As another bill supporting marriage equality prepares to enter the Parliament next week, we thought it’d be a good time to look at the parliamentary oddity that is the conscience vote. Labor’s Tanya Plibersek seems to be calling on Prime Minister Tony Abbott to allow his party one (as is his sister), but what is a conscience vote exactly, and when are they used? What is a conscience vote? A conscience vote is when party members are free to vote on a bill according to their own moral, political, religious or social beliefs, rather than having to vote according to a party line. In most Westminster parliaments, party members are bound to vote in a bloc, with party discipline strictly enforced by the appointed whip. While some choose to break party discipline and cross the floor, party members risk significant punishment for doing so. A conscience vote removes the possibility of party members crossing the floor when voting on contentious issues, by allowing them to vote according to their own personal beliefs. The decision to allow a conscience vote is made by the party leader. The main argument for conscience votes is that they allow parliamentarians the opportunity to better express the pluralism present in society, while critics believe too many conscience votes would make governing nearly impossible. When are they used? Conscience votes are generally used when an issue comes before the Parliament that involves a matter of significant moral, ethical or religious importance. In Australia, conscience votes have been allowed on issues relating to abortion, capital punishment, euthanasia, reproductive and scientific research, divorce, homosexuality and drug reform. Conscience votes may also include issues on which the parties do not have a formal policy, such as parliamentary procedure and parliamentary privilege. A leader might decide on a conscience vote due to pressure from the media, the public, lobby groups, or even his/her own backbench. When have they occurred in Australia? Australian marriage laws were made uniform following a conscience vote in 1961 under Liberal prime minister Robert Menzies, while in 1973 homosexuality was decriminalised after a conscience vote allowed by Gough Whitlam. The death penalty was subject to a conscience on vote on three occasions, in 1968, 1970, and finally 1973. More recently, Australian territories were prevented from introducing euthanasia laws in 1997 when a bill was passed by conscience vote after being introduced by the Liberals. Human cloning was prohibited after a 2002 conscience vote on the issue, while controversial abortion drug RU486 was the subject of a conscience vote in 2006. Former Labor prime minister Julia Gillard allowed a conscience vote on same-sex marriage in 2012 -- the only one in the Rudd-Gillard era -- while Abbott forbade his party members from doing the same. The bill failed to pass, but with acceptance of same-sex marriage spreading around the world, it was only a matter of time before it returned to the Parliament. How often do they occur? Not very. Since 1948, Australia has had only 33 conscience votes in federal Parliament. Some leaders are more partial to them than others: Gough Whitlam held eight during his brief tenure as prime minister, while Paul Keating held none during his time in office from 1991 to 1996. Menzies held five in 16 years, John Gorton six in three years, Bob Hawke three in nine years, and John Howard five in 12 years. On average, conscience votes occur in Australia at the rate of around once every two years -- less often than solar eclipses.