Oppositions need not be consistent. On the same day in 1980, the opposition spokesperson for energy announced that Labour supported the high dam at Clyde in order to maximise the electricity supply, the opposition spokesperson for the environment announced Labour supported the low dam in order to maximise the preservation of the Clutha Valley behind it. (Mike Moore explained his party’s policy was a high dam on one side of the valley sloping down to a low one on the other.)

Governments have no such freedom. Sometimes a minister’s verbal instructions will leave officials greatly puzzled, but once a policy is written down, any inconsistency becomes evident.

Yet, according to a famous choice theorem by economist Kenneth Arrow, the only sure way of getting consistent decisions is for them to be made by a dictator. In the seventeenth century, philosopher Thomas Hobbes had already concluded, without Arrow’s mathematics, that a state was not secure unless it had an absolute sovereign.

But dictators are not always benign. I grieve over those colonial independence leaders who I once admired but subsequently became proper thugs in office – Robert Mugabe is the most recent to die. In Hobbes’ day there was Charles I, perhaps not of the same magnitude of thuggery, but he certainly got offside with many of his subjects.

Half a century later, after the execution of Charles I, John Locke wrote that the power of an absolute dictator should be limited by the consent of the governed. He wrote this before the ‘Glorious Revolution’ which replaced James II (who was not executed) by William and Mary, and limited the power of the monarchy; Locke’s theories were used to justify the changes. Seventy-five-odd years later Locke’s philosophy was key in the formulation of the American Constitution.

Essentially the argument, very evident in the US constitution but also in the unwritten British constitution, is that the substantial powers of the Hobbes-Arrow dictator have to be limited by checks and balances. They include the ability of the governed to dismiss a dictator.

Nowadays, the ruling dictator is no longer the monarch but is a president or prime minister. Not all of today’s ruling dictators are humble. Some seem to think they are absolute monarchs. (Usually, we do not disillusion them by beheading.)

The institutional arrangements in the two countries are currently under severe pressure – perhaps their severest ever. The dictatorships of Donald Trump and Boris Johnson are criticised for breeching the constitution. Both appeal to their rights as a dictator to be able to act unilaterally outside the (informal and even the formal) constitution. Welcome back, Charles I.

Some of the difficulties arise because, as the American experience shows, it is never possible to codify the entire constitution. Rather, it is necessary to have institutions such as Parliament, the courts and the media which can respond to evolving circumstances. Those institutions also evolve. (The UK Supreme Court is only a decade old – before there had been a Judicial Committee of the House of Lords.)

Why are the American and British constitutions under so much stress? A simple answer is that both countries are deeply divided – America by cultural politics, Britain by Brexit.

A more subtle explanation goes back to Locke’s notion of the dictator ruling with the consent of the governed. It is not just that it is difficult to know what the consent is when a nation is deeply divided. In both countries the institutional mechanisms for transmitting that consent are problematic.

Parliaments evolved to reflect that will and to control the ruling dictator. (It was the English Parliament which decided to behead Charles I.) The extent to which a particular parliament really represents the will of the people – Arrow shows it is a nebulous concept – can be debated. However, it seems even less likely in today’s America and Britain, given the way the representatives are selected. In both countries the process is distorted by institutional imperfections including Front Runner elections. In current British circumstances, the existence of a Brexit party and the duality of the Labour and the Liberals means that, likely as not, the next British Parliament will not really resolve the current shambles, even with a different dictator; another referendum on Brexit may help.

New Zealand has not been immune to such difficulties. The country was split deeply over the Springbok Tour in 1981. The election at the end of that year gave more votes to Labour but more seats to National. (It is a nice paradox that, assessed by the popular vote, Rob Muldoon, who ruled for 103 months,  lost three elections and won one, while Bill Rowling, who ruled for 15 months, won two and lost only one.) It is not coincidental that about this time, New Zealand political theorists – descendants of Hobbes, Locke and Arrow – began to think about proportional representation.

So an MMP electoral system produces a parliament which better reflects the demography and ideology of voters than a front runner one. However, Parliament selects the government on a Winner-Takes-All principle rather than a proportional one.

In 2017, perhaps always, there was no combination of parties forming a coalition which reflected the voters' ideology. The choices offered – National-NZF and Labour-NZF-Green (others were ruled out) – were some distance from the political centre. The first coalition was about 0.8 standard deviations to the right of the New Zealand centre on a left-right scale and the second about the same distance to the left. That meant that only about 15 percent of the population were on the right of the first coalition, and only 15 percent of the population were on the left of the second. (On the conservative axis on a social conservative-progressive scale the first coalition was also about 0.85SDs on the conservative side while the second was a slightly closer, 0.65SDs, on the progressive side.) The sharp change in the politics of the government in 2017 did not reflect a sharp change in the politics of the voters but the peculiarities of the way Parliament chooses the government.

The conclusion is that a representative parliamentary system need not choose a ruling dictator who reflects the centre of the population. Which is another reason that we need a system which, over time, switches the leadership from one party to another. In any case, the threat of the change is necessary to ensure a degree of humility on the part of the dictator. But the formal and informal checks and balances are crucial too.