Why do some children succeed while others fail? A new book, How Children Succeed, says cognitive skill - the kind of intelligence that is measured in IQ scores and exam results and that includes the ability to read, write and count numbers - is an important factor, but the qualities children really need are persistence, curiosity conscientiousness, grit and optimism - collectively known as character.

American author Paul Tough says there is growing evidence parents are worrying too much about their children's academic achievements and not doing enough to help develop character-forming traits.

Too many children, he says, lead coddled sub-urban lives, shielded from adversity, and knocked sideways when they have to confront real problems in adulthood.

"In the past couple of decades we've focused way too much on cognitive skills and intelligence as the one predictor of success and I think we've ignored this other set of skills," he says.

"The scientists and teachers that I am writing about in this book are showing evidence, both in the classroom and in the data, that character skills are at least as important as IQ in terms of a child's ultimate success and are quite likely more important."

The good news, he says, is that children can learn character within the family environment and teachers can help as well.

Tough acknowledges that the idea of character being part of education is not new and that the educational establishment has gone back and forth in emphasising the importance of character versus intelligence.

"I think right now that the pendulum has swung about as far as it can go in the direction of emphasising IQ and now I think it's starting to swing back," he says.

In the 1990s, he adds, scientists identified the first few years of life as crucial in brain development, and specifically cognitive development.

"Parents thought they had to start very early in terms of stimulating their children cognitively and so that created the whole industry of Baby Einstein, Mozart CDs in the maternity wards, and flash cards in nurseries.

There are now schools (Junior Kumon) where you can go when you're three and do maths worksheets - a terrible idea, Tough says.

In America, he adds, this desire to give children academic leg-ups as early as possible is called the "rug rat race."

At the same time, standardised tests, which are a feature of the American and British education systems, have also helped keep the focus on cognitive skills.

Parents who may be high achievers themselves, and who want their children to be as successful and driven as they are don't like the idea of failure, but Tough - a writer for The New York Times Magazine - says children growing up in affluent homes need to have a moderate amount of adversity in their lives. "In trying to protect our kids from bad things we're actually harming them," he says. "We're denying them these opportunities to do a little failing and to develop their character."

Example might be a mother rushing to intervene when her toddler is smacked by another in the sandpit, he says, or going to school to deliver homework that a child has forgotten.

"The protective instinct in parents is strong," he says, but sometimes it is better to let children find their own way or make mistakes. "A child might lose some marks if he's left his homework behind but he'll learn something."

The author doesn't want only to address the children of affluent families and how they succeed in life. Children from poorer backgrounds are also disadvantaged by not being encouraged to build character skills.

He has reported on the problems facing children in poverty-stricken neighbourhoods in America, and says such children are surrounded by too much adversity.

"For kids at the bottom end of the income spectrum it's not building their character. It's hurting them, and making it hard for them to feel confident and optimistic and to get through life."

Tough says that it is well-known that disadvantaged children don't do as well as children from more affluent homes.

"The science, which is absolutely compelling, shows the neurological and physical impact of growing up in a chaotic environment and what stress does to the developing brain," he continues.

Stress affects the part of the brain called the prefrontal cortex, he writes in the book, which is critical in self-regulatory abilities. Children in disadvantaged environments find it harder to concentrate, sit still and follow directions. As a result, they often perform badly in schools.

Tough reveals in How Children Succeed that he dropped out of college 27 years ago. "The experience of writing the book and reading all of this research gave me two ways to think about my own dropping out," he says.

"The less generous way to look at it is that kids who don't persist in college are lacking in some key character strength like grit or persistence and I think that was true of me at that time.

I was one of those high-achieving kids who went to a pretty competitive high school and was pushed very hard but never really challenged.

"So I got to Columbia University and I felt that it was going to be more of the same, another four years of working hard and making the next goal, but not pushing myself in any interesting way."

Tough quit university, bought a bicycle and rode 2,000 miles from Atlanta to Halifax in Canada. "Looking back on it, I was trying to give myself a challenge and put myself in the path of failure a little more. I learned a lot in terms of my own character. It felt like a really transformative experience and I think it set me on a positive path."

The evidence in How Children Succeed, Tough says, should convince parents that character really matters.

"I think there are lots of ways to pull back from that overprotective and over-involved parenting that is prevalent right now and I don't think it's hard to change. I think kids want to be more independent.

"The evidence is pretty clear that it's the kids who don't have the most character strengths who end up living on their parents' couches at 30. In terms of wanting success for your kids, I think parents are going to realise that this is what's going to make their children more successful in their jobs and their lives and it's going to make them happier, more fulfilled and more confident."

Tough recounts how his research has affected the way he treats his three-year-old son, Ellignton.

"A few weeks ago it was his first long ride on his scooter and he fell off a few times and wasn't hurt but I did feel myself holding back from rushing over to try to help him and pick him up," he recalls.

"He looked at me as if to say: "hmmmm, this is how we're doing it, huuh? But I think once he got used to the idea that I wasn't going to help him, he was so proud of himself for learning how to do this important new skill on his own."