On May 5 2011, UK voters will be asked to choose between two voting systems for single-member constituencies

the current system, in which first preferences alone count,
and the alternative vote, in which subsequent preferences also count.

Which should one prefer? My answer is: the alternative vote.

That would not have been my view until recently. So what has changed?

The political map of Britain.

Martin Wolf, FinancialTimes 24/2 2011