Nearly 20 years ago, in what would become a landmark piece of research into the way men and women look at themselves, researchers had a series of American college students go into a dressing room, get changed into a swimsuit, and take a maths test.

A second series of students also went into a dressing room to take the same maths test, but instead of swimsuits this group wore sweaters.

What the researchers found was that the students' test results were exactly the same regardless of what they wore - so long as the students were men.

For women, it turns out, the act of wearing a swimsuit rather than a sweater makes them far worse at maths.

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The heavy emphasis our culture puts on the appearance of women's bodies, the researchers theorised, was leading the swimsuit-wearing women in the study to objectify themselves. By putting such a heavy focus on their own bodies, the women were using up much of the mental space they would otherwise have applied to the test.

In other words, the objectification of women is a powerfully negative force.

This objectification, and the marketing and advertising industries that have made it so pervasive are one of Steve Biddulph's key targets in his new book, 10 Things Girls Need Most. He understands these problems, he's angry about them, and he wants to do something about them. At times, he sounds less like a parenting guru and more like a revolutionary.

"It's hyper-capitalism that we want to bring down," he says. "There's nothing wrong with starting a company and looking after your employees and developing wealth but now it's hyper-capitalism, it's massively globalised, it doesn't give a damn about the workforce and doesn't give a damn about trashing kids' lives."

Having sold some four million books worldwide over the past three decades, Biddulph is one of our favourite parenting pop psychologists.

His thoughts drop easily and spread widely, often undigested, into the stream of media-driven parenting chat every time he releases a new book. What he says has the potential to make a difference. We should examine it closely.

Biddulph says that, as a psychologist, his goal is to go where the disasters are. He says that in 1997 when he wrote his best-selling book, Raising Boys, for instance, boys were far worse off than girls.

Then, a decade ago, with a teenage daughter of his own, he says he started noticing the "terrible statistics" illustrating the change in girls' mental health. "I thought, 'I can't let that be,' he says. "It was personal and I was angry at the way the world was treating girls."

Biddulph's new book about girls comes four years after his last book about girls, which he says made no difference to what he describes as the "downturn in girls' mental health". That book, Raising Girls, included frightening claims, like this one:

"About five years ago ... we began to see a sudden and marked plunge in girls' mental health.Problems such as eating disorders and self-harm, which once had been extremely rare, were now happening in every classroom and every street. But more than this, the average girl was stressed and depressed in a way we hadn't seen before."

The new book begins in similar fashion, with Biddulph asking us to picture a classroom full of 15-year-old girls, then outlining the series of problems afflicting them: three or four of them, he says, might roll up their sleeves to show evidence of self-harm; three or four will be in the early stages of an eating disorder - most likely bulimia; half the girls will be on diets; four or five will be sexually active, but only doing it to please boys and to be interesting and special; one in five will be on anti-anxiety medication or anti-depressants.

Current statistics show that about one in five young people have some sort of mental health issue. That figure, says Biddulph should be about one in 30.

In the last 20 years, he writes, "childhood and adolescence has changed. It's more lonely, more pressured, and more unkind."

This sort of frightening talk probably helps sell books to terrified parents, but it is it true? And, not unrelated, is it helpful?

At the beginning of the millennium, the University of Auckland launched a study to track the health and wellbeing of New Zealand secondary students over time. Nationwide surveys of students have now been undertaken in 2001, 2007 and 2012.

Over that period, the percentage of girls reporting what the study calls "significant depressive symptoms" has stayed about the same. Between 2007 and 2012 (the only years for which the study has data on these points) the percentage of girls reporting a suicide attempt decreased, and the proportion of girls reporting "good emotional wellbeing" remained steady.

A 2013 study published in the Journal of Paediatrics and Child Health found that about 20 per cent of New Zealand school students have emotional health concerns or concerning "health risk behaviours". That might sound a lot, but if we're talking about an emerging crisis, consider that a 1990 study from a major American psychiatric journal found a quarter of 15-year-old girls were suffering from psychological disorders as outlined in the Diagnostic and Statistical Manual of Mental Disorders, the bible of mental disorders.

In other words, over a span of almost a quarter century, the proportion of troubled teenagers, at least if we take those studies as indicative, has stayed roughly the same.

"I'm always a little bit sceptical of the claim that things have been fine before and now they're suddenly hugely problematic," says Auckland University of Technology psychology lecturer Dr Pani Farvid, in response to Biddulph's claims of a crisis among girls. "We always have challenges and the challenges are different at different times."

