Martin A. Nowak

Martin A. Nowak works on mathematical models of evolution, including the evolution of cooperation. His 2006 synthesis separated five mechanisms: kin selection, direct reciprocity, indirect reciprocity, network reciprocity and group selection. Each mechanism changes which interactions contribute to an individual's evolutionary success.^[ref]

A simple direct reciprocity calculation shows how repeated encounters can change incentives. Suppose cooperation costs $c$, benefits the partner by $b>c$, and an encounter continues with probability $w$. Two cooperating reciprocators obtain expected payoff $(b-c)/(1-w)$. An unconditional defector against a strategy that stops helping after a defection obtains $b$ initially and no later benefit. The first payoff exceeds the second precisely when $w>c/b$. With $b=3$, $c=1$ and $w=1/2$, these payoffs are four and three.

This calculation depends on the specified repeated-interaction model and retaliation rule. It identifies a mechanism by which cooperation can be favored, without asserting that a single inequality describes every biological system. His institutional research profile also lists mathematical work on infection and cancer.^[ref]

Portrait source: www.oeb.harvard.edu/people/martin-nowak .