How often do you hug?  Do you like to sit close and hold each other’s hands?  Recent research shows it’s good for your health.  Between loving partners, between parents and children, or even between close friends, physical affection can help the brain, the heart and other body systems you might never have imagined.

For centuries, artists have examined love through poetry, painting, music and countless other arts.  In the past few years, scientists supported by NIH have begun to understand the chemistry and biology of love.

At the center of how our bodies respond to love and affection is a hormone called oxytocin.  Most of our oxytocin is made in the area of the brain called the hypothalamus.  Some is released into our bloodstream, but much of its effect is thought to reside in the brain.

Oxytocin makes us feel good when we’re close to family and other loved ones, including pets.  It does this by acting through what scientists call the dopamine reward system.  Dopamine is a brain chemical that plays a crucial part in how we perceive pleasure.  Many drugs of abuse act through this system.  Problems with the system can lead to serious depression and other mental illness.

Oxytocin does more than make us feel good.  It lowers the levels of stress hormones in the body, reducing blood pressure, improving mood, increasing tolerance for pain and perhaps even speeding how fast wounds heal.  It also seems to play an important role in our relationships.  It’s been linked, for example, to how much we trust others.

Dr. Kathleen C. Light of the University of North Carolina at Chapel Hill studies oxytocin in married couples and those permanently living together.  She and her colleagues invite couples into the laboratory and ask them to spend at least 10 minutes holding hands and talking together about a happy memory, usually about how they met and fell in love.

“What we’re trying to do in a lab situation,” Light explains, “is recreate some of the experiences in real life where they felt close.”

The couples then get their blood drawn and fill out a questionnaire about the quality of their relationship.  When the researchers compared their responses to the levels of oxytocin in their blood, they found that people who have a more positive relationship with their partner have higher levels of oxytocin.

Light and her colleagues are now trying to understand how conflict and other factors in relationships affect a couple’s oxytocin levels.  The results of those studies aren’t yet in.

One thing researchers can say with certainty is that physical contact affects oxytocin levels.  Light says that the people who get lots of hugs and other warm contact at home tend to have the highest levels of oxytocin in the laboratory.  She believes that frequent warm contact may somehow prime the oxytocin system and make it quicker to turn on whenever there’s warm contact, even in a laboratory.

The same holds true for mothers and infants:  they both produce higher levels of oxytocin when they have lots of warm contact with each other.  “Those women who hold their babies more at home have higher responses when they hold their baby in the lab,” Light says.

Much of what we know about oxytocin has come from research in animals.  Mother rats, for instance, can stimulate oxytocin in their pups by licking and grooming them.  This loving care has long-term effects.

When researchers separate pups from their mothers for 10-15 minutes a day and then reunite them, many mothers are so glad to see their pups that they lick and groom them intensively.  If the separation lasts for several hours, however, it can have the opposite effect; the mother won’t lick and groom her pups.  Some mothers just never lick and groom their pups when they come back.

Pups that are groomed a lot when they’re reunited with their mothers become more comfortable exploring new environments.  The ignored ones develop more anxiety disorders, produce higher levels of stress hormones and have higher blood pressure.

Research from other animals, including monkeys, confirms that the quality of care a mother gives her offspring can have long-term effects on their personality characteristics and mental health as well as physical problems like heart disease.

Animal research is also shedding light on oxytocin’s role in other social bonds.  Mice that lack oxytocin can’t recognize other mice, even after repeated encounters.  When they’re given oxytocin, however, they can recognize other mice again.

Dr. C. Sue Carter, co-director of the Brain Body Center at the University of Illinois at Chicago, has been studying oxytocin in prairie voles, which form strong bonds with their mates.  When the researchers block oxytocin, the voles don’t form such bonds.  Oxytocin is especially important for females to form bonds with their mates.  In males, a related hormone called vasopressin also plays a role.

Oxytocin and vasopressin aren’t miracle compounds, however.  Giving these hormones to other animals—even other types of voles that don’t normally form social bonds—doesn’t suddenly cause them to form loving bonds.  Animals must have the proper genes to respond to these hormones in the first place.

“Most of us are genetically programmed to form social bonds,” Carter explains, relating the results back to people.  But the ability to form close bonds, she says, is shaped by early experiences.  In the end, a complex interaction of genes and experience makes some people form social bonds more easily than others.