Oxytocin has long been deemed "the love hormone," after its important role in social bonding has been documented. But now, researchers have performed a new experiment that suggests oxytocin stimulates the reward center in the male brain, increasing partner attractiveness and strengthening monogamy.
The researchers, from Bonn University Medical Center in Germany, who published their results in the journal PNAS, are quick to point out that monogamy is not very widespread in mammals.
More the exception than the rule, humans frequently exhibit this trait.
As such, the researchers say science has long tried to uncover the forces that prompt loving couples to practice fidelity.
Dr. René Hurlemann, executive senior physician from the Bonn University Medical Center, notes that "an important role in partner bonding is played by the hormone oxytocin, which is secreted in the brain."
Familiarity 'not enough' to activate reward system
Coupled with oxytocin, only photos of romantic partners - not acquaintances - triggered the reward system for men.
To investigate the effects of this hormone more closely, Dr. Hurlemann and his team, in collaboration with researchers from Ruhr University of Bochum in Germany and the University of Chengdu in China, showed 40 heterosexual men who were in a permanent relationship photos of their female partners.
For comparison, the team also showed the men photos of other women.
All the while, a dose of oxytocin was delivered to the subjects via a nasal spray, though later a placebo was also used.
Additionally, the researchers looked at the participants' brain activity with functional magnetic resonance tomography.
Lead author Dirk Scheele says that when the subjects "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."
In later tests, the scientists looked at whether oxytocin has a similar effect when the subjects looked at photos of acquaintances and female work colleagues, in order to determine whether familiarity enhances the activation of the reward system in light of oxytocin.
However, Scheele explains that the reward system activation "with the aid of oxytocin had a very selective effect with the pictures of the partners."
In other words, familiarity is not enough to prompt the bonding effect of oxytocin. They must be loving couples.
Oxytocin's drug-like effects
Dr. Hurlemann says their findings show how oxytocin's effects are "very similar to a drug" for couples in a permanent relationship.
When drug users take drugs, they are trying to stimulate the brain's reward system, which is a similar effect shown in the experiment.
Dr. Hurlemann adds:
"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."
Despite this finding, the researchers note that therapy with oxytocin could potentially be counterproductive, as a boost of oxytocin could make the longing for the partner even stronger, potentially increasing suffering.
So, do these findings suggest oxytocin drugs men into monogamy? After all, the researchers point out that the classical view of evolutionary biology suggests men have an advantage in "disseminating their genes" as widely as possible, with as many partners as possible.
However, Dr. Hurlemann also points out another biological aspect of monogamy that provides a benefit:
"When oxytocin strengthens the partner bond, it increases the stability of the persons providing nutrition and thus the chances of survival for the progeny."
In turn, both the man's and the woman's genes live on in the children, thus providing a biological incentive for monogamy.
Potential gender differences
When asked whether he and his team have performed the same experiment in women, Dr. Hurlemann told Medical News Today that they have not.
However, he did say that in other studies on female subjects, they "quite often found opposite behavioral effects in females, compared with males."
Dr. Hurlemann continued:
"These gender differences could be related to the fact that oxytocin baseline levels in blood differ between genders, and also the brain oxytocin receptor distribution might differ, but I am not aware of any study documenting this in the living human brain, due to the lack of oxytocin receptor tracers for PET studies."
Medical News Today recently reported on a study from Northwestern University, which suggested oxytocin strengthens the memory of negative social events and elevates fear and anxiety in the future.