Illustration by Steve O'Brien

THE trouble with marriage, some evolutionary psychologists believe, is not immorality but life expectancy. Human beings were indeed designed to be monogamous, runs the argument, but they were supposed to live only to 30: ten years of fidelity is in the genes, but golden anniversaries are not. A similar rule applies to political unions. Voters' devotion to governments, however passionate at first blush, cannot last forever; eventually the couple become rancorous and recriminatory. Gordon Brown and Labour seem to be entering this supernumerary phase—the phase of having stayed too long.

Until April 23rd the government faced a humiliating parliamentary defeat over its abolition of the 10p income-tax band, a change that will leave several million poor people worse off—a botched reform that, say MPs of all parties, voters bitterly resent. Then Mr Brown and Alistair Darling, the chancellor, promised only slightly less humiliatingly to make good some of the losses. As he did so, Mr Brown extolled the poverty-reducing virtues of the minimum wage, introduced in 1999. But the minimum wage, along with the fiddly but generously redistributive tax-credit system he devised, is now just part of the economic furniture. Likewise, when Mr Brown harks back to the early 1990s, lots of voters can scarcely remember the darkness he tries to conjure, and a few were barely born.

Mr Brown's approach to redistribution helps to explain the shortage of credit he gets for it: his neo-Victorian concept of the deserving poor (essentially working parents and pensioners); his gloomy conviction that the sharing had to be surreptitious. But just as, in a long marriage, partners forget how lonesome they were before it, so voters have forgotten or “banked” most of Labour's achievements. Lots of people, for example, tell pollsters that the National Health Service is worse than in 1997, even though on any sensible measure it isn't. Expectations rise and cynicism abounds, but memory fades and patience wears out.

Conversely, if governments have the mixed fortune to survive long enough, their mistakes and myopic compromises catch up with them. The controversial tax reform itself epitomises the fiscal tricksiness with which, as chancellor, Mr Brown raised money through subtle but cumulatively burdensome tax ruses. He also borrowed copiously to finance his spending plans—a technique whose political consequences he might have escaped had Labour left office before the current downturn. Another thing that ultimately catches up with long-lived governments is the economic cycle.

As with voters, so with Mr Brown's cooling romance with his own MPs. Part of the explanation is that parliamentary rebellion, rather like adultery, is habit-forming: at first it feels impossible, then transgressive and finally mundane. There is also, inevitably, a swelling cadre of alienated MPs—has-been ex-ministers and never-going-to-be-and-know-it backbenchers—for whom infidelity seems costless. As Mr Brown's poll ratings wane, self-interest (ie, keeping their seats) actively motivates some MPs to distance themselves from him.

Meanwhile Labour's majority has declined from the landslide proportions of its first two terms to the merely healthy. So whereas Tony Blair comfortably survived early left-wing revolts, over a miserly increase to the basic state pension and cuts to single parents' benefit, the 10p guerrillas rapidly extracted their concessions. So, maybe, will another impending rebellion over bizarrely stubborn plans to increase to 42 days the time those suspected of terrorism can be held without charge. That makes the government look even weaker and more exhausted.

The spiral of indiscipline has also exacerbated another habit that took hold during Mr Brown's resentful decade as chancellor, and is proving its own revenge: regicide. Internecine briefing seems to have become almost a socially acceptable pastime among some ministers, just as it eventually did under the Tories. As the “assassins” who brought down Margaret Thatcher begat the “bastards” who almost did for John Major, so the Brownite-Blairite struggle has been elided, almost uninterruptedly, with intra-Brownite strife. Blood, it turns out, will indeed have blood.

Yet, in this case, the blood is likely to flow in small quantities rather than a great murderous gush. Despite heady speculation about a coup, Mr Brown is probably safe in his job, for now. Labour has already copied the Tory manoeuvre of changing the front man; to do so again would look undemocratic as well as absurd. But another reason Mr Brown is safe is that he has deprived himself of what ought to be one of the few advantages of long incumbency. Almost all the Labour politicians who have developed reputations to match those of, say, the Tories' Ken Clarke or Michael Heseltine have left government, voluntarily or otherwise. There is, for the moment, no obvious alternative to Mr Brown. There is also no one to share his Methuselan burden.

Himself, alone

Because they tend to be built on ideas, which can run out, Labour governments have generally found it especially tough to stay in office. The longevity of this one, after the long Tory imperium, suggests that the life expectancy of British governments, like that of humans, has lengthened. But the basic physics of politics will not change. Eventually the equilibrium between hope and trust on the one hand, and disappointment on the other, must shift against the incumbent.

That is Mr Brown's big problem. It is not that he lacks a “vision” (his egalitarian meritocracy is perfectly serviceable), or that he is indecisive and uncharismatic (though he is). It is not 10p or 42 days. It is that Labour has been in office for 11 years. It will very likely be 13 by the time of the general election—two more years in which Mr Brown must bear the gathering weight of grievance and ingratitude, more or less alone.