Lindy effect

The Lindy effect is a theory that the future life expectancy of some non-perishable things like a technology or an idea is proportional to their current age, so that every additional period of survival implies a longer remaining life expectancy. Where the Lindy effect applies, mortality rate decreases with time.

History

The origin of the term can be traced to Albert Goldman and a 1964 article he had written in The New Republic titled "Lindy's Law". The term Lindy refers to Lindy's delicatessen in New York, where comedians "foregather every night at Lindy's, where ... they conduct post-mortems on recent show business 'action'". In this article, Goldman describes a folkloric belief among New York City media observers that the amount of material comedians have is constant, and therefore, the frequency of output predicts how long their series will last:

... the life expectancy of a television comedian is [inversely] proportional to the total amount of his exposure on the medium. If, pathetically deluded by hubris, he undertakes a regular weekly or even monthly program, his chances of survival beyond the first season are slight; but if he adopts the conservation of resources policy favored by these senescent philosophers of "the Business", and confines himself to "specials" and "guest shots", he may last to the age of Ed Wynn [d. age 79 in 1966 while still acting in movies]

Benoit Mandelbrot defined a different concept with the same name in his 1982 book The Fractal Geometry of Nature. In Mandelbrot's version, comedians do not have a fixed amount of comedic material to spread over TV appearances, but rather, the more appearances they make, the more future appearances they are predicted to make: Mandelbrot expressed mathematically that for certain things bounded by the life of the producer, like human promise, future life expectancy is proportional to the past. He references Lindy's Law and a parable of the young poets' cemetery and then applies to researchers and their publications: "However long a person's past collected works, it will on the average continue for an equal additional amount. When it eventually stops, it breaks off at precisely half of its promise."

Nassim Taleb presented a version of Mandelbrot's idea in The Black Swan: The Impact of the Highly Improbable by extending it to a certain class of non-perishables where life expectancy can be expressed as power laws.

With human projects and ventures we have another story. These are often scalable, as I said in Chapter 3. With scalable variables ... you will witness the exact opposite effect. Let's say a project is expected to terminate in 79 days, the same expectation in days as the newborn female has in years. On the 79th day, if the project is not finished, it will be expected to take another 25 days to complete. But on the 90th day, if the project is still not completed, it should have about 58 days to go. On the 100th, it should have 89 days to go. On the 119th, it should have an extra 149 days. On day 600, if the project is not done, you will be expected to need an extra 1,590 days. As you see, the longer you wait, the longer you will be expected to wait.

In Taleb's 2012 book Antifragile: Things That Gain from Disorder he for the first time explicitly referred to his idea as the Lindy Effect, removed the bounds of the life of the producer to include anything which doesn't have a natural upper bound, and incorporated it into his broader theory of the Antifragile.

If a book has been in print for forty years, I can expect it to be in print for another forty years. But, and that is the main difference, if it survives another decade, then it will be expected to be in print another fifty years. This, simply, as a rule, tells you why things that have been around for a long time are not "aging" like persons, but "aging" in reverse. Every year that passes without extinction doubles the additional life expectancy. This is an indicator of some robustness. The robustness of an item is proportional to its life!

According to Taleb, Mandelbrot agreed with the expanded definition of the Lindy Effect: "I [Taleb] suggested the boundary perishable/nonperishable and he [Mandelbrot] agreed that the nonperishable would be power-law distributed while the perishable (the initial Lindy story) worked as a mere metaphor."

Pareto distribution

Lifetimes following the Pareto distribution (a power-law distribution) demonstrate the Lindy effect. For example with the parameter $\alpha =2$, conditional on reaching an age of $x>x_{\min {}}$, the expected future lifetime is also $x$. In particular, initially the expected lifetime is $2x_{\min {}}$ but if that point is reached then the expected future lifetime is also $2x_{\min {}}$; if that point is reached making the total lifetime so far $4x_{\min {}}$ then the expected future lifetime is $4x_{\min {}}$; and so on.

More generally with proportionality rather than equality, given $m>0$ and using the parameter $\alpha ={\frac {m}{m-1}}$ in the Pareto distribution, conditional on reaching any age of $x>x_{\min {}}$, the expected future lifetime is $(m-1)x$. Example: for $\alpha =2$ or $m=2$ the expected future lifetime is $x$.

Lindy effect is connected to Pareto’s Law, to Zipf’s Law, and to socioeconomic inequality.

See also

• Ergodic theory
• Planning fallacy
• Survivorship curve
• Survivorship bias