This article originally appeared in the February 2005 Harvard Health Letter and is provided courtesy of Harvard Health Publications.

Is it love — or just stress hormones?

People in all societies and cultures fall in love. The common ingredients include the sensation of being in an altered mental state, intrusive thoughts and images of the beloved, and changes in behavior aimed at getting a reciprocal response.

The head-over-heels phenomenon is so universal that some researchers say it may have been programmed into us by evolution. The theory is that offspring may be more likely to grow up and reproduce if they have two adults looking after them, and that falling in love makes such couplings more likely. Of course, it’s also true that love can be fleeting and may not lead to stable relationships and cozy childrearing. In any event, this theory does a nice twist to our usual ideas about the survival of the fittest. If nature loves a lover, maybe it isn’t quite so red in tooth and claw after all.

There are several hypotheses about the biological basis for falling in love. Some say that it comes from surges in phenylethylamine, a signaling molecule in the brain with a chemical structure similar to amphetamines. Chocolate lovers and purveyors like this theory because chocolate contains phenylethylamine. Others trace the intrusive thoughts of the love struck to a dampening of the serotonin systems in our brains.

But Donatella Marazziti and Domenico Canale, researchers at the University of Pisa, argue that falling in love is an emotion related to anxiety and fear. People often forge strong bonds and intimate ties in threatening circumstances, they note.

To test their ideas, Marazziti and Canale conducted a study comparing 24 young people who had recently fallen in love with the same number of control subjects. The criteria for the in-love group included thinking about the partner at least four hours a day and being in the relationship less than six months. The researchers drew about a tablespoon of blood from each person and measured the levels of eight different hormones.

Their main finding was that levels of cortisol, one of the main stress hormones, were higher in the lovestruck group. The Italian researchers believe that was a sign of stress and arousal in the early phases of a relationship.

Interestingly, men who were in love had lower testosterone levels than those who weren’t, but being in love seemed to raise the level of testosterone in women. This stumped the Italians, although they permitted themselves a little conjecture: Maybe falling in love reduces some of the differences between the sexes, softening men and making women more aggressive.

Previous research by Marazziti and others has found that the feeling of being in love tends to last only so long — between 18 months to three years. So in this study, she and Canale measured the hormone levels of 16 subjects in the in-love group who were still in the same relationship after 12–28 months but reported being calmer and no longer obsessed with their partner. Their hormone levels had returned to normal.

The study was published the August 2004 edition of Psychoneuroendocrinology.

Why Jesse likes Jennifer
Even seemingly meaningless similarities may have a pull on us. Shared birthdays, last names, and even first initials make people more attractive to one another. Psychologists chalk this up to implicit egotism: We automatically think positively about objects, places, and people that remind us of ourselves, even if that reminder is something pretty trivial.

John T. Jones, a psychologist at the U.S. Military Academy, and his colleagues published a paper in 2004 in the Journal of Personality and Social Psychology that summed up seven studies they’ve done examining implicit egotism in interpersonal relationships. They used genealogies and other kinds of records to show people are more likely to marry someone whose first or last name resembles their own. Their analysis of phone listings, for example, found that men named Charles were more likely to be married to women named Charlotte than would be expected by chance. The same was true for Robert and Roberta, Paul and Paula, and so on. First and last name similarities may simply be a matter of people marrying within their own ethnic and religious groups, but Jones and his colleagues attempted to correct for that possibility.

They also used experimental psychology to explore the effects of the superficial positive associations we have rattling around in our egos. In one study, people found others more attractive if a pretend code number on a form was switched to match their birthday. In another, participants found people in personal ads more appealing if the bogus names assigned to the ads resembled their own.

We’ll take all this with a grain — maybe a shaker’s worth — of salt. But given the odd reasons people are drawn to one another, who’s to say that a little alliteration might not have an effect: Jesse may have fallen for Jennifer for many reasons, but that shared J may have provided some of the initial spark. Then they just moved to Jacksonville, got jobs as janitors…well, you get the idea.