A green-beard effect occurs when an allele, or linked alleles, produce three phenotypic effects:

As Dawkins points out in The Selfish Gene (Chapter 6) and The Extended Phenotype, this must be distinguished from the green-beard effect.

Since then, 'green-beard effect' has come to refer to forms of genetic self-recognition in which a gene in one individual might direct benefits to other individuals that possess the gene.

Green-beard effects gained their name from a thought-experiment of Richard Dawkins, who considered the possibility of a gene that caused its possessors to develop a green beard and to be nice to other green-bearded individuals.

The green-beard effect is a hypothesis used in evolutional biology to explain altruism between individuals and is linked to the gene-centered view of evolution which postulates that natural selection will increase the frequency of those alleles whose phenotypic effects ensure their successful replication.

Green-beard effect could increase altruism on green-beard phenotypes and therefore its presence in a population even if alleles are assisting other genes that are not exact copies of themselves in a molecular sense; all that is required is that they produce the three phenotypic characteristics described above.

This process produces adaptations for the benefit of genes that promote the reproductive success of the organism, or of other organisms containing the same gene (kin altruism and green-beard effects), or even only its own propagation in detriment to the other genes of the genome (intragenomic conflict).