It is known that the neuropeptide oxytocin affects the formation of friendships and parental feelings in humans. German psychologists have recently discovered another effect of this very simple peptide. It turned out that if you put it in the nose of men who are in a permanent monogamous relationship with a woman, then the comfortable distance to an unfamiliar attractive woman for them increases significantly, which cannot be said about men who do not have a permanent partner. Thus, an increase in the endogenous production of oxytocin in married men can serve as a guarantee of their marital fidelity.
Few species in the animal kingdom are known to form monogamous bonds, and humans are one of them. Monogamy is a rather expensive pleasure from the point of view of evolution, since it leads to a significant decrease in the number and genetic diversity of the offspring of the male. On the other hand, for females, the benefit is obvious, since in monogamous couples, care for the offspring falls on both parents. Therefore, it is logical to assume that in the process of evolution mechanisms have been developed that support monogamy.
One of these mechanisms turned out to be surprisingly conservative — the regulation of social relations with the participation of the neuropeptides oxytocin and vasopressin. Homologues of these neuropeptides are already involved in the regulation of relationships in worms, insects, and molluscs. In prairie voles, rodents known for their marital fidelity, administering oxytocin to females promotes mating, while administering vasopressin to males turns them into loving husbands. In humans, the most well-known function of oxytocin is the regulation of female reproductive function (stimulation of uterine contractions and lactation). Studies with intranasal (nose dripping) oxytocin have shown that oxytocin administration to men increases the degree of gullibility and the ability to understand the mood of other people, as well as more often look the interlocutor in the eyes (see, for example, Genes control behavior, and behavior — genes, «Elements «, 12.11.2008). In the past two years, it has become clear that oxytocin also affects the establishment of romantic relationships between a man and a woman. But until recently, it was not known whether oxytocin is involved in maintaining monogamous bonds in humans.
A group of German psychologists recently conducted a study on 86 young heterosexual men, half of whom were in stable monogamous relationships and the other half were not in a permanent relationship. In the first experiment, subjects were given either oxytocin or a placebo drip into their noses. After that, the subjects were shown an attractive woman and asked to choose a distance that was comfortable for them and slightly uncomfortable (Fig. 1Ai). In one case, the woman approached the subject and he had to stop her; in the other case, the subject approached the woman and stopped himself. In addition, it was proposed to evaluate the degree of attractiveness of the object on a nine-point scale, and also found out the feelings of the subject (valence, arousal, confusion, rejection, etc.).
In the second experiment, the men were also given either oxytocin or a placebo drip into their noses and then presented with different pictures on a monitor. Both “positive” images (an attractive woman or a beautiful landscape) and “negative” images (a distorted photograph of a woman or dirt) were presented as pictures. All images were taken from the database of photographs used for the study of emotions (IAPS). The pictures were presented in four stages, each with five images of each type. To select one or another picture, the subjects had to move the joystick either forward or backward. In this case, the image either increased (imitation of the approach of the object) or decreased (imitation of the removal of the object). The reaction time (time of moving the joystick) and the number of errors were measured when the joystick was moved in the wrong direction. The subjects also had to rate the attractiveness of the object and describe their feelings.
A series of control experiments was also carried out.
In order to make sure that oxytocin affects precisely marital fidelity, and not, for example, sociability, a separate group of heterosexual men with a permanent partner was offered a man, not a woman, as a presented object in the first experiment.
In order to finally make sure that the presented woman is really attractive to the subjects, they were asked to rate her appearance from the photo on a 100-point scale.
The results of the first experiment showed that the optimal distance between a man who is in constant communication with a woman and an object (an attractive woman) increases significantly after oxytocin instillation, and this occurs in all situations (that is, regardless of whether the subject or object is moving, approaching whether they either move away, whether they look into each other’s eyes or not; Fig. 1Aii, Aiii). On the contrary, instillation of oxytocin to loners in almost all situations (seven out of eight) did not change the optimal distance. Only when the object approached the subject and looked into his eyes did oxytocin affect the change in the comfortable distance to the object in loners as well. Interestingly, in all groups of subjects, the comfortable distance was greater if the subject moved away from the object than if he approached it. The authors interpret this result in such a way that a close initial distance is more uncomfortable than a far initial distance. It also turned out that the differences between men who are in a relationship and loners after instillation of oxytocin are stronger if the subject is moving than if the object is moving.
As for the slightly uncomfortable distance, it turned out to be greater in men who had a constant connection after instillation of oxytocin, but only in cases where the woman was moving. In singles, the slightly uncomfortable distance remained the same in all situations. The authors also looked at the relationship between slightly uncomfortable distance and other characteristics of the subjects and found a positive correlation between uncomfortable distance and the degree of social anxiety (see: Augustine Osman et al., 1998. The Social Phobia and Social Interaction Anxiety Scales: Evaluation of Psychometric Properties). This correlation suggests that scoring slightly uncomfortable distances is a good yardstick for reflecting a subject’s motivation to make contact with a stranger.
In a control experiment with a man as an object, no correlations were found between different parameters — in other words, the instillation of oxytocin affected intersexual relations, and not sociability as such. In another control experiment, the subjects rated the attractiveness of the woman who participated as the subject in the first experiment quite highly: she was awarded an average of 64 out of 100 points.
The results of the second experiment showed a significant relationship between oxytocin instillation and marital bonding only for positive female images (Fig. 1B). After instillation of oxytocin, coupled men, looking at an attractive woman, thought much more before answering correctly, and made much more mistakes than single men.
So, this study shows that the behavioral effects of oxytocin depend on the context and the presence of a stable partner. This work is the first direct evidence that oxytocin can change the behavior of married men in a way that maintains a monogamous relationship. Interestingly, the survey of subjects did not reveal any significant changes in feelings or arousal caused by the sight of an attractive woman. This means that the effect of oxytocin cannot be associated with a conscious attitude towards the object. This statement is also confirmed by the lack of association between the effect of oxytocin administration and the direction of gaze. One might expect that the optimal distance should increase if the subject and the object look into each other’s eyes; but it wasn’t.
Oxytocin is produced in the hypothalamus and released into the blood. Measurement of blood oxytocin concentration in people of both sexes showed that this concentration rises at an early stage of romantic love and remains elevated during the first 6 months of marriage (Schneiderman et al., 2012. Oxytocin during the initial stages of romantic attachment: relations to couples ‘ interactive reciprocity). After that, the production of endogenous oxytocin in men generally decreases; it rises only for a short time after the physical proximity of the partners and even after a short stay next to each other.
The authors do not at all encourage suspicious wives to drip oxytocin to husbands who strive to look at the side (although perhaps some readers may take this study as a direct guide to action). However, scientists emphasize the need for further research on the neurochemical mechanism of the effect of oxytocin on behavior, as well as analysis of other aspects of human relationships that may affect the production of oxytocin. This will undoubtedly bring us closer to a better understanding of the extremely complex and mysterious neurobiology of the formation of matrimonial relationships in humans.
A source: Dirk Scheele, Nadine Striepens, Onur Güntürkün, Sandra Deutschländer, Wolfgang Maier, Keith M. Kendrick, René Hurlemann. Oxytocin modulates social distance between males and females // The Journal of Neuroscience. V. 32. P. 16074–16079.