KRISTIN HARPER
gradute student
Emory University

Rasheena, Georgia, USA

I've often heard that love is as scientifically real as it is emotionally real. Is that true? If so, how? And what happens when you lose that love to death or separation?

Kristin Harper
HHMI predoctoral fellow,
gradute student,
Emory University

Everybody knows what first love feels likes. Whenever you are in the presence of the beloved, your heart pounds, your hands get clammy, and you feel lightheaded. These very physical symptoms result from the biochemistry that underlies falling in love. And biochemistry can also help explain why people fall out of love.

Scientists believe there are three stages of love, each one characterized by its own suite of hormones. The first stage is lust, and it is driven by the sex hormones testosterone and estrogen. The second stage, attraction, encompasses the process that we traditionally think of as “falling in love.” During this stage you become a little obsessed with the object of your affection—you would rather daydream about him/her than sleep, eat, or do just about anything else! 

The chemicals that play an important role in the second stage include dopamine, norepinephrine, and serotonin. It is no coincidence that you may have heard of these neurotransmitters for their roles in other processes. For example, dopamine is also activated by addictive drugs such as cocaine. Norepinephrine, also known as adrenaline, gets your heart pounding. And, serotonin, sometimes known as the happiness chemical, can sometimes have the effect of making us temporarily insane. All three of these chemicals are controlled by a substance called phenylethylamine (PEA)— one that is also found in yummy foods like chocolate and strawberries. Its chemical structure is similar to that of amphetamine, commonly used as a stimulant, and, like that substance, it brings excitement that can be addictive. 

The third stage of love is attachment: this stage must be reached if a relationship is to last. One of the chemicals important here is oxytocin, which is also involved in creating the strong bond between mother and child. This chemical gets released each time someone has an orgasm. For this reason, a healthy sex life is an important part of a committed relationship, and some researchers believe that the more sex a couple has, the stronger their bond becomes. The second chemical that has been implicated in this final stage of love is vasopressin. Studies on prairie voles show that suppression of this chemical can cause males to leave their partners and seek out new mates. 

Elucidating the chemicals responsible for love leaves an important question unanswered: What qualities determine with whom you will fall in love? Some researchers believe that romantic infatuation happens more or less randomly, and that it is impossible to predict the “type” of person someone will choose as a mate. However, many studies do support the existence of broad themes in human attraction, trends that are predicted by evolutionary theory. Cross-cultural studies have found that men are drawn to features that convey health and youth in females (perhaps because young, healthy women are likely to bear many future children). In turn, women appear to be attracted to men who exhibit signs of high status and plenty of resources—qualities that would prove useful in successfully rearing children together. Both men and women have been shown to prefer symmetrical faces. Scientists believe this is because a symmetrical face and body indicates good health. 

It is important to note that human decision-making has been shown to differ depending on the stage of love that has been reached. For example, while women are choosier than men initially (with regard to qualities such as a partner’s personality, for example), when the time comes to decide whether to pursue a long-lasting relationship, both males and females value qualities such as intelligence very highly. 

Finally, subtle cues like scent may play an important role in choosing mates. For example, women prefer the smell of males whose major histocompatibility complex (MHC) genes (which play an important role in resisting infection) are the most different from their own. A diverse set of MHC genes would presumably give offspring an edge in combating disease. Lots of research is currently going on in the fascinating field of human attraction!

So what happens, biochemically, when love is lost? We know that the chemical PEA (responsible for the high of falling in love) does not last forever. Researchers estimate that after an initial period of 18 months to 5 years, production of this substance drops. Many relationships end in this time period, and scientists speculate that this may be a result of the biochemical pathway associated with attachment not having kicked in to the extent necessary for a relationship to survive. Changes in the levels of chemicals associated with each of the stages outlined above can modulate the feelings of love (or falling out of love) in a relationship.

Further Reading

The BBC site on love

Thornhill, R., and Gangestad, S. The evolution of human sexuality. Trends in Ecology and Evolution 11(2):. 98-102, 1996. 

02/05/09 20:08