Friday, March 4, 2011

The Nature of Love

Albert Einstein, that seemingly endless fount of quotations, once said, “gravitation is not responsible for people falling in love.” Ok, fair enough, but who or what is responsible? Why do we fall in love? Most of us think of love as an inspired feeling, but it turns out that our passions and attachments owe more to chemistry than to Cupid.

Of the theories I’ve seen on love, the one I find most compelling comes from anthropologist Helen Fisher [1]. Fisher speaks of three varieties of human bonding with the opposite sex, each mediated by a different neurochemical system. There’s the sex drive, linked in both males and females to that infamous hormone testosterone. Next comes romantic love, triggered by a brain chemical called dopamine. And finally there’s that deep, persistent feeling of attachment, governed by a pair of pituitary gland hormones: oxytocin and vasopressin. Think of this passionate trio as lust, love, and longing, respectively.

Why the three flavors of love? Fisher argues that the answer involves an evolutionary solution to the ancient problem of reproduction. Lust motivates us to seek sex with an assortment of partners. Love (of the romance variety) keeps us focused on just one partner at a time. And longing glues us to one partner long enough to raise a child through infancy. The three systems collaborate, . . . well, . . . shall we say, imperfectly, allowing us simultaneously to feel a deep sense of attachment for a spouse and a desire to romance another, while fighting off lustful thoughts of nameless others.

Of course, intersexual relationships are not the only kind of human bonding. There’s also mother-infant bonding as well as bonds with fathers, more distant kin, and nonkin, all of which appear to serve the function of bolstering infant survival rates. (Bonds between kin and nonkin can facilitate the formation of social networks as well). Looking beyond the human realm, we discover that chemically mediated bonding—particularly between mothers and infants, and between males and females—is rampant among birds and mammals, and for the same reasons: reproduction and survival. In the competition to pass one’s genes on to the next generation, evolution, it seems, leaves little to chance [2].

What I find especially fascinating is that some of the same chemicals are involved in these other forms of bonding. For example, oxytocin, the hormone linked to long-term attachment, shows up as a major player in the mother-infant bond. Mothers with higher levels of oxytocin in the first trimester of pregnancy tend to form stronger bonds with their infants postpartum than mothers with lower levels [3]. Oxytocin also turns out to be a primary ingredient of mother’s milk, helping baby bond with Momma. And what hormone is involved with bonding pets to their respective human counterparts? That’s right—oxytocin [4]. Although research into the chemical basis of love remains in its infancy [5], it’s no wonder that some folks call oxytocin “the love drug.”

Now, I am certainly no neurochemist. Indeed chemistry has never been one of my strong suits. (For some inexplicable reason, concepts like covalent bonding and oxidation states have always made my eyes glaze over and my brain turn to mush). Nevertheless, I’m going to make a prediction about the nature and neurochemistry of bonding. I predict that future research will reveal solid evidence of a human bond with (nonhuman) nature—what people have referred to as “biophilia” (see my previous post)—and that this bond will be facilitated by none other than our hormonal friend oxytocin.

Going further out on the limb of theory, I speculate that this effect will turn out to be strongest if strengthened by abundant childhood experience in local nature. In other words, I think that this human-nature bond will have a cultural component (plenty of time as a child exploring local natural environs) and a genetic component (neurochemical mediation by oxytocin). To my knowledge, no solid evidence exists to support any of this, but I’m sticking with it until proven otherwise.

Part of my reasoning relates to the bevy of studies that report the stress-reducing effects of being in nature, or even gazing at it out a window or on a screen [6,7]. I am suspicious that such feelings have a neurochemical basis related to human-nature bonding. But what eco-evolutionary function might such a bond serve? In other words, why on Earth would humans evolve an emotional attachment to the more-than-human world? Reproduction? Survival? Something else? The answer (or at least my current best answer) must await a future post. Thanks for staying tuned.


1. Fisher, H. 2004. Why We Love: The Nature and Chemistry of Romantic Love. Henry Holt, New York.

2. Broad, K. D., Curley, J. P., and Keverne, E. B. 2006. Mother-infant bonding and the evolution of mammalian social relationships. Phil. Trans. Roy. Soc. B., 361:2199-2214.

3. Feldman, R., Weller, A., Zagoory-Sharon, O., and Levine. A. 2007. Evidence for a neuroendocrinological foundation of human affiliation: plasma oxytocin levels across pregnancy and the postpartum period predict mother-infant bonding. Pscyhol. Sci., 18(11):965-70.

4. Odendaal, J. S. J. and Meintjes, R. A. 2003. Neurophysiological correlates of affiliative behaviour between humans and dogs. Veterinary Journal, 165:296301.

5. Campbell, A. 2010. Oxytocin and human social behavior. Personality and Social Psychology Review, 14(3):281-295.

6. Ulrich, R. S. 1993. Biophilia, biophobia, and natural landscapes. Pp. 73-137 in S. R. Kellert, S. R. and E. O. Wilson (eds.), The Biophilia Hypothesis. Island Press, Washington, D.C.

7. Kahn, P. H., Jr. 1999. The Human Relationship with Nature: Development and Culture. MIT Press, Cambridge, MA.

All images derived from National Geographic: