An international team of researchers says they have uncovered a biological mechanism that could explain the attraction between loving couples. If oxytocin is administered to men and if 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, according to the scientists.

The team published their results (“Oxytocin enhances brain reward system responses in men viewing the face of their female partner”) in the Proceedings of the National Academy of Sciences.

Monogamy is not very widespread among mammals. Yet, many Homo sapiens couples have no other partners in a love relationship, said Prof. René Hurlemann, executive senior physician at the inpatient and /outpatient department of psychiatry and psychotherapy of the Bonn University Medical Center. “An important role in partner bonding is played by the hormone oxytocin, which is secreted in the brain,” explained Prof. Dr. Hurlemann.

Scientists at the university under the direction of Prof. Hurlemann and with participation by researchers at the Ruhr University of Bochum and the University of Chengdu (China) examined the effect of the “bonding hormone” more precisely.

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.

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,” noted lead author Dirk Scheele.

“We report the results of a discovery and a replication study, each involving a double-blind, placebo-controlled, within-subject, pharmaco-functional MRI experiment with 20 heterosexual pair-bonded male volunteers,” wrote the investigators in the journal article. “In both experiments, intranasal OXT treatment (24 IU) made subjects perceive their female partner’s face as more attractive compared with unfamiliar women but had no effect on the attractiveness of other familiar women. This enhanced positive partner bias was paralleled by an increased response to partner stimuli compared with unfamiliar women in brain reward regions including the ventral tegmental area and the nucleus accumbens.”

Overall the data showed that oxytocin activates the 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,” continued Prof. Hurlemann, adding that 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,” explained Prof. Hurlemann.

However, therapy with the bonding hormone could possibly be counterproductive. “Administration of oxytocin could possibly even increase the suffering, because it would only make the longing for the beloved partner even greater,” said the Professor.

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