Malapportionment occurs when electoral systems violate the norm of equal representation according to population. In effect, the value of votes in one or more constituency (districts or ridings) will differ from that in one or more other constituencies. Malapportionment is possible only in electoral systems with more than one electoral constituency. Thus a proportional representation electoral system with only one national constituency like those in Israel and the Netherlands cannot be malapportioned. In contrast, majoritarian electoral systems invite both malapportionment and gerry-mandering because they require the drawing of borders for a large number of single member constituencies.

Malapportionment often systematically advantages voters in some constituencies and disadvantages those in others. So for example, the electoral system in the U.S. Senate (2 Senators per state without regard to population size) over-represents or advantages voters in small population states like Maine and Montana, while under-representing or disadvantaging those in large population states like California and Florida. This reduces the individual and collective voting power of racial and ethnic minorities, which are concentrated in the large population states.

Another example is the systematic over-representation of voters in more rural prefectures and under-prepresentation of voters in more urban prefectures in Japanese parliamentary elections. The conservative Liberal Democratic Party thus wins more seats in the Japanese parliament because its voters are concentrated in more rural prefectures.