Author Pamela Haag reckons that modern marriages can be organised into five categories.

In Marriage Confidential: The Post-Romantic Age of Workhorse Wives, Royal Children, Undersexed Spouses and Rebel Couples Who Are Rewriting The Rules, author Pamela Haag outlines five different kinds of modern marriages, revealing how it is often security and children that keep a couple together, rather than love.

Among the “so-so couplings” she describes are the “workhorse wife” - when a woman’s career funds the dreams of her “Tom Sawyer” husband - and “Ed McMahon Syndrome” in which a spouse will agree with their partner just to keep the relationship balanced.

Haag, 45, who herself is married, told the Today Show: “I came up with this field guide to organise all the different ways that marriage is changing today.”

A large part of the book is dedicated to what she calls a semi-happy marriage.

She explained: “A semi-happy marriage lives in a shade of grey. It’s not miserable at all, nor is it all that successful.”

Haag admitted that even she herself was semi-happy in her marriage, writing in the book: “I have a nice marriage, a lovely husband, but you never know. On other days and in other moments I think this could very well be the last year of our marriage.”

But she added in an interview with Glamour that her admission had benefited her relationship.

“I do think that our marriage is better because of this,” she revealed. “We’re paying more attention to each other. I think my husband’s very brave to allow me to write about it!”

She believes that children are the new spouses, having migrated “from chair to throne”, in her section on the Parenting Marriage.

Haag, who has one child, said that while children are often the one remaining reason for couples to marry and stay together, being a parent can overwhelm a marriage because children are so much more socially included these days.

She writes: “Parenthood is swallowing marriage… Children are at the centre of a family now.

“From a historical perspective it’s a departure. Go back to the 1950s and husbands and wives had many different roles - as hostess, decorator, breadwinner, volunteer. They weren’t just parents. Today, parenting is the sole priority… It crowds out other functions.”

Echoing the trend of “semi-happiness”, Haag describes the trend for “Ed McMahon Syndrome”, when a spouse follows the “You are correct, sir!” strategy for getting through disagreements.

(Ed McMahon was Jonny Carson's sidekick for 30 years on NBC's The Tonight Show).

Then there are the “Semi-married”, who are stuck in an uncomfortable limbo between marriage and divorce.

Haag, a historian, based her research on a study of almost 2 000 people and even openly created online dating identities and personal ads with her husband for the book.

Her survey revealed startling findings. It showed that a third of married people say they wish they’d never married, and that as many as 65 percent of wives and more than 85 percent of husbands would have affairs if they knew they wouldn’t be caught.

She told the Huffington Post that due to online dating sites and booming technological advances, trusting your spouse not to cheat has become more difficult.

“When we do get married, social media put new stresses on monogamy - it’s easy to sign on to an online flirtation on Facebook or through email, and you can even use an online affair-finding site to find people to cheat with,” she said.

“If we do slip into infidelity, we could have what I call an ‘Avatar affair’ where the lovers communicate online, but might never meet. It’s all smoke, no mirrors.”

For some couples she says the answer is to become a “new monogamist” - her term for what is effectively an open marriage, or an “Oreo Marriage” where the couple looks traditional from the outside yet might enjoy swinging, in private, for example.

“Many marriages are practising ‘Free Love Version 2.0’, in which spouses are actually trying to be honest and have decided that their marriage can tolerate some other attachments.” - The Independent

* Marriage Confidential: The Post-Romantic Age of Workhorse Wives, Royal Children, Undersexed Spouses, and Rebel Couples Who Are Rewriting the Rules by Pamela Haag (HarperCollins)