Every so often national political parties are convulsed by a contest for the leadership. Inevitably, in a profession as driven by ego and testosterone as politics, these contests are dramatic. And the media love the theatre of it and do everything they can to amplify the drama.
Sometimes a leadership contest is triggered by an existing leader stepping down. Those tend to be less acrimonious, as they are often seen as part of a process of renewal. But even in those circumstances, party stakeholders prefer not to have a genuine contest, as those can be too damaging to the fabric of the party.
Less often, a leadership challenge is a frontal assault on a sitting leader. Such contests are seldom pretty. Leadership contests, when a party is actually in Government, are usually avoided as being too destabilising.
Probably the most exciting British leadership contest of my time in politics was the 1990 challenge by Michael Heseltine to then Prime Minister Margaret Thatcher. This broke one unwritten rule, as it happened when the Tory party was in Government.
But Heseltine was a man of consuming ambition (and a personal fortune, which always helps in politics). And Mrs Thatcher, although she appeared unassailable to the international audience, was increasingly unpopular with her own MPs.
There were various quarrels about policy, including Europe. But the real thing that made her own MPs move against her was their belief that she might lose them the next election. And nothing motivates a politician more than self-interest and self-preservation.
The British Parliament meets five days a week and for most of the day. Being together all the time means that excitement and emotion often sweep through the building. I will never forget the electrifying and theatrical atmosphere in Parliament in the last days of Thatcher.
Heseltine did succeed in forcing Thatcher out, but he did not succeed in being elected leader himself. This added another rule to the lore around leadership contests in British politics, which is that the assassin will never wear the crown.
Sometimes the absence of a leadership contest can be a problem. In 2007, when Tony Blair stepped down as prime minister, Gordon Brown replaced him uncontested. At the time it was seen as a triumph for Brown. But the absence of a contest meant that Brown never had to spell out what he wanted the leadership for, and in the general public's eyes he lacked legitimacy.
Meanwhile, his internal Labour Party opponents festered. In the wreckage of Labour's defeat in 2010, it was widely acknowledged that a leadership contest might have helped Brown by making him define himself and allowing the party to air genuine policy differences.
Sometimes a leadership contest delivers a surprise. In 2005, the Tories were in Opposition, having lost that year's general election. Their leader, Michael Howard, stepped down and the person who seemed most likely to succeed him as party leader was an MP called David Davies.
He was tough, charismatic, an effective parliamentary performer and had the overwhelming support of Tory party members. But during the course of the leadership contest, he was overhauled by the young David Cameron. Nobody had predicted that outcome.
Some leadership contests serve to air genuine policy differences. Others are all about personality. Some contests "lance a boil" so the party can move on. Others create bad feeling that never goes away.
The challenge to party stakeholders and managers is to make these contests constructive. Not just for the health of their party, but for the benefit of the nation as a whole.
— Diane Abbott is a British Labour party MP and spokeswoman on public health