the mean number of genes shared between two related individuals. Identical twins share 100% of their genes and have a coefficient of relatedness of 1. Between siblings and between parents and offspring, the coefficient of relatedness is .5; between uncles or aunts and nieces or nephews and between grandparents and grand-offspring, it is .25; and between first cousins, it is .125. Most studies of
kin recognition and kin-directed behavior show effects only with coefficients of relatedness of 0.25 and greater. See also
inclusive fitness.