Petraeus is not the first high-ranking military man to have an affair, said Farley, who is also a past president of the American Psychological Association. Famously gruff World War II general George Patton had an affair with his wife's step-niece. General Douglas MacArthur had a mistress named Isabel Rosario Cooper, whom he met in the Philippines.
And General Dwight D. Eisenhower, later president, may have had an affair with his World War II chauffer, Kay Summersby, according to the woman's memoirs and some suggestive letters left behind after both parties died.
"The nation should not be surprised at Petraeus having an affair," Farley told LiveScience.
Leaders like Petraeus tend to be bold risk-takers, Farley said, a personality trait that is very helpful when leading soldiers into battle. The same trait may make these leaders more likely to take risks in their personal lives, as well. [10 Easy Paths to Self Destruction]
Broadwell may have some of the same risk-taking traits as the former director. In a January interview with The Charlotte Observer, Broadwell, who is also married, called herself and her husband "adventure junkies."