The one thing all happy marriages have in common, according to an expert

A relationships expert believes she has found the key to a happy marriage and it's one trait all happy couples seem to possess.

Rachel Clements, who is an author focusing on love, psychology and relationships, says she recently came to the realisation about what makes a marriage work.

And she says it's not "quality time" or "healthy communication".

Instead, living happily ever after comes down to treating your partner like your best friend.

Clements, who is due to get married soon, says she came across the piece of advice in the book The Seven Principles for Making Marriage Work by John Gottman and Nan Silver.

The overall message in the book is this, Clements says: "All happily married couples exhibit best friend behaviour".

She includes a quote from the book itself: "Friendship fuels the flames of romance because it offers the best protection against feeling adversarial toward your spouse".

In her column P.S I love You, Clements gives the example of dining out with your best friend.

"If you meet a friend for lunch and she forgot to order your sandwich without onion, would you give her 'the look' or start shouting about how 'selfish and inconsiderate' she is for not remembering the way you like your sandwich?

"Of course not; you would you pick off the onions, make a joke about onion breath at the office, and thank her for buying lunch."

Clements calls this "positivity bias" which is the ability to look past small frustrations.

"Instead of thinking your friend was careless enough to forget that you hate onions, you might attribute the careless mistake to the stress she's been under at work lately instead," Clements writes.

And treating your partner like your best friend also helps to diffuse heated moments.

"Happily married couples find a way to calm each other down when an argument is escalating, either by making a joke, apologizing, offering a warm embrace, or simply by acknowledging that you both need some time to walk away and cool down," Clements writes.

Hear hear!