"Optimal fitness, -, may consist in selecting a mate who is genetically similar but not actually relative. Van den Berghe (1983) speculates that the ideal percentage of relatedness is 12.5 percent identical by descent, or the same as that between first cousins. Other animal species also avoid inbreeding. For example, several experiments have been carried out with japanese quail, birds that, although promiscuous, proved particularly sophisticated. They preferred first cousins to third cousins, and both of these relatives to either unrelated birds or siblings, thus avoiding the dangers of too much or too little inbreeding (Bateson, 1983)"
Rushton: Race, Evolution and Behaviour p.76