Dr. Rene Hurlemann, executive senior physician at the Inpatient and Outpatient Department of Psychiatry and Psychotherapy of the Bonn University Medical Center in Germany, and a team of scientists at the University of Bonn, Ruhr University of Bochum in Germany and the University of Chengdu in China examined the effect of oxytocin on couples.
For a long time, science has been trying to discover the unknown forces that cause loving couples to be faithful, Hurlemann said.
"An important role in partner bonding is played by the hormone oxytocin, which is secreted in the brain," Hurlemann said in a statement.
The researchers showed pictures of their female partners to a total of 40 heterosexual men who were in a permanent relationship as well as pictures of other women for comparison. First a dose of oxytocin was administered to the subjects in a nasal spray; and then a placebo at a later date.
The scientists also studied the brain activity of the subjects with the help of functional magnetic resonance tomography.
"When the men 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," lead author Dirk Scheele said.
In another series of tests, the researchers tested whether oxytocin enhanced the activation of the reward system only when seeing the partner or whether there is a similar effect with pictures of acquaintances and female work colleagues of many years.
"The activation of the reward system with the aid of oxytocin had a very selective effect with the pictures of the partners," Scheele said. "We did not detect this effect with pictures of longstanding acquaintances."
Based on these findings, the scientists said simple familiarity was not enough to stimulate the bonding effect -- they had to be loving couples.
The study, published in the Proceedings of the National Academy of Sciences, found oxytocin activated the brain's reward system, thus maintaining the bond between the lovers and promoting monogamy.
"This biological mechanism in a couple relationship is very similar to a drug," Hurlemann said. "Both in love and in taking drugs, people are striving to stimulate the reward system in the brain. This could also explain why people fall into depression or deep mourning after a separation from their partner -- due to the lack of oxytocin secretion, the reward system is understimulated, and is more or less in a withdrawal state."