Scientists from fields such as evolutionary psychology, evolutionary biology, anthropology, and neuroscience study the biology of romantic love. They examine chemicals in the brain, like dopamine and oxytocin, as well as connected parts of the brain, such as the mesocorticolimbic pathway, which are involved in the feelings and actions linked to romantic love.

Research on romantic love is still in its early stages. As of 2021, only 42 scientific studies had been conducted on this topic.

Definition of romantic love

The meaning of "romantic love" has changed over time, making it hard to describe in simple terms. The term was first used to describe attitudes and behaviors found in a group of writings called courtly love. However, scientists in psychology and biology also study romantic love differently, focusing on brain systems linked to forming bonds or mating, along with related emotions.

In 2021, Bode and Kushnick studied romantic love from a biological viewpoint. They examined its psychology, how it develops over a lifetime, its functions, and its evolutionary background. Based on their findings, they proposed a biological definition:

Romantic love does not always involve two people or social interactions, even though it can be connected to forming bonds. It can occur outside a relationship, such as in unrequited love, where feelings are not returned. A person may feel romantic love for someone they have not yet met, such as in cases of love at first sight or parasocial attachments.

The early stage of romantic love, which can feel obsessive or addictive, is sometimes called "being in love," passionate love, infatuation, limerence, or obsessive love. While scientists have not agreed on a single term or method to study it, this stage is different from the "attachment system" described by researchers like John Bowlby. Some experts once believed attachment theory could replace other theories about love, but others argue romantic love and attachment are not the same. The early stage of romantic love involves brain systems with unique purposes and evolutionary origins. Romantic love is also different from sexual attraction, even though the two often happen together.

Romantic love is expressed in many ways. A study of people who are in love found four groups with different levels of intensity, obsessive thinking, commitment, and frequency of sexual activity. Other research shows romantic love can occur with or without obsessive features. Usually, intense romantic love lasts 12 to 18 months, or up to three years. In rare cases, some people experience strong romantic feelings for 10 years or longer. This later stage is similar to early intense romantic love but includes fewer obsessive traits.

Independent emotion systems

Helen Fisher and her team suggested that the brain systems linked to reproduction in mammals can be divided into at least three parts:

• Lust refers to the desire for sexual activity, also known as libido.
• Attraction (or early romantic love, also called passionate love or infatuation) is connected to feelings of excitement, obsessive thoughts, and a strong wish to form an emotional connection.
• Attachment (from attachment theory, also called companionate love) is linked to feelings of calm, safety, and comfort, but also causes worry or anxiety when separated from someone.

In Fisher’s theory, these systems usually work together, but they can also become separate and function on their own. For example, a person in a long-term relationship might feel a strong bond with their partner, feel intense romantic love for someone else, and be sexually attracted to other people all at the same time. Lisa Diamond has also used the idea of separate emotions to explain how people can experience love without sexual desire, such as in cases of strong, non-romantic feelings for a friend.

Fisher connects each system to specific brain chemicals (lust: estrogen and androgens; attraction: dopamine, norepinephrine, and serotonin; attachment: oxytocin and vasopressin). However, recent research shows these connections are not as clear as Fisher’s theory suggests. Romantic love has also been linked to other brain chemicals, such as endogenous opioids, cortisol, and nerve growth factor, which were not included in Fisher’s earlier model. Modern theories now suggest that the attachment system is active during early romantic love, along with the infatuation part. While Fisher’s model is now seen as outdated, the idea that these systems are connected remains useful.

Evolution of systems

Evolutionary psychology is a way of organizing ideas that explains how psychological functions work, not just how they appear. It also sets rules, like the idea that a trait must help a person survive or reproduce. Evolutionary psychology offers many explanations for romantic love.

• Romantic love helps people stay committed to a partner. It stops people from looking for other mates, even if someone better comes along. It also tells others that a person is taken, making them less likely to try to win them over. Longer relationships in humans began when women's fertility became harder to tell, like when ovulation is not obvious. This meant partners had to stay together for the whole menstrual cycle. Commitment helps with this and with taking care of children. Love might be the brain’s way of rewarding people when they solve the problem of staying committed.

• The strong feelings of romantic love and why people act foolishly for love can be explained by the handicap principle. This idea says that honest signals (like real emotions) and fake signals (like fake smiles) compete. Honest signals can survive if they are too costly to fake. For example, a peacock’s large, colorful tail is expensive to grow and maintain. Only healthy peacocks can afford it, so females use it to find healthy mates. Romantic love may have evolved to be very strong, like being “handcuffed to railroad tracks,” to show that a person is truly committed.

• Romantic love may have evolved to help people reproduce even if it is costly, like raising children. It may also help people choose partners who will stay involved in raising children.

• Romantic love shows that a person will invest in their children. When fathers stay with their partners, children are more likely to be healthy and succeed in life.

• Being in love makes people more creative. Some think art, music, and literature are like a peacock’s tail, showing mental ability to impress a partner. Creativity may be especially important in men’s courtship.

