Modern Man’s twin evils are overconsumption and overproduction. Half the world eats too much, the other half has too many babies. There is just too much of us, human flesh.

Yet no science fiction film has ever been brave enough to show the future as it really will be. Instead, the actors give the lie that we are all going to be childless hotties in skintight spandex. It’s possible, but only if we can learn to conquer our most powerful instincts. And that would truly be going where no man has gone before.

Sure enough, every Trekkie-style show worth its salt will do the “worthy” episode. This is the one where the crew return to Earth, mocked-up to look like the doomsday set from a later-career Michael Jackson video, deliver a portentous eco-sermon and get the hell out of there in time for interplanetary cocktails. No one mentions the problems caused by the fatness. Or the babies.

You see, if science fiction were really to take current trends, and extrapolate, the Starship Enterprise would be a rather different place. The food industry is one of the biggest causes of our planet’s pollution. The Government has predicted that half of all adults will be obese by 2050. Meanwhile, the United Nations reckons that by then the world’s population will have risen by 40 per cent to 9.1 billion. That’s a lot of extra mouths to feed, even if they don’t pig out quite as indecently as we do in the West.

On these trends, Captain Kirk would become so obese, he would burst out of his uniform, moobs first. Spock would be too fat to get out of his double-wide chair, and Bones would spend all his time dealing with heart disease and type 2 diabetes. Oh, and the place would be crawling with children, wreaking havoc with the transporter and getting their fingers stuck in the automatic doors. We’d be more like the people depicted in the children’s film WALL-E, using space age hover chairs because we were too bloated to walk.

So why do we keep on eating and birthing? I was preoccupied by this question last week, when, for another article, I spent a day in an NHS clinic for the morbidly obese. It was there that I read a new report on childlessness from the Office for National Statistics (ONS). I was surrounded by people made utterly miserable by their inability to override their instinct to eat. It got me thinking about the parallel with other primitive urges.

The answer is that our bodies are hardwired for life in East Africa, 20,000BC, not East Grinstead, 2009. Our problem with babies and beer bellies is thanks to how ruthlessly our DNA was then honed for success. Back in our native habitat, refusing food or refusing children would have been suicidal. Both were, for various reasons, scarce. Now both are overabundant. Refusing them is the means to our survival.

But is it even possible for humans to override their instincts like that? Of course, individuals have practised self-denial, but it’s not something we’ve ever undertaken as a collective project. Until now. The ONS report stated that one in five women in Britain is remaining childless — a trend repeated across the Western world. What intrigued the authors was that current childlessness is different from the childlessness of the postwar years, when it was the poorest women, or those left single, who didn’t have children.

By contrast “present-day childlessness is occurring increasingly often among healthy females who are living within marriage or cohabiting”. The more educated and wealthy a woman gets, the less likely she is to have children. Although they could not ask directly, the statisticians were left wondering if increasing numbers of people are actively opting out of parenthood.

If so, they would be doing it for profoundly rational reasons. Study after study on happiness levels shows that while marriage is excellent for mental wellbeing, having children is not. Happiness levels dip after the first born, and are not restored until the last child leaves home.

Sociologists accept this, but as Daniel Gilbert, Professor of Pyschology at Harvard, says, those findings are very hard for the public “to swallow because they fly in the face of our most compelling intuitions”. Children do not make us happy, although surveys also show we believe very strongly that they will. What accounts for this parenting paradox? Well, we are enslaved to our instincts.

When you ask parents exactly what it is they get from their children, they often explain it in terms of “fulfilment” or “selflessness”. I do not exactly enjoy kneeling on the bathroom floor as I act as midwife to my daughter’s toilet activities, but I think, in the scheme of things, I am doing something meangingful.

Yet “fulfilment” and “selflessness” — the feelings may be real, but it’s the kind of vocabulary that shows it’s the DNA talking. We are fulfilling a drive for our genes to repeat themselves, rather than seeking enjoyment in any rational sense.

Some of the childless women in the ONS study may be the first generation to conquer that instinct en masse. It is interesting that it is the more educated women who seem better inclined to fight biology with reason, for a “post-instinct” kind of life (it is also one with more relaxing holidays). Is it a coincidence that the risk factors for getting big with child — lack of education, jobs and other ways of honing higher reason — are the same as those for getting big with food?

At the obesity clinic, I talked to people just after they had had their gastric bands inserted, or, as they liked to think of them, “willpower” in the form of an artificial plastic clamp. Overflowing their hospital chairs, breathless just from standing up, these people were used to society judging them to be weak. They felt the same way about themselves. They hated their bodies and their cravings for food. Their instincts were killing them.

Since medieval times, the idea of the mind ruling the body has been nothing more than a diversion for philosophers. Now, it’s an emergency. Will we manage to live against our genetic drives? What will that world look like? Are we going to fit our spandex spacesuit and save the planet? Tune in for the next exciting instalment in Post-Instinct-Universe to see what happens to the future of mankind. Bring some diet ice cream.

Times Recommends

• Copenhagen will fail – and quite right too
• They’re still wriggling to avoid the flak on Iraq
• Rule the waves? Not any more we don’t