A Couple of Thoughts on Adultery

Robert Ryan and Joan Fontaine in Born to Be Bad (1950)

Mating strategies differ for men and women.  A woman instinctively pursues a quality relationship to assure the continuance of her lineage.  She is focused on finding the best father for her children and assuring the best care for her children.  According to sociologist Joanna Wróblewska-Skrzeks, a man may forgo quality for quantity.  He can be instinctively drawn to an adulterous affair as it allows him to maximize his offspring.  But serious flaws exist in this strategy.  A man who has many children with many women is unable to properly look after his offspring.  There's no benefit in having children if the lack of fatherly attention and support leads them into a poor and dysfunctional existence.       

Ed Harris and Jessica Lange in Sweet Dreams (1985)

The cheaters are, in the end, outside of the norm.  A spouse is more likely to cheat if they are narcissistic.  The narcissist, who is selfish, impulsive and outgoing, will easily be motivated to cheat out of anger or boredom.  

Kirk Douglas and Barbara Stanwyck in The Strange Love of Martha Ivers (1946)

A spouse is more likely to cheat if they are neurotic.  The neurotic will cheat out of insecurity or a desperate need for attention.

https://www.amazon.com/Unfaithful-History-Adultery-Anthony-Balducci/dp/B0C8Y1L7KK