In her memoir When I Grow Up, Bernice Rubens describes an encounter she had, age six, on her way home from school. ‘How’s your Mummy and Daddy?’ asked her neighbour.

‘They’ve gone to South Africa,’ replied the six-year old Bernice. Why did she say it? Her parents don’t have money for the fare to London, let alone Johannesburg, and as soon as the words pop out of her mouth, she knows that trouble lays ahead, that Mrs Comely will tell her mother that ‘little Bernice has a vivid imagination’. Fortunately for Bernice, the Comelys moved away, but the memory stayed. ‘I was simply a liar by nature, and the Comely story was a whopper. Though I didn’t realise it at the time, I was happily at home with menda city. It was less boring than the truth.’

I, too, was a child at home on the range of mendacity. On a train journey to my grandparents I was nine and allowed to travel solo I told a fellow passenger that I was a German orphan. This required a German accent (I’d certainly never heard one) to be sustained for the two hours to the destination of my amazed companion. Waiting for her on the platform was the familiar figure of Bishop Grey, who two weeks before had confirmed my sister and would, God willing, confirm me. I stuck to the truth for the rest of my journey.

But the overactive imagination (a gene-rous definition) has plagued me all my life. In my family, it’s called the ‘Mississippi truth’ and covers terrains as rocky as oaths on passport forms, as dusty as excuses for unanswered letters. Like a gambler in a 12-step programme, I renew regularly my vows of truth, and when I stray, my defence is ‘nobody got hurt’. Which cannot be said about the vaster landscape of lies that surrounds me: Weapons of Mass Destruction, pension promises and marital infidelities. In truth, I feel as virtuous as a nun. But as luck would have it, my family sees through me. Their eyes close shut when they hear me say into the telephone: ‘I’m afraid Carla Carlisle is in a meeting.’

And this week the moral thorn drew blood when a hefty red notebook arrived: University Application Advice 2006-2007. As the mother of a 17-year-old who longs to read history at Oxford, this document was compulsive reading. The bitter truth begins on page 69: ‘Oxbridge applications: Most applicants fail: 75% of applicants to Oxbridge are rejected and 95% of these achieve AAA at A-level. In 2004, Oxbridge rejected 10,000 AAA candidates’. Not that the brutality of the statistics shocked every Christmas is filled with news of friends who have and (mostly) have not got in.

Instantly, I tell Sam that he must not apply to Oxford to read history. ‘You have to apply to read theology.”But I don’t want to read theology’. I remind him of Matthew, ‘most talented economist in the history of Westminster School’, turned down by Oxford to read economics, accepted to read engineering. ‘Anyhow,’ I say with authority, ‘theology is the new history.’ But Sam hasn’t got the Mississippi gene. He insists he’d rather read history at the University of Nowhere than be dishonest. He mutters softly about the Jeffrey Archer School of Motherhood.

Now I worry that my advice won’t be sought when he comes to write his Personal Statement. How his love of fly-fishing doesn’t interfere with his desire to find a cure for leprosy. That his exeats are not spent watching endless episodes of Friends, but reading Gibbon’s Rise and Fall of the Roman Empire. Perhaps this is a sign that he exceeds his breeding, a sign that the truth gene is dominant.

This article first appeared in Country Life magazine on March 23, 2006.