The Nobel Prize winning economist Kenneth Arrow, first came to prominence with his "impossibility theorem". He proved mathematically that when voters have to choose between three or more options, no system of ranking their preferences can produce a single, indisputably best order of precedence. That is, there's plenty of room for argument over which voting system, while not being perfect, is better than the others. This is why we need time – the usual Senate public inquiry would do – to hear from the experts and examine the properties of the voting system one side of politics has come up with and wants to ram through. If the Coalition has proposed a move to optional preferential voting, allowing people to express their preference for up to six party groupings, it's a fair bet it believes such a system will advantage it over its Labor rival.

If the left-leaning Greens and centrist Xenophon party are happy to give the Coalition what it wants, it's a fair bet that's because the deal leaves room for their comfortable survival, while raising the drawbridge against the emergence of new minor-party rivals of either leaning. The benefit to the Liberals is that their coalition (their joint voting ticket) with the Nationals, and the greatly reduced scope for the emergence of new, right-of-centre minor parties, minimises the wastage of right-leaning votes, whereas the lack of a coalition agreement between Labor and the Greens leaves significant scope for the wastage of left-leaning votes. That's the point about optional preferential voting – it leaves a lot of votes being wasted. And this is how it greatly reduces the possibility of new minor parties gaining a foothold. When Xenophon first got himself elected to South Australia's upper house, he did so with a primary vote of less than 3 per cent. It was winning a seat that allowed his profile to grow and ensure much higher primary votes in subsequent (federal) elections. Similarly, it was a quite small initial primary vote that allowed Bob Brown to get known and eventually spread the Greens to all states.

Turnbull's proposal doesn't change the requirement that, to win a seat in a half-Senate election, you need to amass 14 per cent of the votes in your state. For a full-Senate election after a double dissolution, the quota falls to 7.6 per cent. If the indication of preferences becomes optional – meaning many people won't bother – the hurdle facing future Xenophons and Browns will be almost unreachable. They'd need a primary vote not far short of the quota. Behavioural economists know that the way you "frame" a proposition greatly influences how people respond to it. Turnbull has framed his thus: if you want to put an end to micro-parties gaming the system and getting people with a primary vote of as little as 0.5 per cent elected, support my reforms. Sorry, non-sequitur. You can agree with the first part – as I do – without accepting that Turnbull's solution is the only one, or even the best available. A better solution – one that ended gaming by micro parties without stymieing all democratic change – would be to retain the present preference system but simply add the rule that candidates getting a primary vote of less than, say, 2 per cent, would be excluded from election and their preferences redistributed.