Baker, R. and M. Bellis. 1993 "Human sperm competition: ejaculate adjustment by males and the function of masturbation." _Animal Behaviour_ 46: 861-65

"On average, about 10 percent of children are not sired by their supposed fathers. Some men, however, have a higher chance of being deceived in this way than others – and it is those of low wealth and status who fare worst. Actual figures range from 1 percent in high-status areas of the United States and Switzerland, to 5 to 6 percent for moderate-status males in the United States and Great Britain, to 10 to 30 percent for lower-status males in the United States, Great Britain and France. Moreover, the men most likely to sexually hoodwink the lower-status males are men of higher status." ( "Sperm Wars: The science of sex" Basic Books, NY / Fourth Estate, London 1996 pp. 124--5)