An optimist and a pessimist are shipwrecked, and wash up together on a desert island. The optimist decides that, though there's no food or fresh water where they are, it's worth exploring the other side of the island to see whether there's anything edible or potable there.

"Come on!" he says. "Where there's life, there's hope!"

"No there isn't," says the pessimist. "This is a desert island. There's nothing here. We might as well just lie down and die."

"Well, come on, it's worth a try, isn't it?"

"No it isn't. We're going to die."

So the optimist, not a mite discouraged, sets off by himself for the other side of the island, where he is eaten by a crocodile. And the pessimist starves to death.

Those of us who nod with sad recognition at this story have fresh cause for gloom with the arrival on our coffee tables of the latest fun-filled issue of the Proceedings of the Royal Society. Psychologists have conducted a study of more than 100 people and claim to have identified an optimism gene.

"We've shown for the first time that a genetic variation is linked with a tendency to look on the bright side of life," says Elaine Fox of the University of Essex. "This is a key mechanism underlying resilience to general life stress."

The fates of our unfortunate Crusoes are determined not, then, by their characters but by the genetic determinants on their characters. The crocodile victim has two long variants of a particular gene, and that's that. He's always going to be the optimist.

The study sounds, it has to be said, a bit preliminary. A sample size of "more than 100 people", as far as scientific studies goes, isn't a great boast. It's like saying you have "a collection of more than five stamps". The idea that a single gene, even a "bright-side gene", controls your entire outlook on life is - how to put it? - a strong claim. And the research consisted of what? They showed these people some pictures of cheerful stuff and depressing stuff and noted which they spent more time on.

The positive pictures included a couple hugging and someone sailing a boat.The negative images included a photo of someone being mugged. Who's to say which is positive and which is negative? Some people are terrified of sailing boats, and perhaps it was Chris Tarrant being mugged. I'll leave closer scrutiny of the methodology to Ben Goldacre on Saturday.

I choose to believe the result, though, because it confirms what I suspected: we are the victims of our own brains. Actually, I should recast that sentence. Rather than choosing to believe this study because it confirms what I already suspected, I was genetically doomed to believe it because it confirms what I am genetically doomed to believe about the setup of the universe.

I once imagined that optimism was a matter of willpower. When life serves you a shit sandwich, I thought, you can make a positive decision to close your eyes and enjoy the gherkiny bits. My growing feeling of having been wrong about that now enjoys scientific confirmation. I take no pleasure in the knowledge.

Every book I read lately, every news report I come across, seems to add to the sense that not only am I not in charge of my destiny, I'm not even in charge of how I feel about not being in charge of my destiny. The Anglo Saxons were right after all: wyrd bith ful aræd. Hundreds of years of Enlightenment humanism is on the verge of bringing us back to where we started.

I recently read Jonah Lehrer's new book, The Decisive Moment, about the neurology of decision-making, and the lesson was, more or less, that we don't make decisions: our brains make them for us. I might will a particular outcome, but is my will, as I think it was Wittgenstein who wondered, not simply an event in my brain also? How did that man sleep at night?

Be eaten by a crocodile? Or starve to death? The choice is yours. Except it isn't. How bleedin' depressing.

I was in the late-night newsagent in west London the other night, when I was struck by that faint thrill you get when you notice that the man in front of you in a queue is Lord Lamont. The drunk blokes behind me spotted him too. "All right Norman!" shouted one of them in a sort of menacing way. "Yeah, NORMAN!" another one riposted. He mumbled something into the collar of his coat and scuttled off looking miserable.

Do former Tory chancellors spend the rest of their lives as what my little brother used to call "bully magnets"? Anyway, it was a slightly ugly scene and I felt sorry for him. Shame on you, drunk people. Let the poor man buy his Chewits in peace.

Richard Williamson, the Holocaust-denying Catholic bishop, seems to be a subject fit more for curiosity than outrage. First there's his conviction that there were no gas chambers and only 300,000 died in the camps. Then there's his professed willingness to "review the historical evidence". Was he holding it upside-down the first time? Remember the man in the Monty Python sketch who says he wrote the plays attributed to Shakespeare? He is cross-examined: "How is it possible for you to have written plays performed over 300 years before you were born?" "Ah well," he responds good-naturedly. "This is where my claim falls to the ground."

* This week Sam attended the Oldie of the Year awards. "It was bliss. Old celebrities are much friendlier to each other than young ones. I got to meet comedy scriptwriters Galton and Simpson, and Lady West, the wife of the former first sea lord. Terry Wogan got squiffy and told corny jokes, at which everyone laughed fit to bust. I can't wait to be old, but I suppose I have to."