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The Evolution of the Love Hormone

How oxytocin promotes care in humans and other animals.

Key points

  • Oxytocin promotes love, care, and social bonding in humans and other animals.
  • Oxytocin is as old as mammals and first evolved to facilitate the basic muscle contractions of birth and lactation.
  • Evolution co-opted this system for parental care into the love hormone we know today.

Oxytocin has been famously dubbed “the love hormone” for its role in care and social bonding. Psychological studies in humans have shown that oxytocin promotes empathy, increases cooperation in economic games, and is responsible for the warm fuzzy feeling of love.

Oxytocin levels rise when touching or making eye contact with someone you love—even if that someone is your dog. And the feeling is mutual: Dogs’ oxytocin levels rise when cuddling and looking at their owners.

Oxytocin flows through the blood of all mammals, and its role in the psychology of love is rooted in a much more fundamental form of love. Oxytocin chemically facilitates the basic physiological processes that define us as mammals: pregnancy, childbirth, and breastfeeding.

During pregnancy, oxytocin levels rise, and they peak during labor. Oxytocin buffers against the body’s stress response, and helps make a process like pregnancy and childbirth less traumatic. Oxytocin levels also rise in mothers during breastfeeding, and in both parents when in the mere presence of an infant, especially during skin-to-skin contact.

Knowing that oxytocin is implicated in love and care, this makes sense. No doubt evolution has wired parents to ramp up the love hormone to facilitate bonding with infants. Yet these psychological explanations put the cart before the horse.

What Makes Us Mammal

It’s not that mammals evolved first, with their whole toolkit of pregnancy, lactation, and social bonding, and only later evolved a love hormone to help make the whole process go more smoothly. Oxytocin-like peptides are in fact hundreds of millions of years older than mammals themselves, and oxytocin is deeply intertwined with our mammalian evolutionary history.

Oxytocin is not only involved in the psychology of pregnancy and breastfeeding but is critical to the basic physiological processes themselves. Oxytocin levels do not only rise during the birthing process to promote mother-infant bonding and reduce stress; they are also necessary for contractions during delivery themselves. For this reason, oxytocin is sometimes administered to induce or expedite labor.

Similarly, oxytocin does not only promote loving bonding during breastfeeding, but it is necessary for the basic muscle contractions in the breasts that make lactation possible. A recent study using CRISPR gene-editing to produce prairie voles without any oxytocin receptors found that the resultant “oxytocin knockout” mothers had significantly higher rates of infant mortality and greatly decreased milk production.

(Interestingly, it was still a huge surprise that these knockout voles were able to give birth and nurse at all. Comparable studies in mice found that mothers given oxytocin blockers could not do either of those things. Something about the fact that these voles were gene-edited to never have any oxytocin from the start led them to develop a redundant system using some other chemical, even if it was less efficient.)

Parental Care and Social Bonding

Prairie voles are actually the species that brought public awareness to oxytocin’s role as “the love hormone” in the first place. Before the 1980s, oxytocin was only a hormone known for its role in the muscle contractions of birth and lactation. But pioneering research by Sue Carter and other neurobiologists studying prairie voles showed during this decade that oxytocin was in fact central to the psychology of care, as well.

It’s not just that (non-gene-edited) voles given oxytocin blockers can’t produce milk but still want to. Without oxytocin, they won’t even try—the maternal instinct is completely inhibited. Conversely, mothers given extra oxytocin will be hyper-maternal and even nurse offspring that aren’t their own (which would normally never happen). Even male voles given oxytocin will start exhibiting caring for pups—cuddling and licking them—which again, normally would never happen.

Prairie voles are also unique in that they are one of the few monogamous mammals. And here too, Carter and others discovered that oxytocin was responsible not only for the love of parental care but for the romantic love of pair-bonding. Voles given oxytocin blockers would not pair-bond like they normally do, and conversely, voles given extra oxytocin were even more likely to pair-bond and display physical affection.

These foundational studies led to similar studies finding an essential role of oxytocin in parental care and social bonding in mammals ranging from vampire bats to humans. And neuropeptides similar to oxytocin may explain parental care in other vertebrates, as well. Animals as diverse as birds, reptiles, and fish have mesotocin and isotocin, chemical cousins with similar structure and function as oxytocin, and a shared evolutionary history dating back at least 500 million years.

The Evolution of Love

Evolutionary biology has two kinds of theories: ultimate and proximate. The ultimate theory of an evolutionary adaptation is the “grand story” about how something impacted survival or reproduction enough to be adaptive.

It makes sense that parents should love and care for their children, to maximize the likelihood of those children surviving and passing on their genes. It makes sense that sexual partners, especially pair-bonding partners who are parents of the same offspring, should love each other. Again, this will indirectly maximize the chances of passing on one’s genes to the next generation.

And it makes sense that one should love one’s family and friends, and have some capacity for empathy and cooperation in the broader social world. All of these traits increase the likelihood of survival and reproduction.

The proximate theory says: Yes, this all makes sense, but how do these behaviors actually manifest neurobiologically? Proximately, evolutionary psychologists have theorized that the subjective feeling of love directed towards friends, family members, and romantic partners, all evolved as offshoots of the most fundamental form of love: parental care.

Wherever possible, evolution will conserve. The same molecule that was already used for the basic physiological processes of birth and lactation became co-opted for the psychology of parental care. And later, the same molecule motivating care between parent and child became co-opted for the psychology of care and social bonding more generally.

Oxytocin is the proximate explanation for all of these phenomena, chemically signaling everything from the warm fuzzy feeling of love in the brain, to the basic muscle contractions involved in birth and lactation. That is what makes oxytocin the love hormone.


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