This Valentine’s Day, as our collective thoughts shift to tender cards, heart-shaped chocolates, overpriced bouquets and other extravagant gestures of love, I can’t help but wonder what really attracts us to one mate over another. Is it hot sex? Fairy-tale romance? Destiny? Or are we merely at the beck and call of our hormones and brain circuitry?

Online dating sites trumpet their knack at identifying “chemistry,” but it turns out that basic biology may play at least as strong a role in love as do socialization, environment, fate and other factors. “We like to feel independent and free of the brain systems that regulate the mating habits and regimens of animals, but the fact is that we’re not,” says neuroendocrinologist Tom Sherman, an associate professor at Georgetown University School of Medicine. “The latest research indicates that some of our very complex behaviors — like love, courtship and pair bonding — are still regulated, to some degree, by a fairly simple set of neurochemicals.”

Indeed, researchers have now identified three brain systems that are at work in mating and reproduction: lust, which is primarily mediated by the sex hormone testosterone; romantic love, which is primarily mediated by dopamine, a neurotransmitter that drives the brain’s reward and pleasure centers, and is characterized by craving and focused attention for just one person at a time; and attachment, which is primarily mediated by the hormones oxytocin and vasopressin and is associated with the bonding and security you often feel with a long-term partner.

These systems vary from person to person and can function discretely, together, or in all sorts of combinations, says one of the researchers, Helen Fisher, a biological anthropologist at Rutgers University and the author of “Why We Love: The Nature and Chemistry of Romantic Love.”

“That’s why you can feel deep attachment for one person, then swing into wild romantic love for someone else, then switch on the Internet, look at pornography and feel a sex drive that has nothing to do with any of those,” she says. “You can also look across the table and feel all of that for the same person, which is what we want on Valentine’s Day.”

Fisher adds that the interplay of such brain systems, along with the neurotransmitter serotonin, clearly lead to variations in temperament, which help explain why you can walk into a crowded room and fall madly in love with one person rather than another. “The reality is that we’ve got a very strong, powerful brain system for romantic love, and it can get triggered at any moment, but we will not fall in love with everyone that comes along: We have preferences, and those have to do, in part, with the way that our brain is built,” she explains.

However, it remains to be seen how big a part neurochemicals and brain circuits play in love and relationships.

“It’s hard to weed out how much is sociological and how much is biological,” says Lisa Diamond, an associate professor of psychology and gender studies at the University of Utah. “We know that there are huge cultural differences in the way we socialize women and men [about love and relationships], and now we know from animal research that there are biological differences.”

For example, she notes that studies show that oxytocin and vasopressin operate differently in the male and female of several animal species, leading some scientists to theorize that these hormones drive distinctions between how the sexes behave in relationships. “But right now that’s just speculation,” says Diamond.

Still, Georgetown’s Sherman finds the preliminary evidence of a strong biochemical influence on feelings and relationships compelling: He says that studies of male prairie voles show that different levels of brain sensitivity to vasopressin impact the rodents’ ability to bond with a female. He adds that research on the “cuddle hormone” in humans echoed those results, finding that men with weak vasopressin receptors were much less likely to be married and more likely to report a crisis in their relationship.

“This kind of data suggests that part of the way that men respond in a relationship is governed by these wiring patterns in their brains that’s just part of who they are,” he says, noting the intriguing possibility that manipulating such hormones might be able to change how people behave in relationships. (Interestingly, a new study found that monkeys that inhaled oxytocin paid more attention to one another and treated one another more kindly.)

Because I am a romantic at heart, I will point out that one thing that researchers have suggested is that, while it is normal for the passion of initial romance to fade, it is also possible for deep, fervent love to persist. A study published last year in the journal Social Cognitive and Affective Neuroscience found that couples who had been married an average of 21 years and who reported still being intensely in love with their spouses exhibited the same brain activity, including in the dopamine-rich reward system, as those who had just fallen for each other. But there were also some important differences: Only those in the first stages of romance exhibited activity in a brain region linked with anxiety, while the long-term couples showed activity in an area linked with calm.

“That makes sense, because when you’ve just fallen in love, you’re pretty anxious: Will he call? Am I too fat? Why did I say that? But when you’re in love with the person you’ve had children with and been married to for 21 years, you’re not anxious, but you can still want to come home from work, share your night, make love and still be with him or her,” says co-author Fisher. “So love can certainly last, and we found this in the brain.”

Those paramours looking for a last-minute Valentine’s date, take note: Diamond says that other, related research has shown that couples who stay passionate about each other report that they continue to pursue novel, exciting activities together. “The idea of trying something new that’s maybe a little bit stressful with your partner — with his or her support — seems to be a magical combination,” she explains.

Hang gliding with your honey this afternoon, anyone?