Monogamy is a trait exhibited by few species, humans are one of them. When a man falls in love, what is it that changes in him? What makes him stick to that single partner? What keeps them together? The secret is out, it's oxytocin. Oxytocin referred as the “bonding hormone” or the “love hormone”, is secreted by the pituitary gland and is also the reason for maternal bonding.

In a study published by the journal Proceedings of the National Academy of Sciences, Oxytocin enhances brain reward system responses in men viewing the face of their female partner. All overtures of physical intimacy release oxytocin in humans.

The research was conducted on a group of heterosexual committed male volunteers. The first step of the experiment was to dispense oxytocin nasal spray on these men. Next step involved showing pictures of familiar and unfamiliar faces. Using functional magnetic resonance tomography to study the brain activity of these men they found that under the influence of oxytocin men found their partners more appealing than the unfamiliar equally attractive females. When viewing their partners, the reward system in the brain was more active.

Another experiment involved testing whether similar effects were observed with all familiar faces. Result, "The activation of the reward system with the aid of oxytocin had a very selective effect with the pictures of the partners," says psychologist Dirk Scheele. "We did not detect this effect with pictures of longstanding acquaintances." It can be concluded from these observations that oxytocin stimulates bonding for a loving couple.

According to Prof. Hurlemann, who led the study, the couple chemistry is similar to a drug as it stimulates the brain's reward system. “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 under-stimulated, and is more or less in a withdrawal state," said Prof. Hurlemann.