As the 2013 study of New Zealand students reported, "most students attending secondary schools were healthy and have few risk behaviours or emotional health concerns. These results support a great body of research over the previous 50 years that highlights normative development during adolescence and that the majority of adolescents navigate this period successfully."

In other words, over the period in which Biddulph says girls' mental health was plunging, statistics suggest that things were about the same for them as they had been for decades and that most girls are, and have pretty much always been, basically okay.

This is not to say there are no problems facing modern girls or that Biddulph's new book is nonsense - it is full of good sense. There is a whole chapter aimed at convincing doubters and sexists of the importance and value of feminism and the women's movement, which he describes as "the most positive event of the 20th century".

There are also chapters with important calls to arms about how to combat the predatory marketers and advertisers who have helped objectify girls and women to such a powerful extent that their brains no longer function as well in test situations when they're made to think about their bodies.

"Our kids are being educated by people who don't give a damn about their wellbeing," Biddulph told me.

"So we have to not just be feedlot animals that wander through shopping malls and do what we're told."

In the book, he writes about the sexism of the toy industry: "When toys are made to reinforce gender stereotypes - and there is a girls' aisle and a boys' aisle in the toy shops - then we have a problem."

But then he stumbles into casual sexism of his own, writing about how girls are "wired" for social cues in a different way from boys, or describing "angst" as, "a word that seems to have been created just for teenage girls!"

The gender stereotyping also extends to parents: "Dads are exciting and lively and interested in different things to Mum."

Farvid laughs when she reads that line: "Like mothers are limited people?" she says. "Both parents have lots to offer and this will depend on their personal sensibilities, rather than their gender."

Biddulph also writes that a girl needs a dad who treats her as important and a mum who has time to talk. When asked about that claim, and why it's not the other way around, he replies:

"It could be the other way round. Sometimes these are based on the current situation, where it's a corrective, like giving your sheep the missing boron they haven't had in the paddocks. In another year, they might have too much. I'm needing mums to slow down because they're the ones who do the heart-to-heart talking, mostly about friendship and things like that, but it could just as easily be the dad in another family and another time. All of these gendered things, you really should have at the bottom of every page: 'But it could be reversed.'"

If all the gendered things can be reversed, is there a need for a book that's specifically gendered at all? If you're putting a disclaimer at the bottom of every page, might that not be some kind of sign?

Here's a story Biddulph told me about something that happened to him recently:
"I was at a big private school and walking with the school counsellor and we had just come out of a function that was for parents and kids and she said, 'Those two ahead of us: can you tell which is the mum and which is the girl?'

"And I couldn't tell. They both had short skirts and they both had long, blond hair. One of them was 15 and one was 40 and I couldn't tell them apart. I think there's something wrong with that. Us adults are supposed to be frumpy, so our girls can feel beautiful without having to spend an hour in the makeup area."

Here's an alternative scenario: a school counsellor and a psychologist are walking behind a father and son who look more or less the same. It seems reasonable to suggest this has happened before, possibly even more often than it has with a mother and daughter. Do the counsellor and psychologist think there's anything wrong with it? Is it worthy enough for them to pass comment? Do they even notice?

Farvid says, "More than 30 years of psychological research and meta analyses of gender difference research has found that there are no major gender differences between men and women when it comes to pretty much everything - cognitive abilities, leadership abilities, psychological dimensions, personality. And one of the hugest facts that we know and teach our first-year psych students, is that there are more variations between men as a group and between women as a group than there is across men and women.

"It's a proven fallacy that there are huge gender differences in our make-up. A lot of it is socially and culturally produced."

On a chapter-by-chapter level, the 10 things Biddulph says girls need most sound a lot like the things boys need too: a secure and loving start, the time to be a child, friendship skills, the love and respect of a dad, spark, aunts, a happy sexuality, backbone, feminism, and spirit.

"To me," Farvid says, "what is more useful is how to raise a happy child, how to raise a well-adjusted child. The only time you could maybe focus on a girl is when you are bringing a gender analysis in, to go: 'Hey, girls in our society get treated a particular way which disadvantages them in comparison to the boys because of unconscious bias. That's a different book."

Biddulph's project in 10 Things Girls Need Most is first to make us feel fear about what's happening to our girls and then to encourage us to take action to overcome it. This is a noble goal, and the way the world gets changed, but it's important to make sure, before everybody gets unnecessarily freaked out, that the fear is well-founded.

"Let's be clear," he writes in the second chapter, "most girls still turn out fine."

