The 2016 election has left the Liberal-National Coalition with 13 women MPs among its 76 lower house MPs. That’s a mere 17%, down from 20% after the 2013 election.

This contrasts with Labor, which currently has 67 seats including 27 women MPs. A full recount in the seat of Herbert – where it stood a female candidate – will take another two weeks. But it has certainly increased its proportion of women MPs from 36% in 2013 to approximately 40%.

The divide in gender balance between Australia’s two major political parties has steadily increased since Labor introduced quotas for women in 1994.

The Coalition shuns quotas in favour of merit when selecting candidates. Merit sounds fair in theory. But, in practice, evaluations of candidates are open to gender biases that favour men. Without quotas to correct the effects of these gender biases, the under-representation of women Coalition MPs is effectively guaranteed.

More than 40 years of research has shown how women entering traditionally male-dominated occupations, including politics, are disadvantaged by gender biases.

Unfortunately, there is less recognition that the nature of these biases has changed over time. This means we’re focused on tackling outdated forms of gender bias, rather than the biases that are most relevant today.

Without a change in focus to tackle these new gender biases, progress towards gender equality is undermined, and quotas will continue to be needed to ensure fairer outcomes for women in politics.

The traditional bias against assertive women

A long-standing, dominant theory of gender bias is role-congruity theory. This predicts that women are disliked for being confident, assertive and strong – essentially, for acting like we expect leaders to act.

While these leadership qualities are seen as positives for men, they’re negatives for women, because women’s lower social status requires that they be submissive, passive, and deferential.

Tony Abbott’s former chief-of-staff Peta Credlin used this bias to explain why her performance was criticised. She said:

If I was a guy, I wouldn’t be bossy, I would be strong.

This bias also informs the “ban bossy” campaign, led by Facebook COO Sheryl Sandberg, to encourage valuing positive leadership qualities, such as assertiveness and strength, in women and girls.

However, most evidence of this bias against assertive women is more than 25 years old. A widely cited example is a 1990 experiment that found confident, assertive female leaders were less influential and less likeable than tentative female leaders. This bias was not evident for male leaders, who were evaluated similarly regardless of their leadership style.

A new bias against tentative women

Over the last few decades, attitudes towards women’s roles in many western societies have become considerably more egalitarian. There have also been dramatic increases in the number of women who describe themselves in assertive ways; as having “leadership abilities” and “self-confidence”.

Based on these societal shifts, my colleagues and I revisited the classic 1990 experiment to determine whether there is still a preference for tentative rather than assertive women leaders.

Our results revealed the opposite. Instead of bias against assertive women, tentative women were singled out for disapproval: they were considered less likeable and less influential than assertive women.

As in the 1990 experiment, men were equally likeable and influential regardless of whether their leadership style was assertive or tentative.

This means women are now singled out if they don’t act with the strength and assertiveness expected of leaders. This might seem consistent with merit, but only when seen in isolation from the greater leeway given to men.

As all leaders stumble from time to time, this new form of bias suggests women are not given the same opportunity to “grow” in leadership roles as men. To make a bad situation worse, women are also more likely than men to be interrupted when they speak, so being evaluated as confident and assertive is already more challenging for women than men.

A bias against unfriendly women

Our findings are straightforward, but they were difficult to publish. Even though our research followed the same procedure as the 1990 study to ensure reliable comparisons, many reviewers argued that bias against assertive women still exists. The problem was, we hadn’t portrayed assertive female leaders as unfriendly.

More recent research purporting to show continued bias against assertive women has focused on women who are not just assertive, but unfriendly or even ruthless – willing to step on toes in order to get ahead. But when these women were evaluated more negatively than men, it was their unfriendliness, not their assertiveness, that was the source of bias.

Women are held to higher standards of kindness and friendliness than men. This places them at a disadvantage in domains such as politics, where stepping on toes is believed to be critical for getting ahead; as the saying goes:

If you want a friend in politics, get a dog.

But bias against women for being unfriendly should not be confused with bias against women for being assertive.

Like tentativeness, this bias will be hard to overcome. While being unfriendly or ruthless might be important, it’s not a celebrated leader attribute.

So, believing that Credlin suffered bias for being assertive seems unfair in a way that believing she suffered bias for being unfriendly does not – it’s a negative quality, so it seems a valid reason for criticism. The problem, and bias, is that men are not criticised as harshly as women for the same behaviour.

Challenging new forms of bias against women

The belief that bias against assertive women is responsible for continued gender inequality has persisted beyond researchers’ ability to find more evidence for it today.

Critically, continuing to focus on overcoming bias against assertive women may actually be counterproductive. This is because positive messages about the need for “strong women”, exemplified by Sandberg’s “ban bossy” campaign and others like it, may be feeding into the new intolerance for women’s tentativeness.

It’s obviously unfair to penalise women for acting assertively, a quality we expect from our leaders. But it is precisely because bias against women for being tentative and for being unkind are harder to see as unfair, that they are likely to be more difficult to overcome than bias against assertive women.

We may think we are judging women on merit when we penalise them for tentativeness and unfriendliness, but we’re still being biased because we’re not applying the same standards to men.

If we believe women should be able to compete on an equal footing with men, finding ways to challenge bias against tentative and unkind women should now be a focus. Until then, relying on “merit” won’t work, so quotas may be the only way for the Coalition to ensure their party gives women a fair chance of becoming MPs.

This piece has been updated since publication to amend the number of women Coalition and Labor MPs.