We use the word love in many ways: to describe the attraction between two individuals, to describe the physical sexual act between two individuals, to describe the bond between the mother and the father and their child, or to describe the bonds between members of a family or closed society. These different types of love between human beings are, of course, in many ways interrelated. Love has been a theme for poets/authors since mankind started to communicate. We have enjoyed reading how it is to be in love, but have been spared a molecular explanation of what happens in our brains! During the last century, the knowledge of human physiology and chemistry has expanded from a rather vague knowledge of basic processes of the body to a detailed knowledge down to the level of molecules and atoms. Subjects like human reproduction have been studied in detail. The field of love, and the chemical and neurological processes in the end leading to a wish to reproduce, has apparently, and not surprisingly, been a field of research of low priority. However, during the last ten years, scientists and journalists have discussed various aspects on the chemistry of love, and the importance of pheromones, phenylethylamine and other compounds and physiological events for human love in television programmes, newspapers and magazines. When scrutinizing the various claims, it is apparent, that many “true facts” are hypothetical theories, and that in many cases observations made on animals have directly been transferred to the human world. It is sometimes surprisingly difficult to find scientific information and scientific publications related to this field in research bases like Pubmed and PsycINFO. The information in this paper should be read with this knowledge, but also with an open and humble mind. This is a difficult but very important field of research.
We can identify several components in love between living creatures. Some scientists have chosen to describe the process of falling in love as a three-stage rocket: Lust, Romantic attraction and Attachment. During this process several neurological and biochemical systems will be engaged and interact. Although these three components are here described as interrelated, they may also be described as isolated phenomena.
It appears that the initial attraction, Lust, between two individuals is basically influenced by some simple anatomic facts. Thus males are attracted to women with, e.g., low waist to hip ratio and low age of face. The attraction of women to males varies across the menstrual cycle, a woman, e.g., preferring a symmetrical “masculine” face during the fertile phase of her menstrual cycle. This immediate interest is modified as soon as the couples get contact by all well-known factors like body language, speech, culture, shared interests etc. This would add the process of Romantic Attraction to the feeling of Lust.
Phenylethylamine (PEA), dopamine and serotonin
PEA is an amphetamine-related substance belonging to the “Ecstasy” group and has in numerous popular publications been named “the molecule of love” (not to be confused with the party drug MDA, “love”). Few, if any, up-to-date scientific publications can be found related to this subject, with the exception of a recent essay. It is suggested that PEA increases “warmth, affection, sexuality and the feeling of physical energy”. It is suggested that this neurotransmitter increases when an individual falls in love, and that this amphetamine-like substance will give the individual who has fallen in love a kick, a reward. It has also been suggested that chocolate, which contains PEA, will give the same kick. This is unlikely, taking the small amounts of PEA in chocolate in consideration, and the short half-life of the substance. It has also been suggested that the amphetamine-like effect of PEA will diminish with time, explaining why some relations fade away in some years (1-4 years, cf. “7-year-itch”). In summary, the thought of a role of PEA in human love is fascinating, but so far speculative.
Dopamine levels have been reported to be increased when an individual is in love, findings in agreement with neural correlates in functional MRI scanning, see below. The role of serotonin is poorly understood. It is known that selective serotonin uptake inhibitors (SSRI) may affect libido and orgasm; this problem may be less pronounced in highly selective SSRI.
Brain activity when in love, as investigated by Functional Magnetic Resonance (fMRI)
The activity of the brains of 17 individuals who were deeply in love was investigated using the technique of fMRI. The individuals were investigated while viewing pictures of their partners, and of pictures of three friends of similar age, sex and duration of friendship. The activity recorded while watching their partners was similar to all individuals and surprisingly restricted and unique, being located to foci in the medial insula and the anterior cingulated cortex, and subcortically, in the caudate nucleus and the putamen, all bilaterally. Some of these regions are implicated in happy states and social interactions. Similar studies have been performed with mothers and their children. Interestingly, brain areas rich in oxytocin and vasopressin receptors (see below) are shown to be active when the mothers look upon pictures of their own children but not the children of other women.
