According to Roberts, one in five couples divorce by the time their children are five years old. He thinks this figure may be conservative as it doesn't take into account couples who aren't married. Roberts, who is married with two children, wrote the book with his sister, Sydney-based divorce lawyer Melanie Roberts-Fraser, who is also married with two children. They based many of their observations on interviews with 150 people from around the world.

At the time of this interview, Roberts, a former journalist, was in the midst of relocating to his home country of New Zealand from Britain, where he had worked for the government in media relations and community policy development.

'One of the strengths of the book is the balance between male and female points of view,' says Roberts. 'Mel and I were able to argue out our viewpoints. Sometimes the geographical distance helped. If we fought, we could just go and stew for 24 hours.' Roberts says they argued the most about chapter three, called Mr and Mrs 50s Revival. 'We were coming at it from a male versus female point of view, but we also had to agree with everything that went into the book,' he says.

At the start of the project, they did not understand each other. But by writing the book, the siblings gained a perspective on what parenting was like for the opposite sex. Once children are born, the book says, the equality couples develop over the years can be disturbed by the realisation that their lives are beginning to resemble a cliched 1950s American sitcom. Worse, it's a sitcom without any comedy.

'Maybe it's simply that this is the most obvious way to share the workload,' the authors say. 'Someone needs to earn money and someone needs to look after the baby. It could be that, once kids have come along, people can't help their instinctive reactions to nurture [women] and provide [men].

'There are two issues here. The first is about how much you want - or are able - to juggle work alongside sharing the roles of parenthood. The second is whether you're happy with that split. There's a world of difference between making your decision because that's the way you want to live your life, and feeling forced into it by finances, family expectations, or a fear of 'damaging' your children.'

Roberts says they found that one of the biggest problems was a lack of understanding between the sexes. This leads to resentment. 'I was shocked just how prevalent that resentment is between couples,' he says. It is often difficult for women to understand the huge pressures that men experience, Roberts adds.

'They feel an overwhelming need to provide financially. But they may not enjoy their jobs any more and they may want to spend more time with their child. The man might enjoy his weekends at home with his kids, and resent the fact that his wife is spending more time with them while he's out working.

'On the other hand, women can feel that they are obliged to look after the child all the time. They can start to resent the fact the husband can go to work, enjoy adult conversations and even go to the toilet whenever he wants to.

'Both partners are busy, and when the child comes along, the amount of stuff that needs to be done increases at an exponential rate. It becomes very easy for both partners to look at each other and say, 'You're not pulling your weight'.' Roberts advocates sharing the parental load as much as possible, as well as building a support network and paying for a cleaner. For those who live in Hong Kong, a helper is a good idea.

'The more you can share responsibilities with your partner, the more you'll understand, value and - probably - fancy each other,' he explains. 'Work out ways to increase the amount of time you have, and then sort your priorities to use that time in the best way you possibly can.'

Spending time with each other is one of the most important things, he says. 'Women tend to have higher expectations for their relationships in terms of intimacy and romance. But what both men and women all too often forget is that romance and intimacy need work. You have to prioritise them.'

Marriage counsellor Elizabeth Martyn warns of the troubles that she and many of her colleagues see when parents have put their children first. She stresses the importance of regular chats, developing rituals of being together and snatching moments whenever you can, however low-key or short.

'Taking time together is not selfish, because when your relationship with your partner is strong your children benefit,' say the authors.

'Seeing parents who enjoy being together, who hug and hold hands, helps children feel secure, teaches them about relationships and will help them to find good, enriching relationships themselves.

'It is important to give your children a positive blueprint of being together with another person. The sight of loving parents can lead to a well-balanced child.

'You need time together, time to yourself, time with friends, and each of you needs time on your own with the baby,' says Roberts. 'Ensuring you have time for all these things can be difficult, but it's critical for your own mental health, for your relationship and for your kids.

'Parenthood, and motherhood, in particular, no longer gets the sort of respect it needs. As a result, parents undervalue each other and themselves,' says Roberts.

The Partner-Parent Principle is an e-book published by Untreed Reads 2011. It retails for US$4.99