Scientists at the Bonn University Medical Center found that if oxytocin is administered to men and they are shown pictures of their partner, the bonding hormone stimulates the reward center in the brain, increasing the attractiveness of the partner, and strengthening monogamy.
Monogamy is not very widespread among mammals; human beings represent an exception. Comparatively many couples of the species Homo sapiens have no other partners in a love relationship. For a long time, science has therefore been trying to discover what causes loving couples to be faithful.
"An important role in partner bonding is played by the hormone oxytocin, which is secreted in the brain," said Professor Dr Rene Hurlemann, Executive Senior Physician at the Inpatient and Outpatient Department of Psychiatry and Psychotherapy of the Bonn University Medical Center.
The researchers showed pictures of their female partners to a total of 40 heterosexual men who were in a permanent relationship - and 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. Furthermore, 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," said lead author Dirk Scheele.
In another series of tests, the researchers tested whether oxytocin enhances 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," said Scheele.
"We did not detect this effect with pictures of longstanding acquaintances," he said. Based on these results, therefore, simple familiarity is not enough to stimulate the bonding effect. They have to be loving couples; of that the scientists are convinced.
"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", said Hurlemann.
The results are published in the Proceedings of the National Academy of Sciences (PNAS).


Latest News from Lifestyle News Desk