In cryptography, a tontine is a secret sharing algorithm which allows n people to share secret data, such that any k of them can reconstruct it by combining their keys.

For the Australian pillow manufacturer, see Tontine Group.

A tontine is an investment vehicle which combines features of a group annuity, group life insurance, and a lottery.

The scheme is named after Neapolitan banker Lorenzo de Tonti, who is generally credited with inventing it in France in 1653. Some sources claim that similar schemes already existed in Italy, but there is no dispute that the popularity of the form was due to Tonti.

The basic concept is simple. Each investor pays a sum into the tontine. The funds are invested and each investor receives dividends. As each investor dies, his or her share is divided amongst the surviving investors. This process continues until only one investor survives. Originally, the last surviving subscriber received only the dividends: the capital reverted to the state upon his or her death and was used to fund public works projects, which often contained the word "tontine" in their name. In a later variation, the capital would devolve upon the last survivor, effectively dissolving the trust and usually making the survivor very wealthy; it is this version that has often been the plot device for mysteries and detective stories.

While once very popular in France, Britain, and the United States, tontines have been banned in Britain and the United States due to the incentive for investors to kill one another, thereby increasing their shares. Geneva, in Switzerland, was known for its active market in tontines in the 17th and 18th centuries. Nevertheless, there are underground organizations in the US that still use the tontine, and ownership of a business or property by joint tenancy with right of survivorship has much the same effect.

Tontines were the first government bonds issued anywhere in the world, and the British government first issued tontines in 1693 to fund a war against France. However, tontines soon caused problems for their issuing governments, as they would increasingly underestimate the longevity of the population. At first, tontine holders included men and women of all ages. However, by the mid-18th century, investors had caught on how to play the system, and it became increasingly common to buy tontines for young children, especially for girls around the age of 5 (since girls lived longer than boys, and by which age they were less at risk of infant mortality). This created the possibility to produce great returns for the holders, but it proved to be quite costly for the governments (not unlike modern day "pay-as-you-go" pension schemes). As a result, the tontine scheme was eventually abandoned, and as of the mid-1850s, the tontines had been replaced by other investment vehicles such as "penny policies", a predecessor to the 20th century invention of the pension scheme.

A tontine also may refer to a bottle of liquor a group of individuals keeps for the last of the group to drink when the others have died. A reverse tontine may refer to a bottle of liquor kept by a group of individuals to drink when the first of its members passes away.

Tontines have often appeared in fiction, usually, but not always, as a motive for murder.

• In The Wrong Box, a novel by Robert Louis Stevenson, the object is to conceal the death of one of the last two investors.
• In an episode of The Simpsons, Grampa and Mr. Burns enter into a tontine during World War II, involving a treasure of antique paintings stolen from a German castle. When the two of them become the only surviving members, they compete for the rights to the prize. Eventually they both lose once the US State Department interferes and takes the paintings back to the German baron (and stereotypical Eurotrash) who is the rightful owner.
• A tontine is also used as a plot device in the novel Seventy Seven Clocks by Christopher Fowler, and in one of the Dr. Syn books.
• P.G. Wodehouse used a similar idea in his novel Something Fishy (titled The Butler Did It in the U.S.). In Wodehouse's version, the money did not go to the last survivor, but to the last son of the investors who remained unmarried.
• The plot of the Agatha Christie murder mystery 4.50 From Paddington, with Miss Marple, is based on a tontine will.
• The French writer Alain-René Lesage wrote a play called La Tontine in 1709.
• The Wild Wild West, the 1967 TV series, season 2 episode 16 is titled: The Night of the Tottering Tontine. In a Ten Little Indians-style plot, James West and Artemus Gordon, assigned to protect a prominent professor, are trapped as a tontine collapses at a seaside mansion during a storm.
• The 2001 comedy film Tomcats features a similar arrangement, where several young men put money into a fund to be paid to the last unmarried man among them. The comic interest stems from the efforts of one of the last two to get the other one married.
• The Being of Sound Mind episode of Diagnosis Murder uses a tontine as the motivation for multiple murders.
• Thomas B. Costain, a popular writer in the 1950s, wrote The Tontine around 1955. This novel, set in the 19th century, gives a good overview of how a tontine works or worked in history.
• In an episode of Barney Miller, which aired 7 January 1982, an old man attempts suicide to allow his cousin to become the survivor of a tontine.
• Tontine is the name of a reality TV game show based on the premise of a tontine: competitors contribute to a common fund, which is awarded to the person who remains in the competition the longest.
• In one of the last seasons of the television show "M*A*S*H*" a more innocuous version of a tontine was portrayed. Col. Sherman Potter (Harry Morgan) is very depressed for much of one episode "Old Soldiers". At episode's end he discloses, to the men and women under his command, he has learned he is the last survivor of a tontine, a group of World War One soldiers/close friends who pledged the last of them would drink a toast to the others. The tontine award is the last bottle of brandy from a cache the unit had discovered. In the last scene he and his campmates drink a toast to all his departed comrades.
• Lawrence Block's Matt Scudder mystery A Long Line Of Dead Men features a fraternal society which, while it is not a Tontine itself, is repeatedly described in terms of a tontine in the novel. It is also described as a tontine on the blurb to the paperback edition. Here, the privelege given to the last surviving member of the group is to choose a disparate group of men to replace all those who have died during his own time in the organisation, and then, at their first meeting, to read a list of the deceased for a final time. Meeting once a year for a ceremonial meal, the sole purpose of the society is to mark the passing of its members.

• The Straight Dope: What's with tontines, the odd annuities in which you benefit when others die?
• The Wild Wild West: The Night of the Tottering Tontine.
• Template:En icon The Tontine Agreement