• Romantic love helps people focus their attention on one partner instead of many. This saves time and energy.

• Happy, long-term relationships improve health and survival. When relationships end, like through divorce, people are more likely to get sick, feel sad, or use drugs. People who live alone are also more likely to have heart attacks again.

• Monogamous relationships (where people have only one partner) help prevent diseases that harm fertility, like syphilis. These diseases can cause miscarriages or harm unborn children. Having fewer partners reduces the risk of getting such diseases.

• Romantic love encourages people to stay with one partner, which helps prevent jealousy. Jealousy evolved to protect people from losing their partner. However, too much jealousy can be harmful.

Romantic love likely evolved during or after humans began walking on two legs. The earliest human-like species with evidence of walking upright and some signs of pair bonding is Ardipithecus ramidus, from about 4.4 million years ago. Some scientists think monogamy (having one partner) evolved because walking upright made it harder for mothers to carry babies, so fathers helped. Another idea is that human babies are born less developed than other animals, which made fathers more important for survival.

Some scientists think romantic love may have started even earlier, around the time humans and chimpanzees shared a common ancestor, 5–8 million years ago. While chimpanzees mostly mate when they find a chance, they sometimes show behaviors like guarding a mate, which is similar to romantic love.

Helen Fisher’s theory says romantic love is a system that helps people choose a mate and focus on them. She thinks this system evolved in mammals for finding partners, like how animals choose mates based on looks or behavior. In humans, this also includes personality traits. Romantic love and courtship in animals both use brain circuits that give pleasure. While courtship in most animals is short, romantic love in humans can last much longer.

A different view, by Adam Bode, says Fisher’s theory only explains short-term attraction, like love at first sight or a crush.

Mechanics

The early stage of romantic love is sometimes compared to a behavioral addiction, which is an addiction to something that is not a drug. In this case, the "addictive substance" is the person you love. Addiction involves a process called incentive salience, which is also known as "wanting." This is when things in the environment catch a person's attention and become very appealing, like a strong magnet that pulls someone toward a reward. Incentive salience is different from craving because craving is something a person is aware of, but incentive salience may not always be. While incentive salience can make people feel a strong need to act, it can also drive actions without a person realizing it. For example, in an experiment, people who used cocaine did not know they were choosing a low dose of the drug more often than a fake version.

In the incentive-sensitization theory of addiction, repeated use of drugs makes the brain more sensitive to drugs and the things that remind a person of them, leading to extreme "wanting" to use the drug. People in love are thought to feel incentive salience when they think about the person they love. Lovers also share other similarities with addicts, such as tolerance (needing more of something to feel the same effect), dependence (needing something to feel normal), withdrawal (feeling bad when something is taken away), relapse (returning to old habits), craving, and changes in mood.

Incentive salience is controlled by a brain chemical called dopamine, which is sent through a pathway called the mesocorticolimbic pathway. This pathway is involved in rewards, motivation, and learning. Dopamine signals for incentive salience start in an area of the brain called the ventral tegmental area (VTA) and travel to areas like the nucleus accumbens (NAc) in the ventral striatum. The VTA is one of two main areas in the brain that produce dopamine (the other is the substantia nigra pars compacta). Dopamine activity in the NAc helps the brain notice the importance of things linked to rewards. Brain scans of people in love using fMRI (a type of imaging) often show activity in these areas, such as the VTA and NAc, when they look at a picture of someone they love. Another area of the brain involved in reward and motivation, called the caudate nucleus, is also active in romantic love. This area has many dopamine receptors and is linked to learning from rewards, such as money or drugs. These brain activities suggest that early romantic love is a strong motivation system, similar to addiction.

In addiction research, scientists separate "wanting" a reward (incentive salience, linked to dopamine) from "liking" a reward (pleasure, linked to other brain areas). People can become addicted to drugs even if using them no longer feels good or harms their lives. They may also strongly "want" something they don’t consciously desire. Similarly, people in love may "want" a loved person even if being with them is painful. For example, someone might want to contact an ex-partner after a breakup, even if it causes sadness. It is also possible to be "in love" with someone you do not like or who treats you poorly.

Scientists have proposed different theories to explain how addictions start and continue. One theory by Wolfram Schultz suggests that dopamine signals help the brain learn by showing the difference between what a person expects to happen and what actually happens (called a reward prediction error). Drugs like cocaine trick this system by overstimulating dopamine neurons, creating a stronger signal than natural rewards. Another theory by Kent Berridge and Terry Robinson says that dopamine signals are linked to motivation but may not directly cause learning. However, research shows that dopamine is important for both learning and motivation. Studies using fMRI have found that people in long-term romantic relationships show similar brain activity to addiction when their expectations about how a partner views them are confirmed or challenged. This activity is seen in areas like the VTA and striatum, which are also active in addiction research.

While romantic love and addiction share some similarities, they are not the same. One major difference is that the addictive feelings in romantic love often fade over time as a relationship develops. In contrast, drug addiction usually gets worse over time, leading to loss of control and negative emotions. Scientists think this difference might be related to a hormone called oxytocin, which is active in romantic love but not in addiction. Oxytocin may help reduce the effects of drug withdrawal and prevent long-term addiction problems.

