Cuddle hormone Oxytocin makes monogamous

Hormone activates reward system only at the sight of one's own partner

We humans are more or less monogamous - and thus an exception among mammals. © SXC
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A small spray dose of oxytocin and already the man is faithful. Sounds simple and is it - at least in the experiment - also: If researchers gave men the cuddle hormone oxytocin to men, they found pictures of foreign women less attractive than that of their own partner. The hormone activates the reward system only at the sight of the partner and thus promotes monogamy, as the researchers report in the journal "Proceedings of the National Academy of Sciences".

Monogamy is not very common in mammals, humans are more of an exception. For a long time, science has been puzzling over which factors cause love between lovers of love. "The hormone oxytocin, which is released in the brain, plays an important role in partner binding, " explains René Hurlemann from the University Hospital Bonn. For example, certain vole mice are known to promote monogamy and tight pair bonding. In addition, there are indications that this hormone, which is released, for example, when cuddling and also during orgasm, also plays an important role in human relationships.

Hormone makes partner more attractive

Hurlemann and his colleagues have now investigated in an experiment whether the "bonding hormone" also reduces the temptation to go alien. For this purpose, the researchers showed 40 heterosexual, firmly engaged men pictures of their partners and for comparison of foreign women. Subjects were previously given either an oxytocin dose with a nasal spray or a placebo. While the participants looked at the images, the scientists recorded their brain activity using functional magnetic resonance imaging (fMRI).

The result was clear: "If the men received oxytocin instead of the placebo, their reward system in the brain was very active at the sight of the partner, " says first author Dirk Scheele. The men found her partner more attractive and desirable than the foreign women. But is this effect really specific to the partner, or does the hormone just make acquaintances and friends more attractive than strangers?

The researchers tested this in a further test by showing their subjects pictures of longtime acquaintances and co-workers instead of the pictures of the partner and alternately the strangers. Again, participants previously received either oxytocin or a placebo. "Activation of the reward system using oxytocin was very selective with the pictures of the partners, " reports Scheele. "We did not notice this effect when it came to pictures of longtime acquaintances." Display

Togetherness works like a drug

According to these results, mere familiarity is not enough to stimulate the binding effect. It has to be love couples, the scientists are convinced. Overall, the data confirmed that oxytocin activates the reward system, thereby maintaining the bond between the lovers and promoting monogamy.

"This biological mechanism of togetherness is very similar to a drug, " says Hurlemann. In both the love and consumption of drugs, people sought to stimulate the reward system in the brain. This might also explain why people fall into depression or deep mourning after being separated from their partner: the reward system is under-stimulated due to a lack of oxytocin release and quasi withdrawal the researcher. However, therapy with the binding hormone might be counterproductive: oxytocin supplements could possibly increase the suffering, because the longing for the beloved partner would only be greater would recommend.

At first glance, monogamy is not biologically meaningful. Because from the classical viewpoint of evolutionary biology men have an advantage, if they spread their genes through many different partners as much as possible. But another aspect also plays a major role: If oxytocin strengthens the pair bond, it increases the stability of the nurses and thus the chance of survival of the offspring, explains Hurlemann. And the children are in turn spread their own genes. (Proceedings of the National Academy of Sciences, 2013; doi: 10.1073 / pnas.1314190110)

(Rheinische Friedrich-Wilhelms-University Bonn, 26.11.2013 - NPO)