Many reports of extrapair paternity are little more than quotes based on hearsay, anecdotes, and unpublished findings.
Simmons, Firman, Rhodes, and Peters reviewed 11 published studies of extra-pair paternity from various locations in the United States, France, Switzerland, the United Kingdom, Mexico, and among the Yanomamo Indians of South America.
The Standard Cross-Cultural Sample describes the amount of extramarital sex by men and women in over 50 pre-industrial cultures.
The amount of extramarital sex by men is described as "universal" in 6 cultures, "moderate" in 29 cultures, "occasional" in 6 cultures, and "uncommon" in 10 cultures.
The number of married partners who engage in extramarital sex never exceeds 50% in studies using large or nationally representative samples.