We mammals have it rough. Especially the humans among us. All mammals are born helpless — their survival hinges on maternal care — but compared to many other species, Homo sapiens require a prolonged childhood. Raising a human being is no joke.

The beneficial effects of “the bonding hormone” on maternal blood pressure are now being measured. From the collection of U.S. Children’s Bureau c. 1945. Photo: Georg Jones. Courtesy National Library of Medicine.

Consider the mice of the field. A prairie vole (a critter resembling a mouse) weans its young at the tender age of 3 weeks or so, when the youngster departs to seek its fortune. But weaned human tots can't be cast out, so a lack of maternal bonding can be dire. It is, to put it mildly, a risk factor.

So, too, is an unhappy marriage.

This is a true-love story. It's about the rapport between mother and infant. It's also about romantic love — not the kind that made Romeo and Juliet risk everything, double or nothing, but rather the contentment they might have found with a longer-term deal. And it's about how maybe, if we're lucky, we can cultivate that.

How do scientists analyze the effects of loving care on health? One way is to look at the role of oxytocin, a pituitary hormone, and measure its relation to blood pressure and heart rate, as well as to blood levels of stress hormones.

Oxytocin, C₄₃H₆₆N₁₂O₁₂S₂, is produced only in mammals. A peptide hormone which exhibits neurotransmitter-like actions, it has receptors in various sites, including the cardiovascular system. In the female body it stimulates the contraction of uterine muscle and the secretion of milk. Discovered at the turn of the 20th century, it was eventually synthesized in the 1950s and found clinical uses in labor, delivery and postpartum recovery.

Now Dr. Kathleen Light stands among the first scientists worldwide to study oxytocin in human social relationships. Funded through an NIH grant at the University of North Carolina, Chapel Hill, she recently traveled here to deliver her lecture "Loving Family Relationships and Oxytocin: New Human Findings."

Light described oxytocin as "the bonding hormone" known to enhance pair-bonding in the 5 percent of mammals that are monogamous. Humans, she reminded us, are serially monogamous.

"If you are in love," she explained, "that contact becomes rewarding because of pathways in the brain activated by oxytocin." So if you give oxytocin to the females in a monogamous species like prairie voles, they are more likely to pair-bond with males. (Vasopressin, a related hormone, works this way on the males.) Oxytocin is also involved with social recognition and behavior, pain and stress regulation, mood and affect.

Light noted that there is extensive literature on animals and oxytocin, but "few studies look at oxytocin in humans, and even fewer in the context of social relations." There have been human studies showing that social support is good for you, that marriage has a salutary effect on health, but these didn't look at oxytocin.

She designed studies using "warm contact" in which couples sit close together, hips touching, in a love seat. They hold hands, talk to each other about a happy memory, then hug for 20 seconds.

Light found that the warm contact raised oxytocin blood levels in both men and women if they had supportive partners. These elevations were related to a decrease in blood pressure (BP) in women, but not, she said, in men.

Le Mari Malade [The Sick Husband]. Lithographic print by Charles Motte, after A. Maurin, c. 1815. Courtesy National Library of Medicine.

Study Design Crucial in Couples Work Studying human subjects can be tricky, since folks being observed may feel uncomfortable and behave in ways that are altered by the observation itself. When asked how she designed her studies, UNC's Dr. Kathleen Light offered these details: "There are lots of different ways to design the couple studies. We tried for a naturalistic setting, like couples sitting together at the movies. We also left the couples alone during their warm contact period so they would feel comfortable and behave normally. For the mother/baby interactions, we told the moms to behave as they usually do at home with their babies. This is one reason why our findings in the lab are linked to what happens at home — why how often the couples hug at home or how often the mom picks up her baby at home is related to the oxytocin and blood pressure measures we see in the lab studies. For these initial studies, being naturalistic was our highest priority. In future studies, we can examine whether it was the positive verbal interaction or the physical closeness and warm touch that was the specific component needed to see oxytocin differences."

Oxytocin levels were also linked to a reduction in women's levels of the stress hormone norepinephrine but, again, not in men's levels. "This makes us wonder: is oxytocin a more important regulator of stress responses in women?" she queried.

Her study couples were no strangers. They were monogamous co-habiting couples (not necessarily married) who had lived together for more than one year. Forty percent were African American; the rest were white.

"The benefit to the women's blood pressure was linked to partner relationship quality, and to physical affection expressed as frequent partner hugs," said Light. "Not all marriages are equally advantageous. It's the relationship quality that determines if marriage has health benefits over being single."

In a separate study monitoring BP and heart rate, Light found that warm contact had a positive effect across all race and gender groups. Men and women benefited equally, and even if the partner was no longer present, warm contact had an enduring effect.

Asked why she is among the first to study people this way, Light replied: "Many studies failed because they were trying to find an oxytocin change in response to a single event. It takes more than one exposure; it's a cumulative effect of the way supportive couples habitually interact together and hug each other."

Studying infants and mothers, Light found that mothers whose oxytocin increased after they held their infants closely showed lasting positive effects on the moms' blood pressure. Even without the babies present, the oxytocin's effect on maternal BP endured. She replicated the study and found that there was a much stronger effect if the infant was under 5 months.

Looking at prenatally cocaine-exposed versus non-exposed mother/infant pairs, she found that the cocaine-exposed mothers had lower oxytocin levels before and after baby contact, held their babies less at home and ran higher blood pressures. "They also had higher BPs in the lab except when they were holding their babies," Light noted. "Many of the mothers were bottle-feeders, but they were different from other bottle-feeders," she said. When interacting with their infants, or when separated briefly from them, as well as when exposed to tape-recorded baby cries or pictures of babies in danger, a second preliminary study showed that their oxytocin levels were consistently lower. They were also not as stressed during separation from their infants, but more stressed and overwhelmed when they listened to the tape-recorded baby cries, and interestingly, when seeing pictures of happy babies.

Because their oxytocin levels were relatively low, Light concluded, holding their babies was less rewarding and separation less aversive.

So why not give people oxytocin to enhance social behavior? "Last year's article in Nature by M. Kosfeld et al. showed that inhaled oxytocin in male subjects increased one kind of trusting behavior," she said, "but remember, oxytocin makes the uterus contract. And an uncontrolled decrease in blood pressure can be dangerous." She noted that nasal oxytocin administration approved for use in Germany may minimize adverse peripheral effects of oxytocin while allowing it to influence the brain.

But there are other, more hopeful, implications.

"There are implications for the mother-child bond," Light said. "Even in the cocaine-exposed mothers, the system was not broken. Their BPs decreased when instructed to hold their babies." Intervention is possible, she said.

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