A book by a founder of this site.
Research on Infidelity and Affairs
Research on "infidelity" and other "relational betrayals" (i.e., doing things you are not supposed to do like breaking promises) also indicates that lying and deception is common among lovers (see Buss; Barash & Lipton).
Research reveals that infidelity is a real problem—anywhere from 30 to 60% of all married people have cheated on a spouse (see Buss & Shackelord for a review of research on infidelity). You can also calculate the risk of your partner cheating on you (see cheater risk assessment).
Lending further support to the idea that infidelity is common, roughly 2% of all children born are not the biological offspring of the assumed father (see stats about infidelity).
More importantly, close to 50% of all first marriages end in divorce and many couples who stay together are not happy. This is important to mention because deception, though ever present, is much more common as relationship begin to fall apart.
Finally, other types of betrayals such as, breaking promises, revealing confidences, forgetting special occasions (i.e., like an anniversary), and so on, happen all the time (see Metts). And it is common for people to cover such mistakes through the use of deception.