Manipulation of electoral district borders to favor certain outcomes of an election
The image from above appearing in a news article by Elkanah Tisdale in 1813

While the wasted vote effect is strongest when a party wins by narrow margins across multiple districts, gerrymandering narrow margins can be risky when voters are less predictable. To minimize the risk of demographic or political shifts swinging a district to the opposition, politicians can create more packed districts, leading to more comfortable margins in unpacked ones.

Gerrymandering of state legislative districts can effectively guarantee an incumbent's victory by 'shoring up' a district with higher levels of partisan support, without disproportionately benefiting a particular political party. This can be highly problematic from a governance perspective, because forming districts to ensure high levels of partisanship often leads to higher levels of partisanship in legislative bodies. If a substantial number of districts are designed to be polarized, then those districts' representation will also likely act in a heavily partisan manner, which can create and perpetuate partisan gridlock.

Gerrymandering may be advocated to improve representation within the legislature among otherwise underrepresented minority groups by packing them into a single district. This can be controversial, as it may lead to those groups' remaining marginalized in the government as they become confined to a single district. Candidates outside that district no longer need to represent them to win elections.

As an example, much of the redistricting conducted in the United States in the early 1990s involved the intentional creation of additional "majority-minority" districts where racial minorities such as African Americans were packed into the majority. This "maximization policy" drew support by both the Republican Party (who had limited support among African Americans and could concentrate their power elsewhere) and by minority representatives elected as Democrats from these constituencies, who then had safe seats.

Gerrymandering can also be done to help incumbents as a whole, effectively turning every district into a packed one and greatly reducing the potential for competitive elections. This is particularly likely to occur when the minority party has significant obstruction power—unable to enact a partisan gerrymander, the legislature instead agrees on ensuring their own mutual reelection.

In 2006, the term "70/30 District" came to signify the equitable split of two evenly split (i.e. 50/50) districts. The resulting districts gave each party a guaranteed seat and retained their respective power base.

In the United States, however, such reforms are controversial and frequently meet particularly strong opposition from groups that benefit from gerrymandering. In a more neutral system, they might lose considerable influence.

The most commonly advocated electoral reform proposal targeted at gerrymandering is to change the redistricting process. Under these proposals, an independent and presumably objective commission is created specifically for redistricting, rather than having the legislature do it.

To help ensure neutrality, members of a redistricting agency may be appointed from relatively apolitical sources such as retired judges or longstanding members of the civil service, possibly with requirements for adequate representation among competing political parties. Additionally, members of the board can be denied access to information that might aid in gerrymandering, such as the demographic makeup or voting patterns of the population.

Another way to avoid gerrymandering is simply to stop redistricting altogether and use existing political boundaries such as state, county, or provincial lines. While this prevents future gerrymandering, any existing advantage may become deeply ingrained. The United States Senate, for instance, has more competitive elections than the House of Representatives due to the use of existing state borders rather than gerrymandered districts—Senators are elected by their entire state, while Representatives are elected in legislatively drawn districts.

The advantages of this method are that it allows a certain amount of human intervention to take place (thus solving the Colorado problem of splitline districting); it allows the borders of the district to follow existing jagged subdivisions, such as neighbourhoods or voting districts (something isoperimetric rules would discourage); and it allows concave coastline districts, such as the Florida gulf coast area. It would mostly eliminate bent districts, but still permit long, straight ones. However, since human intervention is still allowed, the gerrymandering issues of packing and cracking would still occur, just to a lesser extent.

This district-drawing algorithm has the advantages of simplicity, ultra-low cost, a single possible result (thus no possibility of human interference), lack of intentional bias, and it produces simple boundaries that do not meander needlessly. It has the disadvantage of ignoring geographic features such as rivers, cliffs, and highways and cultural features such as tribal boundaries. This landscape oversight causes it to produce districts different from those a human would produce. Ignoring geographic features can induce very simple boundaries.

While most districts produced by the method will be fairly compact and either roughly rectangular or triangular, some of the resulting districts can still be long and narrow strips (or triangles) of land.

Like most automatic redistricting rules, the shortest splitline algorithm will fail to create majority-minority districts, for both ethnic and political minorities, if the minority populations are not very compact. This might reduce minority representation.

The introduction of modern computers alongside the development of elaborate voter databases and special districting software has made gerrymandering a far more precise science. Using such databases, political parties can obtain detailed information about every household including political party registration, previous campaign donations, and the number of times residents voted in previous elections and combine it with other predictors of voting behavior such as age, income, race, or education level. With this data, gerrymandering politicians can predict the voting behavior of each potential district with an astonishing degree of precision, leaving little chance for creating an accidentally competitive district.

On the other hand, the introduction of modern computers would allow the United States Census Bureau to calculate more equal populations in every voting district that are based only on districts being the most compact and equal populations. This could be done easily using their Block Centers based on the Global Positioning System rather than street addresses. With this data, gerrymandering politicians will not be in charge, thus allowing competitive districts again.

The malapportionment had been originally designed to favour rural areas in the 1930s-1950s by a Labor government who drew their support from agricultural and mine workers in rural areas. This helped Labor to stay in government from 1932–1957. As demographics and political views shifted over time, this system came to favour the Country Party instead.

Senate constituencies were created by grouping all lower-chamber districts in a region, or by dividing a region into two constituencies of contiguous lower-chamber districts. The 1980 Constitution allocated a number of seats to appointed senators, making it harder for one side to change the Constitution by itself. The opposition won 22 senate seats in the 1989 election, taking both seats in three out of 19 constituencies, controlling 58% of the elected Senate, but only 47% of the full Senate. The unelected senators were eliminated in the 2005 constitutional reforms, but the electoral map has remained largely untouched (two new regions were created in 2007, one of which altered the composition of two senatorial constituencies; the first election to be affected by this minor change took place in 2013).

In the election of 2005, The Left (successor of the PDS) gained 8.7% of the votes and thus qualified for top-up seats.

This allows Congress to create new districts once a place reaches 250,000 inhabitants, the minimum required for its creation. With this, local dynasties, through congressmen, can exert influence in the district-making process by creating bills carving new districts from old ones. In time, as the population of the Philippines increases, these districts, or groups of it, will be the basis of carving new provinces out of existing ones.

The United States, among the first countries with an elected representative government, was the source of the term gerrymander as stated above.