Researchers from the Bonn University Medical Center in Germany performed a new experiment that suggest oxytocin is responsible for strengthening monogamy in men. Dr. René Hurlemann, executive senior physician from the Bonn University Medical Center, notes that "an important role in partner bonding is played by the hormone oxytocin, which is secreted in the brain."
In the study, 40 heterosexual men in LTRs were shown photos of their partners as well as photos of stranger ladies, all while a dosing themselves with oxytocin (or, alternately, a placebo) via a nasal spray.
Lead author Dirk Scheele says that when the subjects "received oxytocin instead of the placebo, their reward system in the brain when viewing the partner was very active, and they perceived them as more attractive than the other women."
Interestingly, when the dudes were later shown pictures of female coworkers or other women they knew, the oxytocin wasn't as effective.
So, I guess if you're worried about your dude's wang wandering out in search of brighter pastures (filled with vaginas?), maybe you need to bathe in oxytocin more regularly. Or, you know, dump the dude.
They haven't tested this on women yet, but Dr. Hurlemann said they "quite often found opposite behavioral effects in females, compared with males."