Britain is catching up with most other Western democracies in having televised leaders’ debates during the general election. Overseas evidence suggests that such debates are more often talking points than turning points, attracting a lot of media interest but seldom changing the outcome, except when the election is already very close.

Politicians and broadcasters have always looked to the US, where debates started, on radio, in the Oregon Republican presidential primary in May 1948 when Thomas Dewey, the eventual nominee, and Harold Stassen discussed the single topic of outlawing the Communist Party. The first televised debates between presidential candidates were the famous Kennedy-Nixon matches in 1960. However, they became regular only from 1976, varying in number, but usually three, with one between vice-presidential candidates. These follow a large number of TV debates within each party in the previous year.

More relevant to Britain, TV debates have become the norm in comparable parliamentary, as opposed to presidential, systems. They have been held in Australia since 1984; in Canada regularly since 1979 after a one-off in 1968; and now in most European countries (in Sweden and Norway even before the US).

Within Britain, party leaders in Scotland have held a series of debates during the campaigns for the Scottish Parliament, as did the candidates for London Mayor in 2008. During general election campaigns, leading spokesmen often appear on television programmes together.

Their impact is harder to judge. John F. Kennedy got a crucial edge in a very close election in 1960 after being seen as winning his first debate against Richard Nixon, who was sweating and looked uneasy, although radio listeners thought the latter did better. In 1976, Gerald Ford’s blunder in saying that there was “no Soviet domination of Eastern Europe” hit his campaign against Jimmy Carter. Since then, debates have had steadily declining viewing figures in the US. This may partly be because the heavily rehearsed and coached candidates play it safe, avoiding risks.

Defensive tactics have resulted in more score-draws, in which neither main candidate is seen as a winner; prior views tend to be reinforced. However, with live television there is always the scope for slip-ups. That is why, until now in Britain, frontunners have been reluctant to face the risk — and uncertainty — of a debate. But, whatever their impact on the result, TV debates will dominate the lives of the party leaders and be the most talked about events of the campaign.