Dr Catherine Conlon: How brain science explains sexual chemistry

Magnetic resonance imaging shows parts of the brain are more active when we are in love

Renée Zellweger and Leo Woodall in Bridget Jones: Mad About the Boy (2025)

Bridget Jones returns to movie theatres this Valentine’s Day. In Mad About the Boy, Bridget is a widowed mother of two and again looking for love. 

A chance encounter with a younger male leads to an unlikely romance and a chemistry so intense it could warrant a new entry in the periodic table.

Love may be one of the most studied, but least understood, human emotions. In 1992, two American anthropologists William Jankowiak and Edward Fischer surveyed 166 societies and found evidence of romantic love in 88.5% of them. The researchers concluded that romantic love, which they equate with ‘passionate love, is universal.

In 2005, US anthropologist, human behaviour researcher and author of Anatomy of Love, Helen Fisher led a research team that published a groundbreaking study that included the first functional magnetic resonance imaging (fMRI) of the brains of individuals in the throes of romantic love.

The researchers analysed 2,500 brain scans of college students who viewed pictures of someone special to them and compared the scans to images when the students viewed pictures of acquaintances.

Looking at people they romantically loved caused participants’ brains to become active in the regions rich in dopamine: The feel-good neurotransmitter.

Two Harvard Medical School (HMS) psychiatry professors, Richard Schwartz and Jacqueline Olds, have spent a lifetime studying how love evolves. They have also been happily married for more than four decades.

“We know that primitive areas of the brain are involved in romantic love,” Olds told Harvard Medical School in 2015. “And these areas of the brain light up on brain scans when talking about a loved one. These areas can stay lit up for a long time for some couples.”

Schwartz described how falling in love causes chemicals associated with the reward circuit to flood the brain, producing a variety of physical and emotional responses: Racing hearts, sweaty palms, flushed cheeks, feelings of passion and anxiety.

The stress hormone cortisol increases during the initial phase of romantic love. Rising levels of it cause the neurotransmitter serotonin to be depleted.

It is this depletion in serotonin that causes what Schwartz describes as “the intrusive, maddeningly preoccupying thoughts, terrors, hopes of early love”, the all-too-familiar symptoms of early infatuation.

Renée Zellweger and Leo Woodall in Bridget Jones: Mad About the Boy

High dopamine levels also get “the reward system going”, explains Olds. (Dopamine is released following the use of other pleasure-inducing stimulants, including foods high in fat, salt and sugar, alcohol and cocaine.)

Evidence for this high dopamine release is found in many studies, including one published in Science (2012), where male fruit flies that were sexually rejected drank four times as much alcohol as fruit flies that mated with female fruit flies.

Other chemicals that run high during romantic love include oxytocin and vasopressin. Released during sex and heightened by skin-to-skin contact, oxytocin heightens feelings of attachment, as well as the contentment, calmness, and security often associated with bonding. Vasopressin is linked to behaviour that produces long-term relationships. These two hormones influence the evolving changes in behaviour, Swartz suggests, and may explain why passionate love fades as attachment grows.

In addition to the positive feelings of romance, love also deactivates the neural pathway responsible for negative emotions, such as fear and social judgment.

These positive and negative emotions involve distinct neural pathways in the brain. When we are engaged in romantic love, the critical-assessment pathways tend to shut down.

Schwartz describes this change as the neural basis for the ancient wisdom that ‘love is blind’.

If love endures over time, the initial angst will calm. Schwartz says this is also explained by brain chemistry. Cortisol and serotonin levels return to normal.

Brain areas associated with reward and pleasure are still activated as loving relationships proceed, but the constant craving and desire inherent in romantic love often lessen.

However, the initial euphoria of love does not always decrease over time.

A 2011 study conducted at Stony Brook University in New York state found that it is possible to be madly in love with someone even after decades of marriage. The research team, which included Fisher, performed fMRI scans on couples who had been married for an average of 21 years. They found the same intensity of dopamine-rich areas of the brain as in couples who were newly in love.

“A state-of-the-art investigation of love has confirmed, for the very first time, that people are not lying when they say that after 10 to 30 years of marriage, they are still madly in love with their partners,” concluded Schwartz.

More recent research, published in Cerebral Cortex (2024), found that different types of love light up different brain parts. The researchers, from Aalto University in Finland, used functional magnetic resonance imaging (fMRI) to measure brain activity. At the same time, subjects mulled over brief stories related to different types of love, including romantic love, parental love and love for pets or nature. Love for one’s children generated the most intense brain activity, followed by romantic love.

This Valentine’s Day, love is not just in the air or the heart — it’s in the brain and can last a lifetime.

• Dr Catherine Conlon is a public health doctor