The natural history of the sitcom

The lifespan of TV comedy can be anywhere between 30 minutes and 30 years. Can science tell us which is more likely

What do sitcoms and fruit flies have in common? Answer: they're both associated with rotten material and they both have very short life expectancy.

What do sitcoms and fruit flies have in common? Answer: they're both associated with rotten material and they both have very short life expectancy.

They also both come in swarms. Mark Lewisohn's Radio Times Guide to TV Comedy, a work of almost certifiable scholarship, includes entries for a staggering 2,600 programmes, 90 per cent of which have left no greater trace behind them than the average Drosophila melanogaster expiring unmourned in a laboratory test tube. With such a prodigious rate of breeding, though, successful mutations are bound to occur - leading to those rare individuals that far exceed the general lifespan and raise the tantalising possibility that the secret of longevity, for human and television series alike, might be unlocked. What exactly is it about Friends, for example, that, seven years after it launched and as its 145th episode airs tonight on E4, it can still have British schedulers battling for the rights? And why should Cold Feet, an enormously successful British comedy drama, be murmuring about calling it a day after just four short series?

As it happens, the fruit fly has proved rather helpful to scientists studying the process of ageing in animals - one recent experiment established that tinkering with just one gene could double an individual's lifespan without affecting its vigour or fertility. Scientists nicknamed it the Indy gene (for "I'm not dead yet"). But while television executives would probably give up their company cars to discover the secrets of a sitcom's Indy gene, it isn't likely to happen in a field in which life expectancy bears no relation to merit and in which commissioner's whim and accidents of scheduling far outweigh any genetic effects. What gerontology can do, on the other hand, is offer some instructive metaphors that help you to understand why one programme is senile when it's just six episodes old and another is still young when it's well over 100.

1 The Hayflick effect

One of the few established certainties in the science of ageing was established by a researcher called Leonard Hayflick, who discovered that some human cells, far from being able to divide indefinitely, go through only a finite number of divisions before they give up the ghost. Given that many sitcoms replicate by cell division, generating numerous episodes from one original, this looks a promising avenue, but any attempt to establish dependable numbers soon runs into trouble. Some broad generalisations are possible - American series, if they survive the crucial first run, generally run much longer than British ones. That is partly down to the powerful incentive of syndication - which will, in effect, mean that the creators never have to work again, but which requires at least 70 episodes. But caution is needed in analysing the figures. The longest-running American sitcom ever, for example, was My Three Sons, starring Fred MacMurray, which ran for an awesome 380 episodes but aired for only 12 years in total. By contrast, Britain's oldest comedy, Last of the Summer Wine, has now delivered something like 170 episodes (exact figures are hard to come by) but has run for 29 years. Which should count as the longer-lived? Last of the Summer Wine raises another problem, which is the difficulty of establishing exactly when death can be formally certified. Brain death and actual interment are often separated by years, even decades. The programme may continue to exist as a kind of comedy vegetable, running through a series of tics and familiar gestures. To uninitiated outsiders, they can look startlingly like evidence of continuing life - but they are often entirely reflexive and unconnected to anything that could be called sentience. Those who love the programme will resist all attempts to disconnect the life-support system; those who don't can't understand why it wasn't allowed to die with dignity years ago. Last of the Summer Wine is apparently due for another series.

2 Low Caloric Consumption

Studies have shown that rats whose caloric intake is 30 per cent lower than that of a control group tend to live 30 to 40 per cent longer. The problem on television, though, is that sitcoms are prodigious gluttons, consuming comic plots and narrative situations at a rate that is almost bound to induce cardiac arrest in the scriptwriter. Again, a broad distinction is possible. Sitcoms with modest nutritional needs in terms of farcical machinery ( Seinfeld, the comedy "about nothing", would be a prime example) could theoretically run for ever, while those that depend on elaborate plotting are generally more short-lived. Even a writer as prodigiously inventive as David Renwick eventually found himself exhausted by One Foot in the Grave's unappeasable appetite for comic complication. In America, where almost all comedy writing is a team effort, the effects of plot-consumption are slightly mitigated, but here it can render a sitcom prematurely grey. Mike Bullen, the writer of Cold Feet, recently identified that as one of the reasons for invoking the right to die on behalf of his programme, otherwise in the prime of life in terms of audience response: "The problem was in finding the situations," he said. "Abortion, birth, cancer, drugs... we'd done them all - and the rest of the alphabet. Was there anything left to say?"

3 The royal jelly syndrome

Paradoxically, feeding may also prolong life, rather than shorten it. Some insect species include queens that live hundreds of times longer than the workers, despite there being no essential genetic difference between the two forms, only a difference in what they feed on. Matters are more complicated in television, where a queen ( Fawlty Towers, with just 12 episodes) may expire long before the drones ( Terry and June, on 65), but a similar effect does operate in some cases. In 1998, the chief writer on Seinfeld earned just under $250m - the kind of royal jelly that can prove almost miraculously rejuvenating to a writer feeling the effects of age. In Britain, such sustenance is increasingly difficult to come by, though - indeed feeding will often be suspended even if the patient looks likely to live a long and productive life. "They don't now give you a chance beyond the first series," says Peter Bennett-Jones, head of the independent production company Tiger Aspect, which produces The Vicar of Dibley. Either it works first go, or it dies there and then, a Darwinian harshness that would, incidentally, have snuffed out some of Britain's longest-running comedies in their infancy.

4 The accumulated damage theory

This holds that ageing is the result of accumulating errors in our genetic information, leading to defects in tissue flexibility and immune-system response. It is clearly applicable to television programmes, which can easily see off parasites in their first flush of youth (the critical response to the first series of Friends was lukewarm at best, but the show proved a hit with viewers) but become increasingly susceptible to attack as they age. A vigorous grip on your interest (will Daphne and Niles in Frasier get together?) becomes arthritic or too feeble to continue. Other toxins can limit a programme's lifespan too - most notably the egos of its stars, who may begin a series as non-entities and end it with the conviction that they are the secret ingredient that alone makes it work. When the cost of keeping a cast together becomes greater than the return, the programme will die. The in-built obsolescence of the components is an issue too - if The Simpsons were a human family, Bart would have started shaving years ago. Because they aren't, there is no theoretical limit on the programme's lifespan, beyond the exhaustion of the audience or its creators.

There is a consolation for the bleak fact that no safe prediction can be made about the life expectancy of any television comedy, and that is that the afterlife is not just a fantasy for sitcoms - it's a proven certainty. When Steptoe and Son emerged from cryonic suspension to become one of BBC 2's ratings hits a few years ago, one of its stars was no longer around to see the resurrection. Similarly, Fawlty Towers, laid to rest on 25 October 1979, lives on in repeats and video sales. Did it really have a shorter life than Bootsie and Snudge (104 episodes in total)? The truth is that if you really want to know how long a television programme will live for, you'll have to come back after all of us are dead.