A week ago I appeared before
the Commonwealth Parliament’s Joint Standing Committee on Electoral Matters and
presented a brief submission that tried to assess in what circumstances a
candidate is advantaged or disadvantaged by optional compared to compulsory
preferential voting.

Under compulsory preferential voting, the
party that receives the majority of preferences is always the party that
receives an advantage. But with optional preferential voting, assessing the
advantage is always more complex because some votes “exhaust” their preferences
and are essentially excluded from the final count.

In essence, there are two effects at play
that need to be understood in assessing who receives advantage under optional
preferential voting.

The first is what I have called the “exhaustion effect”. Every vote that exhausts preferences advantages the
candidate that starts off with the highest vote. Under all circumstances, the
leading candidate is advantaged by the exhaustion effect.

In reality, this effect is quite small with
marginal seats. In general, marginal seats have a lower vote for third parties,
and the two major parties tend to have closer primary votes. The exhaustion
effect is more important in safe seats, where it always acts to inflate the
margin of the candidate who already has over half of the primary vote.

But the second effect is what I have called
the “missing preferences effect”. If preferences would have split 50:50 under
both optional and compulsory preferences, then the missing preferences effect
is zero. Under a 50:50 split, no second running candidate can ever pass the
leader, even under compulsory preferences. But under optional preferential
voting, a 50:50 split still advantages the leading candidate through the
exhaustion effect, lowering the winning post and effectively inflating the final
percentage of the leading candidate.

Read more on the site here.