Experts do not all agree on whether love is always an addiction or when it should be treated. The term "love addiction" has not been clearly defined and is not currently a recognized mental health condition. However, one definition suggests that people addicted to love may feel bad when separated from a partner and strongly want to see them to cope with stress. Some researchers include people who have been rejected in love as love addicts, while others say love is an addiction only when it causes harm. A broader view is that all love is a type of addiction, like how people depend on food. Research on behavioral addictions (like gambling) is less developed than research on drug addiction, but some studies show that people can have brain patterns similar to drug addicts when responding to natural rewards like food or sex. Romantic love may be a "natural" addiction, different from drug addiction because it can help people form relationships and is not harmful when it is mutual. Scientists like Helen Fisher and Arthur Aron suggest that romantic love is a "positive addiction" when it is mutual and a "negative addiction" when it is unrequited or harmful.

Oxytocin is sometimes called the "love hormone" because it is involved in bonding between parents and children and in forming lasting relationships between adults. Oxytocin is made in the body and plays a role in social connections.

Brain imaging

Brain imaging methods like functional magnetic resonance imaging (fMRI) and positron emission tomography (PET) have been used to find out which parts of the brain are involved in romantic love. Most of these studies have had people look at a picture of their loved one during an fMRI scan, though some studies used different methods. Differences in how the studies were designed, such as how long the participants had been in love or what tasks they did during the scan, may explain why results sometimes vary.

In 2000, a study by Andreas Bartels and Semir Zeki from University College London was the first fMRI study of romantic love. The 17 participants were "deeply in love," had been together for an average of 2.4 years, and saw one or two photos of their loved one during the scan. Two main brain areas were active: the middle insular cortex, linked to feelings like "butterflies in the stomach," and part of the anterior cingulate cortex, connected to happiness. Other areas that showed activity included the cerebrum, caudate nucleus, putamen, and cerebellum. A later study in 2004 by the same authors found activity in the ventral tegmental area (VTA), which makes dopamine. The study also found that some brain areas were less active in romantic love compared to love for friends, such as the amygdala (involved in fear and risk detection) and the medial prefrontal cortex (mPFC, involved in understanding others’ intentions). These findings suggest that people in love may overlook risks and misunderstand others’ intentions.

In 2005, a study by Arthur Aron, Helen Fisher, Debra Mashek, Greg Strong, Haifang Li, and Lucy Brown was the first fMRI study of early-stage romantic love. This study was praised for improving understanding of intense, new love, even by critics of fMRI research. The 17 participants had been in love for about 7.4 months on average and thought about their loved ones most of the time. They also looked at a photo of their loved one during the scan. Active brain areas included the VTA and caudate, linked to reward and motivation, as well as the insular and cingulate cortex, involved in emotions. Some areas, like the ventral pallidum (linked to attachment in animals) and the anterior cingulate (linked to obsessive thinking), showed activity that changed with the length of the relationship. The study also found that the right VTA was more active from romantic passion than from seeing an attractive face, while the left VTA was more active from seeing something pleasant. In 2011, Xu and others repeated this study with Chinese participants.

Ortigue and others used fMRI to study how romantic love affects motivation without people being aware of the stimulus. In their study, participants saw a word for 26 milliseconds (like a loved one’s name, a friend’s name, or a hobby) and then symbols for 150 milliseconds, followed by a target word. They had to decide if the target was a word. Participants who saw love-related or passion-related words responded faster, and this was linked to their scores on a love scale. The study suggests that love activates motivation systems, not just emotions. Active brain areas included the VTA and caudate, as in earlier studies, plus the fusiform and angular gyri, which help integrate abstract ideas. This was tied to a theory where feeling close to a loved one is rewarding.

Brain scans of long-term romantic love (people who said they were "madly" in love but had been with their partner for 10 years or more) showed similar activity to early-stage love in the dopamine system ("wanting"), but long-term love was linked to the globus pallidus, a brain area involved in pleasure ("liking"). People in long-term love also showed less obsession than those in early stages.

A study by Sandra Langeslag looked at how attention affects brain activity related to loved ones. Unlike other studies where people just looked at photos, this experiment used a task where participants had to pay attention to either their loved one or a friend. Participants saw faces for 250 milliseconds and pressed a button if the face was the target. The study found that the dorsal striatum (a reward system area) was more active when the loved one was the target, not when they were a distraction. This suggests that the dorsal striatum responds to loved ones only when they are the focus of attention. Activity was stronger in people who had been in love for a shorter time. The authors think this reflects learning from past social experiences that makes people pay more attention to their loved ones. Participants also made more mistakes when a loved one was a distraction than when a friend was.

Some studies on early-stage romantic love found activity in the posterior cingulate cortex, which helps with memory related to social events (like remembering a partner’s name) and attention. Most studies, including those on long-term love, showed activity in the hippocampus and parahippocampal gyrus, areas linked to learning and memory.