BRITAIN has changed in many ways since the election in 2010. One of the most striking is the fall in violent crime, which is now as low as it has been since reliable records began. Yet if politicians have noticed this happy trend, they are not letting on. Since April the coalition has been arguing over a Conservative proposal to toughen the law on knife crime. This, says Nick de Bois, the Conservative MP sponsoring the change, will send “a clear, unambiguous message” to criminals. Its other consequences will be less benign. Knife crime has long been a problem in Britain. Stabbings continued to rise long after other crimes receded. Calls for tougher laws became louder following the recent killing of a teacher, Ann Maguire, in her classroom in Leeds. Such tragedies notwithstanding, the problem has diminished substantially in recent years. Between 2007 and 2013 the annual number of fatal stabbings in England and Wales fell from 272 to 194. Admissions to NHS hospitals for knife wounds fell slightly more.

Mr de Bois proposes that adult criminals convicted of a second knife offence should automatically be sent to prison for at least six months. Juveniles will get at least four months. The Labour Party apparently backs the proposal, which suggests it will become law despite Liberal Democrat resistance.

Yet there is little reason to think that the current law is too soft. Between 2008 and 2013 the proportion of people caught in possession of a knife who were immediately jailed increased, from around 20% to 28%, even as the total number of convictions fell (see chart). The average sentence length rose from 156 days to 221. Of those people convicted of a second offence, 37% were immediately sent to jail. Many offenders are not violent: in Cambridgeshire, an apparent spike in knife offences was caused by eastern European farm workers carrying home their tools.

Mandatory sentences are an ineffective way of toughening the law, argues Lyndon Harris, the editor of “Banks on Sentence”, a sentencing handbook. When judges lose their discretion, the results can be perverse. For example, under one of Britain’s existing mandatory sentences, people convicted of certain firearms offences get an automatic five-year prison term. That may act as a deterrent, but since defendants now typically get the same sentence however they act, they have little incentive to plead guilty or to co-operate with police.

In America, mandatory sentences are being repealed as politicians realise that their main effect is to fill already overcrowded prisons. In Britain, however, the coalition government’s early plans to cut the number of people in prison were ditched after an intemperate tabloid campaign. Kenneth Clarke, a liberal Tory, was demoted from his post as justice secretary. With the next election nearing, his replacement is determined to appear tough. And if the law must serve as a megaphone to get that message across, so be it.