Penn Biologists Reveal That Generosity Is Linked to Evolutionary Success

Associate professor Joshua Plotkin and postdoctoral researcher Alexander Stewart, both of the Department of Biology, examined the outcome of the classic game theory match-up known as the “Prisoner’s Dilemma,” as played repeatedly by a large, evolving population of players. While other researchers have previously suggested that cooperative strategies can be successful in such a scenario, Stewart and Plotkin offer mathematical proof that the only strategies that succeed in the long term are generous ones. They reported their findings in the Proceedings of the National Academy of Sciences.

“Ever since Darwin,” Plotkin says, “biologists have been puzzled about why there is so much apparent cooperation, and even flat-out generosity and altruism, in nature. Our paper provides an explanation for why we see so much generosity in front of us.”

The Prisoner’s Dilemma is a way of studying how individuals choose whether or not to cooperate. In the game, if both players cooperate, they both receive a payoff. If one cooperates and the other does not, the cooperating player receives the smallest possible payoff, and the defecting player the largest. If both players do not cooperate, they receive a payoff, but it is less than what they would gain if both had cooperated. In other words, it pays to cooperate, but it can pay even more to be selfish.

Their work builds upon the seminal findings of economist John Nash, who advanced the field of game theory in the 1950s, as well as those of computational biologist William Press and physicist-mathematician Freeman Dyson, who last year identified a new class of strategies for succeeding in the Prisoner’s Dilemma: an extortion strategy.

Stewart and Plotkin became intrigued by Press and Dyson’s work, and began to explore a different approach to the Prisoner’s Dilemma. Instead of a head-to-head competition, they envisioned a population of players matching up against one another, as might occur in a human or animal society in nature. The most successful players would get to “reproduce” more, passing on their strategies to the next generation of players.

Stewart noticed the first of these generous approaches among the strategies that Press and Dyson had defined. After simulating how some generous strategies would fare in an evolving population, he and Plotkin crafted a mathematical proof showing that not only can generous strategies succeed in the evolutionary version of the Prisoner’s Dilemma, but in fact these are the only approaches that resist defectors—those who do not cooperate—over the long term, because an extortion strategy doesn’t succeed if played against itself.

The discovery, while abstract, helps explain the presence of generosity in nature, an inclination that can sometimes seem counter to the Darwinian notion of survival of the fittest.

Read the full story here.

Arts & Sciences News

University of Pennsylvania, Neubauer Family Foundation, and Philadelphia Police Department Partner to Support Police Leadership Education

The first-of-its-kind graduate degree in the U.S. for police leaders launches this fall at the School of Arts & Sciences.

View Article >
Marisa C. Kozlowski Named Next Associate Dean for the Natural Sciences

Kozlowski, who joined the Penn faculty in 1997, succeeds Mark Trodden, who transitions to the Dean of Penn Arts & Sciences on June 1.

View Article >
One Fourth Year, One Alum Receive 2025 Hertz Fellowship

Eric Tao, C’25, Gr’25 (left), and Suraj Chandran, C’23, were awarded the honor, part of a group of 19 fellows selected this year. Each one receives five years of funding toward a doctoral program.

View Article >
Benjamin Nathans Wins 2025 Pulitzer Prize in General Nonfiction

Nathans, Alan Charles Kors Endowed Term Professor of History, won for his book “To the Success of Our Hopeless Cause: The Many Lives of the Soviet Dissident Movement.”

View Article >
Mark Devlin Elected to National Academy of Sciences

He joins three others from Penn to receive the honor this year, all recognized for “distinguished and continuing achievements in original research.”

View Article >
Michael Jones-Correa and Sophia Rosenfeld Elected to American Academy of Arts & Sciences

They join three others from the University of Pennsylvania, selected as part of the Academy’s mission to convene leaders from “every field of human endeavor to examine new ideas, address issues of importance to the nation and the world, and work together.”

View Article >