I don’t know why you’re not there
I give you my love, but you don’t care
So what is right?
And what is wrong?
Gimme a sign
What is love?
Baby, don’t hurt me
Don’t hurt me no more
Valentine’s Day is the ideal opportunity for us, who talk science, to use the moment and write something on neurochemistry of love, passion, attraction, sex, and relationships.

Why do we need love? Evolutionary biology has given us some answers: love, and more precisely, the connection between partners, is an important advantage for the Homo sapiens species, because the species’ young grow up slowly, and their childhood and adolescence last a lot longer than with other species. So, the children need their parents much longer, and the child has the best chance of survival if both parents are present.



Some species are monogamous, like a lot of birds species, and it’s a rarity to see the pair ‘separate’, and if one of the partners dies, it is a common thing for the other partner to become depressed. Cases of self-harm are also not a rarity, especially with parrots, who sometimes start to pluck their feathers after their partner dies. Swans, eagles, crows – all are monogamous birds which, when once commit, it’s for life. Monogamous birds we’re probably the most familiar with are pigeons (all types) which have a pretty extensive courtship process, the male pigeon is the one to build the nest and most of the time lay on the eggs and feed the young. Pigeons have probably, due to this, become the symbol of love.



What is it that motivates some species to be monogamous, and others to be polygamous? Evolutionary biology gives some more explanations: e.g. polygamy increases the risk of sexually transmitted diseases, and infections decrease fertility and pose a risk to offspring survival (according to The Handbook of Evolutionary Psychology, edited by David M. Buss, John Wiley & Sones, Inc., 2005. Chapter 14, Commitment, Love, and Mate Retention by Lorne Campbell B. and Bruce J. Ellis.). 

Own work


However, all this is still not the answer to what love and passion are. Some say that love is a thing of the brain and neurotransmitters, and not heart, however, brain and the heart have equally important roles: a cocktail of neurotransmitters plus the heart is strong enough to pump the blood into the genitalia, that is the right recipe.
The main “forces” of love are sexual drive, commitment and preferences for partner selection. The first two segments are controlled by neurotransmitters and hormones. It is not hard to guess that the main hormones in this process steroid sexual hormones: estrogens and androgens (testosterone) hormones. In fact, it would be more precise to say that these are the hormones that control lust. All sexual hormones are by their chemical nature steroids and are synthesized in the organism from cholesterol, and all steroids have a common structure of the so-called  cyclopentanoperhydrophenanthrene ring.
But lust is only one face of love: chemical forces that create attraction, that is, falling in love are more complex from the sole existence of estrogen and androgen – if it were to gonadal hormones, we would jump at each others, rape and hate each others. Even raw passion needs a bit more than just the drive for procreation and erection. Other components of falling in love are neurotransmitters dopamine, noradrenaline (norepinephrine) and serotonin.
Dopamine, or 3,4 dihydroxyphenethylamine is a catecholamine, a signal molecule that has a big role in the behavior occurring in cases where there is an award (or punishment), that is, dopamine controls the first lessons (the “carrot and stick” system). If a certain behavior results in an award, of course we will try and replicate the behavior. If the award is, let’s say an orgasm, it is obvious that we will try to repeat the behavior. Animal training is based on this type of learning. However, if a certain experience produces a punishment as a consequence or it produces a trauma, we will instinctively avoid situations similar to the one that caused the trauma. Dopamine is one of the molecules important in pain processing – physical and psychological pain.
Noradrenaline is another catecholamine, and the main postganglionic neurotransmitter in the sympathicus. It accelerates the heart rate and dilates the pupils, it affects sleep regulation and mood, and is the main neurotransmitter in “fight or flight” stressful situations.
Serotonin or 5-hydroxytryptamine is a monoamine neurotransmitter, however, around 90% of serotonin is not created in the brain… but in the stomach. Serotonin has a role in cognitive functions like memory and learning, it regulates the mood and appetite, and is known as the “molecule of happiness” in popular science. A low level of serotonin is tied to depression and anxiety, so some medicines used in these cases act to prevent the decomposition of serotonin blocking the monoamine oxidase enzyme and increasing the serotonin concentration in the organism. A lowered level of serotonin can cause heart and breathing failure, and more recently low levels of serotonin are tied to the “crib death” syndrome (SIDS, Sudden infant death syndrome). However, serotonin is also tied to the obsessive-compulsive disorder (OCD) and lowered levels of serotonin lead to obsessive behavior. Nevertheless, the science community is split on the issue of how much is serotonin linked to OCD. Donatella Marizziti from the University of Pisa studied the levels of serotonin in those newly in love and compared them to the levels of serotonin in individuals suffering from OCD and a group of people who are neither in love nor suffer from OCD. The levels of serotonin in those in love and suffering from OCD were around 40% lower than in individuals that don’t suffer from OCD and are not in love. In translation – its very hard to differentiate love from a mental disorder.
Also, it was shown that people in love have higher cortisol levels, the hormone of stress.
However, partner selection is just another pair of shoes. It seems that we choose our partners almost subconsciously, based on how much they differ from us. Claus Wedekind from the Lausanne University has, in 1995, done an interesting experiment (Wedekind C, Füri S. “Body odour preferences in men and women: do they aim for specific MHC combinations or simply heterozygosity?” Proc R Soc Lond B1997; 264: 1471–1479): he asked 49 women to smell some sweaty shirts from unknown men with different genotypes which affected the subtle sweat smell nuances. He asked them then to tell him which shirt smelled the best and which one smelled the worst. Sweat smells that women chose to be the “best” were genotypically the most different than their own genotypes – based on these “pheromones” we seek a person different from us, that may have a different immune system, meaning something in their immune system that ours lack. In that way, the chances of a healthier, stronger and more resilient offspring are higher. That is the moment when all Freudian theories on how individuals seek a partner who resembles their father or mother, because of the subconscious sexual desire towards the parent of the opposite sex, fall into water.
Nevertheless, all these are molecules that affect our desire to have sex with someone. Which molecules connect us and “make” us to stay together?
Oxytocin is a neuropeptide (peptides are smaller, shorter amino-acid chains) also known as the “love hormone”. Oxytocin is important for the creation of the so-called mother’s instinct and the connection between the mother and child, but the molecule has the same effect on both sexes: oxytocin is what in part creates positive impressions and affects the connection of a group, which makes it, in a way, important for sociological phenomena, but also for empathy. Also, oxytocin seems to be one of the “award” neurotransmitters, and the scheme of the path activation is similar to cocaine euphoria. Also, oxytocin causes our tendencies to trust someone.
chemical structure of oxytocin
There are more and more evidence for another neurotransmitter, vasopressin or the antidiuretic hormone (ADH) impacts the schemes of social behavior, sexual motivations and connections.
Despite all written here, however, we should not think that love is just a mix of a few molecules and demean the importance of happiness, feeling of fulfillment, connection and obligations to our partners, the feeling that love is a miracle, but use the information here as a reason to love more, harder and deeper.


Jelena Kalinić, MA in comparative literature and graduate biologist, science journalist and science communicator, has a WHO infodemic manager certificate and Health metrics Study design & Evidence based medicine training. Winner of the 2020 EurekaAlert (AAAS) Fellowship for Science Journalists. Short-runner, second place in the selection for European Science journalist of the year for 2022.

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