中间选民理论（又称为中间选民定理或中间选民模型）是一个理论，用来分析选民投票的行为。该理论模型的假设如下：在多数决的选举中，所有选民的选择偏好都可以在座标平面上找到对应的一点，这样的政策偏好图（也就是民意分布图）呈现在座标平面上时大部分会是一个常态分配的钟形曲线；且所有的选民都只会将他手中的选票投给所提政见与自己的政策偏好最接近的候选人（即单峰偏好 "single-peaked preference"），并没有双峰偏好（"double-peaked preference"）的情形；在一个选举只有两个候选人竞争的情况下，候选人要得胜出的方法，就是要得票数极大化；为了获得更多的票数，候选人所提出的政见必须更往此图的中间靠拢。如果双方候选人为了胜选，皆采用将政见向中间靠拢的策略，在激烈的竞争下，会使得双方所提出的政见最后都座落于选民政策偏好图的中点之上，双方所提的政见并不会有任何的差异，而使得双方所获得的票数都会一样多（各获得半数选民的票数）；倘若候选人所提的政见有任何的偏离，而另外一方未偏离中点时，会导致自己可获得的票数比对方少而落选。

The median voter theorem states that "a majority rule voting system will select the outcome most preferred by the median voter".

The median voter theorem makes two key assumptions. First, the theorem assumes that voters can place all election alternatives along a one-dimensional political spectrum. It seems plausible that voters could do this if they can clearly place political candidates on a left-to-right continuum, but this is often not the case as each party will have its own policy on each of many different issues. Similarly, in the case of a referendum, the alternatives on offer may cover more than one issue. Second, the theorem assumes that voters' preferences are single-peaked, which means that voters choose the alternative closest to their own view. This assumption predicts that the further away the outcome is from the voter's most preferred outcome, the less likely the voter is to select that alternative. It also assumes that voters always vote, regardless of how far the alternatives are from their own views. The median voter theorem implies that voters have an incentive to vote for their true preferences. Finally, the median voter theorem applies best to a majoritarian election system.