Don't throw away your second vote for Mayor

Take today's YouGov/Standard poll. Forget the first-preference choices for a moment. Skip to the second-preference votes. Just like all the other polls, this one reports that most of us are proposing effectively to throw away our second preferences, casting second-preference votes that will never even be counted.

What, you ask? But yes. How it works is this: first, they count all the first-preference votes. Every candidate bar the top two (almost certainly Boris and Ken) is eliminated. Then they look at the second preferences of those who voted Green, Lib-Dem, and the other eliminated candidates. Any for Ken are added on to Ken's first-round total; any for Boris are added on to his first-round total. Second preferences for any other candidate are ignored.

That means two things, neither widely understood. To begin with, there is no point at all in giving anyone except Boris or Ken your second preference. Second preferences for Sian, Brian or the others (favoured by 67 per cent of Londoners for their second pref in today's poll) are a waste of time.

And Point B is this: if you vote for Brian, or Sian, or one of the other lesser parties as your first choice, and Boris or Ken as your second, it is your second choice vote that will count, not your first.

Across London, there does seem to be a substantial desire to kick the Mayor. But Livingstone activists' road to victory may be to claim that you can somehow satisfy that desire, and "register a protest", by giving someone else your first preference, and Ken only your second.

In practice, a second preference vote for Ken or Boris is worth precisely the same to them as a first preference vote. The only way to register a protest against either man is not to vote for them, first or second. (There is, incidentally, no obligation to cast a second-preference vote.)

But relatively few people seem to understand this, perhaps including the brave gay activist Peter Tatchell. He was added to Ken's bulging list of "racists" after opposing the visit of Yusuf al-Qaradawi, the homophobic, fundamentalist Muslim cleric. So last week I was sorry to see him saying that, though he regarded Ken as "tarnished" and wouldn't put him first, he would put Livingstone second.

Perhaps Mr Tatchell sees Boris as bad for gay Londoners (surely wrongly: half of Boris's campaign team are gay.) Perhaps he thinks a Ken second-pref is somehow worth less than a first preference. Wrong again.

In a tight race, second preferences could be really important. So it's vital that people don't cast them without fully understanding what they're doing. It's equally important that they don't spoil their votes by mistake. Last time, an amazing 57,000 ballot papers were rejected.

In theory, the first-pref/second-pref system is fairer than the old first-past-the-post. But only if it's properly explained, and it hasn't been.