This is 40 pages after he's presented his hypothetical classroom of bulimic, self-harming, unhappy, sexually-active-but-not-because-they-enjoy-it, anxious and depressed girls, and it's just a couple of sentences before he writes that one out of five girls will go through a crisis, but will recover with the help of her family, while another will go "so far off the rails that her adult life is really impaired".

Farvid says, "Humans are hugely resilient. This includes teen girls who are struggling. It is usually those with little family/social/community support who go 'so far off the rails that her adult life is really impaired'. Not your average person who has generally had a normal-ish life."

It's important to note, she says, that a history of neglect or any form of psychological, physical or sexual abuse is the biggest risk factor leading to the negative outcomes he discusses.

Near the end of my conversation with Biddulph, he said to me, "You come from 300,000 years of successful fathers behind you and you've got that in your genes. One thousand generations of Bruce dads that at least kept their kids alive, and you've got that genetic heritage, and all the built in sense and intelligence and heart for that. So, if you just keep listening to that, your brain will just piece the things together. Some things will feel right; some will feel wrong."

That's a hopeful message, an uplifting one in difficult times - that we've got what we need to raise good kids already within us - but it's not a message that's going to sell many books.

THE 10 THINGS A GIRL NEEDS MOST

A secure and loving start:

The most important thing for children under 2 is to feel loved and secure.

The chance to be wild, and time to be a child:
Encourage your girl to get outside, be adventurous. Avoid exposing her to the pressures of television and the media in general.

Friendship skills:
Learn the key skills of friendship and be there for your girl - coach her through friendship difficulties and bullying.

The love and respect of a dad:
Fathers make a huge difference to girls. Good fathering involves continual learning. It's important to be vulnerable.

Spark:
Children with a strong interest cope better in almost every part of their lives. Find your child's passion and help foster it.

Aunts:
The advice and influence of older women plays a crucial part in girls' wellbeing.

A happy sexuality:
Pornography is giving kids the wrong message about sex. We need to talk to our girls about sex and make it clear that they don't have to settle for anything less than great.

Backbone and strength:
Girls are born strong. We need to nurture that through role-modelling and taking charge of our own emotions.

Feminism:
Introduce your girl to the feminist movement. Feminism has achieved a lot but there is still a need to fight for change.

Spirit:
Help your girl see the big picture, that life is more than just a pimple or a difficult relationship, that she is part of a larger whole.

WHAT GIRLS THINK

Lauren Parsons, 15

How do you think life would be different if you were a boy?

"I do think the social aspect of it could be quite different [for boys]."

Why do you think that?
"I think there's a lot of pressure on girls to perform well and do well in school and look good, like physical appearance. I'm not really sure about guys, but I think it would be slightly easier."

The pressure on girls to look good - do you think that's harmful?
"Of course it is. Of course it is. In the media, you see all these perfect girls and when we aspire to be like them, it's hard."

Do you feel the need to conform or do you try to fight against it?
I buy the newest clothes when they come out or I buy - I don't know - a cool phone case that I think everybody else would like, but I don't try to change my body. I don't try to make myself look like those kinds of girls. I'm happy with who I am. I'm fine with what I look like."

Do you feel like a lot of people your age struggle with that?
"I think a lot of girls do struggle with it. Like the aspect of not fitting in or being ignored."

Melissa Uren, 13

Are there differences between girls and boys your age?

"There are difficulties both ways."

Is your life harder in some ways than a boy's? Easier?
"I don't know. This is probably being sexist, but girls are slightly more emotional than boys. I guess everything that happens in their lives they times it by 100 and that's how they react."

Do you feel personally you're like that?
"Sometimes I feel like I couldn't overreact anymore than I already have and sometimes I'm like, 'Why don't I care about this?'"

If you were born a boy how do you think you'd be different?
"I think I'm quite competitive as it is but I think I'd go to the next level with that. I'd probably keep going until I've like won every single thing because maybe I'd have been brought up differently and my parents would have introduced me to different things. I don't know."

Isabella Brimming, 12

Would your life have been different if you were born a boy?

"Yeah, it would be a lot better because boys get up and throw on whatever and then run their hand through their hair, brush their teeth and then they're good."

How long do you spend getting ready?
"If I'm just staying at home, 10 minutes; if I'm going to school, half an hour; if I'm going out to the mall or something like that, my friend's house, 40 minutes."

Why do you think girls have to do that and boys don't?
"Tough question. I think it's because girls like being praised a lot. so they have to look their best, so people are like, 'I like your shoes, I like your hair,' and stuff like that. They like attention."

And boys?
"Yeah, they don't care."

10 Things Girls Need Most by Steve Biddulph (Finch/HarperCollins, $30) is available now.