The Liberal Democrats are right to advocate voting reform. Unfortunately the voting system they favour – the alternative vote (AV) – has serious weaknesses. AV, which involves ranking candidates by order of preference, is opaque, potentially complex and does a poor job of reflecting voters' preferences. It can exclude the candidate preferred to every other candidate by a majority of voters early on in the runoff process.

Worse, a candidate who would have won can wind up losing if he or she garners more first place support in the course of a campaign. This happens because the increased support the candidate receives in a campaign can change the order in which candidates will be dropped and votes reallocated in arriving at a result. In the technical literature, this is referred to as violating the monotonicity condition – the idea that if a candidate wins more support, this should not adversely affect his or her prospects.

For instance, suppose 21 voters are voting for three candidates: Alice, Bob and Charlie. Eight voters rank the candidates Alice 1, Bob 2, Charlie 3; two rank them Bob 1, Alice 2, Charlie 3; five rank them Bob 1, Charlie 2, Alice 3; and six rank them Charlie 1, Alice 2, Bob 3. Since Charlie has the fewest first-place votes, he is eliminated, and those six votes now have Alice in first place, so she wins 14 to 7.

But suppose the vote were slightly different, and the two voters who put Bob first had instead ranked Alice top (Alice 1, Bob 2, Charlie 3). Now Bob, with only five first-place votes, is eliminated and those five rankings then have Charlie in first place, so Charlie wins 11 to 10. Moving Alice up in a few rankings converts her from a winner to a loser, because in doing so there is a change in which candidate is eliminated.

The upshot is that under AV voters cannot know for sure whether a vote for the candidate they favour will help or hurt that candidate. Even first-past-the-post, with all of its flaws, does not suffer from this problem, since under that system no candidate set to win an election could lose by gaining more support at the expense of other candidates.

Approval voting does not encounter these difficulties. In this system, voters are allowed to give one vote each to the candidate or candidates which they support, with the candidate having the most votes winning. The main advantage approval voting has for the voter is that it allows a person to vote both sincerely and strategically at the same time to better reflect his or her preferences. As such, it has the additional advantage of getting rid of the wasted vote and the spoiler role in elections, which is necessary to open up the political process to third parties.

Had approval voting been in place in Florida during the 2000 presidential election, Al Gore surely would have won. In the British general election, approval voting would probably have yielded a less skewed result in terms of parliamentary seats. For instance, in those constituencies with a Conservative plurality alongside a majority divided between Labour and Liberal Democrats which did not prefer the Conservative candidate, the majority under approval voting would have been able to reflect this fact in their voting patterns and the outcome.

This is why the new government should embrace approval voting in single-member constituencies when the time comes for a referendum on electoral reform. It would be fairer than the current system, and the Lib Dems' presence in government gives it a decent chance of being enacted.

• Professor Robert Z Norman contributed to the research for this article.