To read the news lately, certain folks in the U.S. military, the CIA and Congress should probably pay attention to new research that suggests the hormone oxytocin may promote fidelity within monogamous relationships.
Blow, A.J. & Hartnett, K. (2005). Infidelity in Committed Relationships II: A Substantive Review. Journal of Marital and Family , 31, 217-233.