In the Election Questions 2025 series, UT researchers explore how science helps us understand the dilemmas voters face. This time, mathematician Jasper de Jong looks at what mathematics can tell us about tactical voting. "I find it fascinating that with pure logic you can prove what is possible in any voting system," he says. "And one of those proofs shows that in every conceivable system, strategic voting is theoretically possible."
In theory, it’s always possible
De Jong points to one of the most remarkable mathematical results about elections: a proof showing that strategic voting can never be fully eliminated. He refers to the Gibbard theorem from the 1970s.
No matter what kind of voting system you design – whether it’s the Dutch proportional system or something entirely different – there will always be situations in which voters can do better for themselves by not voting sincerely. Only in very simple systems, with just two options or one person making all the decisions, does that effect disappear.
"What people come up with in terms of voting rules or variations, there’s always a possibility for strategic voting," says De Jong. "That’s a mathematical fact. But that doesn’t mean it’s actually useful in practice."
In the Netherlands, the odds are small
The Dutch electoral system is proportional: every vote counts in the allocation of 150 seats. That makes tactical voting particularly difficult.
Only if you knew exactly how everyone else in the country would vote could you calculate whether switching between two parties would yield one extra seat, and of course, you never do. Polls give you a general direction, but not whether a party is just over or under the threshold for an additional seat.
That’s why there are hardly any convincing examples of large-scale tactical voting in the Netherlands. The chance that a single vote will make the difference is negligible.
Why tactical voting rarely pays off
Even if you think you’ve found a clever strategy, the uncertainties quickly multiply. "So much can go wrong. You don’t know whether your preferred party will actually join a coalition because of that one extra seat – and even if it does, you don’t know if they’ll push through the issue you care about most. The chain of ‘my vote → seat → coalition → policy’ is full of uncertainties."
Mathematically, it’s a probability calculation with extremely low odds. The expected influence of one tactical vote is so small that, rationally speaking, it’s hardly worth the mental effort.
The spectrum dilemma
De Jong describes a situation where tactical voting might seem mathematically possible: someone who sits just to the right of the political centre voting for a far-right party, hoping to pull the eventual coalition slightly in that direction. In theory, that can make sense, especially for issues that can be placed along a scale, such as how much money we should spend on climate policy.
In practice, though, it often backfires. Some parties deny climate change altogether, which means you could end up strengthening the influence of groups that want the very opposite of what you do. In coalition talks, that can work against you. And not every political issue can be plotted neatly on a single spectrum.
For De Jong, it remains mostly a thought experiment. "You can model it as a mathematical scenario," he explains, "but the uncertainties quickly pile up, from whether a party even wins a seat, to whether it joins a coalition and defends your position. The chance that things play out exactly as you hope is very small."
What the mathematician says
De Jong’s conclusion is straightforward: "In theory you can vote tactically, but in practice the chance that it really makes a difference is very small."
That’s why he simply votes for the party that best matches his views. If you want to optimise purely for personal gain, it’s hardly worth it. But voting itself, as an act of democratic responsibility, remains important.
He stresses that this is not voting advice. "I’m looking at this as a mathematician, not a political commentator. Tactical voting sounds clever, but the probability rarely works in your favour. So vote based on substance, on the party that reflects your values."
