In study after study on human sexuality, in particular marriage fidelity, men are found to cheat more often than women. You can find dozens of papers and articles that ultimately explain this in terms of the evolutionary drive to “spread our genes.” This line of reasoning goes like this: men use two mating strategies to spread their genes. Make lots of babies with random women and some will be good at making lots more babies, thus spreading your genes around, or make a few babies with one woman and stay to raise, train and protect them to ensure quality.
It is easy to understand how both strategies can lead to spreading one’s genetic material. And in studies of other species, even ones like birds who take a single mate and stay together for a season or for life and jointly raise their offspring, male infidelity is not uncommon.
The American divorce rate of 50% certainly implies that one or another of the couple is seeking a “better” mate. Many studies show that such divorces are not uncommonly cause by male infidelity.
Those who study this subject say that female infidelity doesn’t make nearly as much evolutionary sense. A female can only have one child at a time, regardless of how many males she might be sexually active with (although this isn’t universally true across all species; with some animals, a female can have a single litter with different fathers). Her best strategy is to pick the fittest male she can - hence the sometime dramatic courtship displays by many animal species - and try to keep him around to protect and help raise the infants.
But female infidelity DOES exist. In a study by Jane Reid, a biologist at the University of Aberdeen in Scotland, in a group of wild song sparrows, 28% of the hatchlings were from fathers other than the female’s mate. A suggested motive for this female infidelity was that perhaps she mated with males even better fit than her mate. Reid’s study showed that wasn’t true. The “bastard” hatchlings actually produced fewer offspring creating somewhat of a genetic bottleneck for the founding female’s gene line.
In the end, it may all be due to male promiscuity after all. If there is a gene that causes some males to choose the “promiscuous” route to spreading his genes, as opposed to the “nurturing” route, perhaps those genes are spread to his female offspring, making more likely to engage in similar behavior.
To bring this to a personal level, I’d like to suggest that if humans, and possibly other species, find the right mate, all this infidelity might vanish. Perhaps the 50% of marriages that DON’T end in divorce mean that at least half of us are better are picking the right mate.