Summary
Cooperation has been a longstanding evolutionary problem because cooperators suffer an individual fitness loss to provide benefits to other individuals in the group. A notable solution to this problem involves the greenbeard effect, first conceived of by W.D. Hamilton in the early 1960s as a thought experiment to explain the evolution of altruism, and later popularized by R. Dawkins. The main tenet of the greenbeard effect is that there exists a hypothetical gene, or set of linked genes, that expresses a perceptible trait (e.g., green beard), enables the bearer to recognize this trait in other
