The most serious threat to a monogamous relationship is the number of people involved in affairs. A conservative estimate of 60% of men and 40% of women are at some point engaging in extramarital affairs.
While affairs happen in non-married but committed relationships as well as within married relationships, most of the statistics deal only with extramarital affairs.
These statistics began with Kinsey's reports in the 1940s and 1950s.
Report Author Year men women
Kinsey's 40's-50's 50% 26%
Hunt's 1972 N/A 52%
The Hite's 1980 72% N/A
Cosmopolitan 1980 N/A 50%-69.2%
Shere Hite's 1987 N/A 70%
Jan Halper 1988 82% N/A
* N/A means the study was a single gender study.
Since then we've basically resided to the notion that it's a 50 - 50 chance that someone would cheat on you.