Due to death, resignation, or expulsion of elected representatives from a party, parliamentary seats sometimes become vacant between elections. How often vacancies arise largely depends on the term of the office: the shorter the term, the less frequently there are likely to be vacancies.
Related to this is the question of whether or not elected representatives can keep their seats if they leave or are expelled from their political party, which can be dealt with in different ways in legislation. If they are allowed to keep their seat, there is no need for a by-election. For more information about this, read the file about Floor-Crossing.
How to fill the vacant seat
If a seat becomes vacant between elections, List PR systems often simply fill it with the next candidate on the list of the party of the former representative, thus eliminating the need to hold another election.
However, plurality/majority systems often have provisions for filling vacant seats through a by-election. A by-election offers the opportunity to fill the vacant seat with a fresh candidate, and it allows the voters to express their will again. Voters may vote differently and thereby express discontent with the government. In a FPTP, single-member district system, by-elections are held in the constituency with the same rules applying as at general elections but often with other types of candidates, e.g., more publicity-seeking individuals who gain more attention in by-elections than they would in a countrywide general election.
Effects of by-elections
By-elections are smaller and therefore less costly than normal elections, but in some countries they will nevertheless put a significant burden on the budget, and seats are sometimes left vacant for long periods because of a lack of capacity to arrange by-elections. This is an especially salient problem in some countries in Africa, where the HIV/AIDS epidemic often leads to a large number of vacant seats between elections.
Since it usually takes time to hold and organise by-elections, the seat remains vacant for a while. This may cause problems for party groups in parliament, if they need every single member to vote for passing or rejecting legislation.
In some circumstances, by-elections can have a wider political impact than merely replacing individual members, and are seen as a mid-term test of the performance of the government. If the number of vacancies to be filled during a parliamentary term is large, this can lead to a change in the composition of the legislature and an altered power base for the government, or even a change in government.
Voting in by-elections generally differs from that in a general election, in that voters tend to express discontent with the current government in by-elections. The more the by-election is in the middle of the legislative term, the more losses a government in power usually has to expect. Furthermore, since only one member of the legislature is being elected in the by-elections, voters sometimes feel encouraged to vote for parties that would not gain a lot of support in a general election. Another difference between by-elections and general elections is the fact that by-elections usually receive less publicity and therefore voter turnout rate is mostly lower.
It is possible to avoid by-elections by electing substitutes at the same time as the ordinary representatives. If a member of parliament needs to leave the seat, the elected substitute simply replaces her/him. These substitutes may also replace the elected representative and therefore participate in voting and parliamentary work if the principal suffers from a sickness, travels, or is on parental leave.
