Bonding in the Brain: Oxytocin

Some genes have such a long and extensive history of research that it is difficult to decide where to start. This article will be about oxytocin, and also about love and caring, but also xenophobia and dishonesty. As a starting point for this story, I choose a cute little rodent from North America, the prairie vole. The prairie vole lives distinctly monogamous, which is rather unusual in the animal kingdom. Even the closely related and almost identical-looking montane vole is not monogamous. This has left researchers wondering why these behaviors are so different.

Monogamous and not so Monogamous Voles

In the early 1990s, researchers compared the brains of these two species that are so similar, yet exhibit such different sexual behaviors. They found that two receptor types in particular were distributed quite differently in the two species: those for oxytocin and those for vasopressin. Oxytocin and vasopressin act as hormones and each consist of nine amino acids. They are extremely similar: the two neuropeptides differ in only in two of these amino acids.

Prarie voles usually mate quite extensively and then have a strong lifelong bond with their partner. However, if the animals are only allowed to meet for a short time, this pair bond does not develop – unless we intervene: If you inject vasopressin into the brain of the males in the process, they show the usual strong preference behavior toward their mate. This also works with females, but after injection of oxytocin. Conversely, this partner bonding fails to occur when vasopressin or oxytocin release is suppressed in the animals. Further studies could show that oxytocin udnd vasopressin in this type of partner bonding mainly activates the mesolimbic dopamine system, which is considered the central “reward system” of the brain.

Prairie voles drawn by Profprestos with permission from Todd Ahern, Emory University based on his photograph using CC BY 2.0. Drawing reproduced from wikimedia based on Creative Commons Attribution-Share Alike 4.0 International.

Oxytocin from Nasal Sprays

The two vole species have since become the gold standard of monogamy research, and the number of publications on the subject has become unmanageable. However, more public attention was then attracted by those studies in which the system was investigated in humans. One particularly sensational study in 2005 showed that oxytocin administered as a nasal spray increased trust in humans. Specifically, the study involved subjects giving money to other people, who were able to return that money at a profit – or not. This, combined with the accumulating evidence that oxytocin increases couple bonding and caring, has led to booming sales of oxytocin nasal sprays.

In whichever species we looked, oxytocin just always seemed to increase bonding between individuals. Besides couple bonding, this includes mother-child bonding, and bonding between individuals in a social group. As a result, oxytocin became known as the quintessential cuddle, love-, and partnership fidelity hormone. Men who are in long-term relationships maintain a distance of about 10-15 cm further from an attractive woman after taking oxytocin by nasal spray than men who received a placebo nasal spray. In single men, oxytocin administration does not affect distance.

Plot Twist: Oxytocin and Xenophobia

In 2011, however, the story took an unexpected turn when Carsten De Dreu and his colleagues revealed the other side of the coin. For the stronger the behavior for the benefit of one’s own relatives, the stronger is the aggressive behavior towards outsiders in certain scenarios. Oxytocin can reinforce unfair behavior toward individuals outside one’s own group. Oxytocin also seems to increase the willingness to lie if it can be assumed that the lie serves the good of one’s own group.

Then again, researchers in Bonn conducted an interesting experiment immediately after the refugee crisis in Germany started in 2015. They gave students 50 euros and left it open as to how much of this should be donated to local or refugee people in need. In addition, they used a questionnaire to determine the degree of rejection towards migrants. The administration of oxytocin increased the willingness to donate only in those test persons who indicated a rather positive attitude toward the refugees, not in those who were rather negative toward them. However, another round of the experiment was really exciting: if, in addition to the oxytocin administration, the test persons were told how high the willingness to donate to the refugees was among other participants, this significantly increased their own willingness to donate to the migrants. One could interpret this result again as the already repeatedly demonstrated effect of oxytocin to adapt our behavior more to that of our own group. Perhaps we should simply hug our xenophobic neighbors more often and at the same time tell them that we welcome refugees with open arms. But what was that again about hugs and oxytocin?

Correlation, causality, doesn’t really matter on the Internet

It is proven that oxytocin levels are increased in women and men immediately after orgasm. For a few years now, the Internet has also been abuzz with reports that long hugs (many sources state a minimum time of 20 seconds) also trigger the release of oxytocin. On social media platforms, people like to advertise hugs with this. Well, I can only recommend hugs and that they lead to oxytocin release is certainly not excluded. Only: according to my research, it is rather not proven. In this regard, I am like this author since after quite some researching, I still could not find any study that would provide evidence for it. In fact, in some places (e.g. here) the claim is wrongly supported by this study. However, this study only showed that women who reported hugging their partners more often and longer had higher oxytocin levels. However, this correlation does not allow us to conclude causality: do hugs increase oxytocin levels or do women with higher oxytocin levels tend to hug more?

So the research story on oxytocin is certainly far away from being over. The question of the exact context in which oxytocin is more likely to promote pro-social or anti-social behavior currently has to be answered with: it’s complicated. A study from 2022 examined the generosity and satisfaction of test persons between the ages of 18 and 99 and is titled: “Oxytocin Release Increases With Age and Is Associated With Life Satisfaction and Prosocial Behaviors” – but again, we don’t know which is cause and effect here. 😊

Theresa Schredelseker

molecular biologist turned research manager and science communicator

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