Vasopressin and oxytocin. The hypothalamic-pituitary–adrenal (HPA) axis
Recently, these pituitary hormones have attracted the attention as being of possible importance in establishing long-lasting bonds between humans, Attachment. If we discuss Attachment in the simple terms of the popular three-stage love theory it has been suggested that vasopressin (for males) and oxytocin (for females) will create a bond to keep the man and women together when the Romantic Attraction has faded away, and, according to the speculative PEA theory, when there is no longer any amphetamine-like reward in the relationship. While oxytocin is secreted during labour and vasopressin is of importance in fluid balance, these hormones may also increase during touch and caress, and, maybe in humans, during intercourse (see above). Positive social behaviours, including social bonds and attachment may, furthermore, diminish the activity of the HPA axis. Interestingly, vasopressin and oxytocin have been implicated in the control of the HPA axis. The great interest in vasopressin and oxytocin in relation to attachment has originated in a series of fascinating studies of the prairie vole. Vole species display a wide range of social behaviour, ranging from being highly social and monogamous to being solitary and promiscuous. Social bonds form after sexual activity, and behaviors associated with monogamy, including pair-bond formation are facilitated by vasopressin binding to a receptor in the ventral pallidal region. If sexual activity is prevented, a bond can be created by injecting vasopressin. In one experiment, administration of vasopressin receptor antibodies prevented the creation of bonds. In another experiment, the density of vasopressin receptors in the ventral pallidal region was increased by delivering the receptor gene using an adeno-associated vector.
These males exhibited an increase in pair-bond formation compared to controls. Thus, these and other experiments show that vasopressin and oxytocin are important in the creation of bonds in non-human mammals, and for the first time, scientist have succeeded in changing the behavioral pattern by introducing hormone receptors in the brain. The question is – do vasopressin and oxytocin circuits play the same role in humans. Does the surge in oxytocin during labour play a role in creating maternal love and bonds between mother and child? Has a possible increase in vasopressin/oxytocin during intercourse any significance? More important, will couples who live together, have children together, care for each other, use these circuits in creating bonds and long-lasting relations? As pointed out in an essay in Nature commenting the vole studies of the Larry Young laboratory, there are other important questions related to this subject. Lower-quality day care may alter infant attachment and cortisol patterns; what will happen later in life? Severely deprived children, who have lived their entire life without love, may exhibit abnormalities in later relationships. We may in cases like this improve our knowledge and possibility to help the children by brain imaging and neurobiochemical studies.
Conclusion
We are just in the beginning in understanding the chemistry of love in humans and there are many question marks. Which role do pheromones, PEA and oxytocin/vasopressin play in human love life? We know, however, that the power of love in humans is great. Lust and romantic attraction can affect us in unpredictable ways, and make everyone act irrationally. It is also worth mentioning that this research has narrowed down the theory of “love at first sight”, lust and love. Therefore, one can now conclude that chemical attraction brings two humans of the opposite sex together, allows them to behave in affectionate ways and then provides the avenue for real love to grow as time goes by. It is my opinion now, that there is no such thing as “love at first sight”. Human beings have an addictive nature, hence the reason, when the levels of catecholamines decrease they “fall out of love” with their mate and long for such a feeling again, they go in search of it elsewhere. Seldom, does a couple resurrect the “feelings” in their relationship by repeating something that brought the chemical attraction in the first place. Relationships should not be based on feelings for when the feelings fade away, one who is “hooked” on it, will also fade with it. When one is “hooked” on the feelings brought about by catecholamines (PEA), I refer to this person as being in “lust” and when one has developed bonds with their mate even after the catecholamines have faded away, I refer to this person as being in “love”.
The studies of attachment are of interest in a broader sense, in that we not only are dealing with human beings in love, but also with attachment between individuals in the society. These studies have focused interest on the chemistry and brain function in interaction between individuals, a new and important field of research.
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