It's no secret that daughters have a special relationship with their dads. But their influence may be even more profound. Two new studies suggest that daughters - who are more likely to favour higher taxes and public spending - cause fathers to become more compassionate and move to the left

Long before he had a tribe of children to call his own, Brad Pitt broke down in tears on primetime television when talking about how much he wanted to have a daughter. "Little girls, they just crush me - they break my heart," he said.

Now with three daughters - and three sons - Pitt has more than achieved his dream. He will also have had ample opportunity to experience the powerful influence that little girls have on their fathers: the most masculine man will learn to love pink, take part in endless games of dressing up, and even bake fairy cakes if that's what his little princess desires.

According to new research, however, daughters have an even more profound effect on their daddies: fathers, say Professor Andrew Oswald, from Warwick University, and Dr Nattavudh Powdthavee, of York University, will shift their political allegiance for their daughters. Using research from the British Household Panel Survey, the two economists found that the more daughters there are in a household, the more likely their father is to vote Labour or Liberal Democrat.

In an unpublished article that has been submitted to an economics journal, the pair state: "This paper provides evidence that daughters make people more leftwing, while having sons, by contrast, makes them more rightwing." The academics go on to speculate that leftwing families become so through a predominance of females down successive generations, as anecdotally evidenced by Tony Booth and his many daughters, or the late John Smith and his three.

Compared with males, females tend to be more in favour of higher taxes to fund provision such as the NHS. Higher taxation also affects them less since they tend to be in a lower income bracket. "As men acquire female children," said Oswald, "those men gradually shift their political stance and become more sympathetic to the 'female' desire for a ... larger amount for the public good. They become more leftwing. Similarly, a mother with sons becomes sympathetic to the 'male' case for lower taxes and a smaller supply of public goods. Political feelings are much less independently chosen than people realise," he added. "Children mould their parents. It's so scientifically attractive because it's out of the parents' control - whether they have a boy or a girl."

The researchers have been accused of propagating gender stereotypes and of perpetuating the idea that women go in for softer politics than men. But their work mirrors recent findings by American researchers, who looked at the voting records of US congressmen before and after having children. In a joint paper, sociologist Rebecca Warner from Oregon State University and the economist Ebonya Washington from Yale University found that support for policies designed to address gender equity is greater among parents with daughters. The result, they say, is particularly strong for fathers.

Because parents invest a significant amount of themselves in their children, the authors argue, the anticipated and actual struggles that offspring face, and the public policies that tackle those, begin to matter more to those parents.

They add that people who parent only daughters are more likely to hold feminist views, with congressmen who have female children tending to vote liberally on issues from reproductive rights and teenage access to contraceptives to flexibility for working families and education. Oswald said: "I argue that these results generalise to voting for entire political parties. We document evidence that having daughters leads people to be more sympathetic to leftwing parties. Giving birth to sons, by contrast, seems to make people more likely to vote for a rightwing party."

Oswald found that, among parents with two children who voted for the left (Labour or Lib Dem), the mean number of daughters was higher than the mean number of sons. The same applied to parents with three or four children. Of those parents with three sons and no daughters, 67% voted for the left. In households with three daughters and no sons, the figure was 77%.

There are those who dispute the interpretation of the findings, but evidence nonetheless abounds of daughters who have tamed the most manly of men. When rapper Sean (P Diddy) Combs and his girlfriend Kim Porter had identical twin daughters two years ago, the New York musician admitted that "having girls changes you for the better".

Sylvester Stallone, star of the Rambo and Rocky films, altered his career path after the birth of his daughter, Sophia, in 1996. "The birth of my daughter was a subtle indication of which way I should go. I want to get back to more emotional, character-driven films," he said.

While not claiming to have shared the testosterone-defined personalities of Combs or Stallone, Colin Brazier, a presenter for Sky News, also admits his five daughters have softened him. "I am definitely a softer man because of my daughters," he said. "I think being the father of girls has made me more empathetic and more skilled in certain forms of negotiation that are particular to bringing up girls. There is something about watching my daughters interact with each other. Girls are so co-operative with each other whereas boys have more kinetic energy. Because of this, I think it's possible to enjoy daughters in ways you don't enjoy boys, who need so much more intervention. Because of this, having daughters has made me more reflective."

Margaret McAllister, a psychologist who has spent the past 25 years specialising in child development and family functioning, said the studies revealed the importance of home environment over work and social influences, in forming an individual's personal and political views. "It makes perfect sense that, if a parent has children of one gender, they are more likely to be aware, alert and sensitive to issues that affect that gender."

But Arthur Mayne, a biologist who has three sons aged from 18 to 12 years, disagrees. "This is a simplistic scientific that could be accused of gender stereotyping, especially the idea that women are more likely to be softer politically than men," he said. "While it is true that men biologically determine the sex of their children, recent studies seem to prove that women with higher levels of testosterone - who are more likely to display dominant, positive behaviours - seem to produce more sons than daughters.

"Women with lower levels of testosterone, who are more likely to be empathetic and better listeners, tend to produce more daughters. It could be that the women who are most likely to produce daughters pick a partner who is closer to her more empathetic attitude to life. Hence people who are already more liberal may produce more daughters and those who are already conservative may produce more sons."