A cuckold is the husband of an adulterous wife. In evolutionary biology, the term is also applied to males who are unwittingly investing parental effort in offspring that are not genetically their own.

In evolutionary biology, the term cuckold is also applied to males who are unwittingly investing parental effort in offspring that are not genetically their own. As noted above, the term cuckold is derivative of the mis-directed parental investment of birds who direct parental investment to the eggs that cuckoo birds have laid in their nests.

In his book "Sperm Wars", biologist Robin Baker speculated that the excitement and stimulation of the cuckolding fetish emerges from the biology of sexuality and the effects of sexual arousal on the brain, although it is important to note the word "cuckold" does not appear in his book. According to one of his theories, Baker believes that when a man thinks that his female mate may have been sexual with another man, the man is prompted by biological urges to copulate with the female in an effort to "compete" with the other man's sperm. Baker is also one of the few proponents of the theory of "Killer Sperm", the idea that sperm compete not only for first access to the egg but by "attacking" other sperm. Although this idea appears frequently in cuckold fetish material, very few biologists share this view.

Baker and his proponents' views conflict with the hypothesized foundations for sexual jealousy in evolutionary psychology, which is rooted in the idea that men, specifically, will react jealously to sexual infidelity on the parts of their mates. Infidelity is also the number one cause for divorce.

The cuckold’s urge to thrust, through intercourse or masturbation, is often enhanced by the presence of the bull, whether real or fantasized. A study by Gordon Gallup and coworkers (2003) concluded that one evolutionary purpose of the thrusting motion characteristic of intense intercourse is for the penis to “upsuck” another man’s semen before depositing its own.