Between you and me - Sydney Morning Herald

Careless whispers … why is it so hard to keep a secret? Photo: Getty Images

When I was six, I had a secret. A big one.

I remember the superior, impatient feeling it gave me. The way the words quivered at the back of my throat. The secret made me feel kindly and patronising towards my younger sisters. Their darling, clueless little faces. They didn't know. But I did know. All I had to do was speak and I could change their worlds. It was exhilarating, and also excruciating.

I went to school and shared the secret with my whole class. "Just don't tell your little brothers and sisters," I warned my classmates. The next day, my teacher phoned my mother to say there had been complaints from parents. Some kids went home in tears.

(I won't reveal the secret here in case any gifted and talented four-year-olds are reading. I will say only this: it involved highly defamatory remarks relating to the identity of Santa Claus.)

I had failed my first secret. I was deeply ashamed, so much so that never again in my life did I ever divulge ... well.

It wasn't my fault. Apparently, the brain simply doesn't like keeping secrets. American neuroscientist David Eagleman explains that secrets create a "neural conflict". One part of the brain is desperate to spill the beans. The other part wants to do the right thing.

Research has found that carrying a secret actually feels like you're carrying a physical burden. When people confess or write down their deepest-held secrets, their stress-hormone levels drop measurably.

The reason people share secrets with strangers on aeroplanes is because they can alleviate the conflict without consequences.

I can attest to this. I once sat next to a man on a flight from Sydney to LA. The things I could tell you about David's girlfriend and that time she ... well.

Anonymity also accounts for the success of the PostSecret website, which began as a community art project and became a global phenomenon. Its creator, Frank Warren, handed out postcards to strangers and asked them to write down a secret, and mail it to him anonymously. Since 2004, he has received more than half a million secrets, ranging from the mildly amusing - "I gave my vegetarian sister a meal with beef" - to the mildly horrific: "Sometimes when I'm having sex with my wife, I'm thinking of my mom." (Maybe he's just thinking he needs to call his mother?)

I'm the eldest of six, and we've always been slapdash with our secrets. "Oh, by the way," someone will casually mention, "I think that was meant to be a secret." Eventually we discover everyone knows the secret except one sibling, and then we have to keep secret how long the rest of us knew the secret.

One research study of 3000 women (strangely enough, carried out by a wine company) found that the average woman can't keep a secret for longer than 47 hours. (They didn't include men in the study, possibly because men are often just not that interested. It can be unsatisfactory to share secrets with men. They don't gasp in the right places. "Do I even know this person?" yawns my husband.)

"I always have to tell at least one person," admits a friend. "Just for the sheer visceral thrill of it."

I understand. Once a friend sat at my dining-room table and revealed a juicy secret she'd kept for a decade. I gasped. It was such an unexpected plot twist in the narrative of our lives. It was like finding out that Luke Skywalker's father was Darth Vader. The words had barely left her mouth before I was dialling the number of a mutual friend. You couldn't have paid me to keep that secret.

Some people do take secrets almost to the grave. In researching my new novel, I've been reading about deathbed confessions. There was Christian Spurling, who confessed on his deathbed that he helped to fake a notorious photo of the Loch Ness Monster.

A famous songwriter dying of cancer wrote a letter admitting, after years of adamant denials, that she had plagiarised a lullaby melody.

Then there was the hapless man who, after suffering a stroke, confessed he'd killed his neighbour 30 years earlier. The only problem was that he didn't end up dying. After he was released from hospital, he went straight to jail.

I wonder, did these people confess to clear their conscience, or was it just for the pure pleasure of finally sharing a really good secret?

I can't imagine I'll have many secrets left by the time I'm on my deathbed. "Oh, I've known that for years!" one sister will say. "Seriously, that was meant to be a secret?" someone else will ask. And just as I slip away, I'll hear the outraged shriek of the sibling who missed out: "How come nobody ever told me?!"

Liane Moriarty's latest book, The Husband's Secret, is published by Pan Macmillan.