for three terms, and was keen to demonstrate that it was fizzing with ideas about how to make the nation better.

The second stage came in parallel with Lab our's terms in office. With the party securely in power, the priority for centre-left think tanks was to provide realistic, grounded, sensible policy advice. This technocratic era was unsurprisingly marked by a revolving-door relationship between the think tanks and the government. Geoff Mulgan, the founding head of Demos, went on to run the prime minister's Strategy Unit and then the No 10 Policy Unit; Phil Collins, a former director of the Social Market Foundation, became chief speechwriter to Tony Blair.

IPPR acted as a training college for Labour politicians and advisers. Patricia Hewitt, a former deputy director, became an MP and then cabinet minister. So did David Mili band. And James Purnell. Miliband was formerly Blair's head of policy, and that job was subsequently filled by Matthew Taylor, who had previously run IPPR. It is now held by Dan Corry, IPPR's former head of economics. Indeed, to list the number of IPPR staff who have worked for a Labour government, or vice versa, would take the rest of this article. (And yes, to declare an interest, me too.) Chris Powell, when chairman of trustees of the IPPR, boasted that IPPR actually had two departments: the research and development section in Southampton Street (the think tank's HQ) and the "applied department" in government itself.

Now, however, the progressive think tanks are entering a third stage, one resembling the first in its emphasis on free thinking. Now that Labour's hold on power is tenuous the rules have changed again. The value of ideas has risen, both for a government in desperate need of intellectual reju venation and for an opposition anxious to prove itself fresh, modern and ready to govern.

This requires a different style of leadership, and especially a greater openness to work across party lines. It is likely that this would be happening under male leadership, although perhaps to a lesser extent. "Let's be honest - it is partly a reflection of the political situation," admits Fieschi. "Having said that, I do think that women may be slightly better at handling ambiguity, acting as critical friends, and perhaps working with different partners in a different way."

Oppenheim agrees, though she is wary of being too deterministic about the gender element. "I think there is a likelihood that women are more consensual in their approach, and less bound to a particular political party." The job-share arrangement at IPPR (unprecedented in UK think tanks) is itself a powerful symbol. "Lisa and I are often asked how we can possibly share leadership," she says. "It is a different way of leading, and for us it is a very powerful one."

One of the other factors reducing the level of tribalism in the think tanks is the career stages and ambitions of the new breed of women leaders. According to Fieschi: "The women who run these think tanks have no ambition to end up in the government's Strategy Unit. They have either already been in government, or have no interest in being in government."

It is certainly true that Rossiter, Oppenheim and Harker have all been government advisers, but they can now be considered as in the post-hack, rather than pre-hack, stages of their careers. Consequently, these female leaders have more latitude. They tremble less when a Labour minister rings to complain about a critical report.

As a result, the Conservatives, energetically triangulating to prove their changed condition, are mustard-keen to work with the centre-left think tanks. IPPR submitted substantial evidence to the Conservative "quality of life" task force, is working with the Liberal Democrats on immigration and is trying to build a cross-party consensus on climate change. Demos, while anchored in progressive politics, is also less prescriptive about where it is to be found: "I am quite happy to work with Steve Hilton [David Cameron's key strategist]," says Fieschi.

IPPR had a major presence at the Conservative party conference for the first time last year. Demos is taking a more dramatic step away from party politics and eschewing the drunken party conference scene altogether. Instead, there will be Demos events at the Hay and other literary and cultural festivals around the country. For Fieschi, shaping radical ideas and building a consensus for progressive change is now a more subtle and complex game that reaches far beyond political party.

In 1997, the era of the Blair Babes, it seemed as if politics itself might be on the cusp of a new, more feminine era. It hasn't quite worked out like that; women are in short supply in senior government roles. But they have scaled the commanding heights of the progressive intellectual powerhouses. Rather than being a "plague", these brilliant women may be the medicine that progressive politics urgently needs.