The hot cup of tea. Its powers to soothe are as legendary as the supposed antiviral properties of chicken soup. Examples abound in literature. For instance, after Mrs. Inglethorp quarrels with her husband in Agatha Christie's 1920 novel The Mysterious Affair at Styles, the maid is quick to suggest, “You will feel better after a nice hot cup of tea, m'm.” In real life, too, many of us turn to a piping-hot panacea—be it chamomile or cocoa—when we feel in need of comfort.
According to a growing body of research, there may be real merit to this popular remedy. During the past decade scientists have discovered that our physical temperature can affect how “warm” or “cold” we feel toward other people. For instance, studies have found that when we are hurt, isolated or betrayed, a short dose of heat—in the form of a hot beverage, warm bath or even the sun—may help restore feelings of trust and bonhomie. Likewise, other investigations have shown that a chill in the air can raise our suspicions.
In general, this line of inquiry belongs to a larger research field called embodied cognition, which holds that our body—and not just our brain—plays a role in our thinking, emotions and memories. The field has its critics, but when it comes to temperature, there is little doubt that the link between physical and psychological warmth and coolness is built on more than just metaphor. Researchers have uncovered overlapping mechanisms that govern both the system that regulates body temperature and the one that governs our emotional state. Imaging studies have tracked both systems to the insula in the cerebral cortex. And as neuroscientists and psychologists begin to understand this circuitry better, they are looking for ways to manipulate it to treat depression and other disorders that can put a freeze on our social connections.
Warm Hands, Warm Heart
Yale University psychologist John A. Bargh first began exploring the links between physical and psychological temperatures in 2008. At the time, he says, his laboratory was “scouting into a new territory about the warm-cold effect.” As part of that initial foray, he paired up with psychologist Lawrence E. Williams, now at the University of Colorado Boulder. They invited 41 undergraduate students to visit their fourth-floor psychology lab. During the elevator ride up, the students all encountered a woman carrying an armful of books, a clipboard and a coffee cup. She asked each one to hold her cup, which was either steaming hot or icy cold, while she scribbled something down on her clipboard. Once in the lab, the students read a short description about a fictitious “person A” and then had to rate the warmth of his or her personality. When the scientists analyzed the results, a clear pattern emerged: most of the students who held the hot cup had judged “person A” to be significantly more generous and caring than those who held the chilly cup.
Many similar experiments soon followed, extending the association. For instance, in 2013 psychologists Simon Storey of Bath Spa University in England and Lance Workman of the University of South Wales found that merely holding a gel-based hand warmer made some students more trusting. They asked 30 pairs of volunteers to hold either a hand warmer or a freezer pack and then to play repeated rounds of the Prisoner's Dilemma game, a classic test of cooperation between two people. When they analyzed the results, they found that those primed with warmth chose to cooperate with a partner more often than those primed with cold.
A study the following year found that people waiting in a heated room would also turn a kinder eye toward their fellow human. Scientists from Germany and Switzerland gave study participants eight mugshots and asked them to guess what crimes these individuals had committed. If the temperature of the room was set at around 79 degrees Fahrenheit, participants were more likely to think of lesser or white-collar crimes, such as drug possession or tax evasion. But if the thermostat was turned some 11 degrees colder, they jumped to cold-blooded accusations, including murder and kidnapping.
Subsequent studies demonstrated that this temperature effect also works in reverse. When scientists analyzed data from an online movie rental company and surveyed students about their film choices, they found evidence to suggest that we rent more romantic movies when we are cold (perhaps to feel more connected to others). Similarly, studies suggest that drinking icy water threatens our feelings of belonging. Both eating alone or reminiscing about a time we were left out socially can make us judge the surrounding air temperature as colder. In contrast, thinking of a friendly person can boost our perception of a room's temperature by as much as 3.6 degrees F. “There are tons more of these kinds of effects, and the theory behind these findings seems quite robust,” says psychologist Hans IJzerman of Free University Amsterdam, who has studied similar links himself.
Some temperature-related results have met with criticism. For example, in 2012 Bargh and psychologist Idit Shalev of Ben-Gurion University in Israel reported that lonely people took more warm baths or showers. But when psychologists Brent Donnellan, Richard Lucas and Joseph Cesario of Michigan State University tried to reproduce the same results two years later, they failed. Bargh offers one possible explanation for why studies may not always replicate: “Researchers make significant changes to the original procedures.”
For definitive proof that physical and psychological temperatures are linked, scientists have turned to neuroimaging. “Neuroscience has confirmed the reality of these phenomena, using much more powerful measurement tools,” Bargh says. These tools have tracked the source of the connection to the insula, a small, pyramid-shaped structure deep within the cerebral cortex. This region plays a role in how much we trust others and how much empathy we feel toward them. A 2015 study, for example, showed that damage to the insula causes people to misplace their trust and be overly naive in some situations but cagey in others.
Critically, studies also reveal that the insula is important in temperature perception. In 2010 neurologist Hans Lüders of University Hospitals Case Medical Center in Ohio and his colleagues investigated the cases of five women with intractable epilepsy. In hopes of better understanding their seizures, they surgically placed electrodes in the women's insulae, among other brain structures. They reported that stimulating regions within the insula made these patients experience sensations of warmth in different body parts.
That same year, working with his colleagues at Yale and Boulder, Bargh conducted an experiment that linked both feelings of interpersonal trust and temperature perception to the insula at the same time. They asked 23 participants to play a game inside a functional MRI scanner. The game required players to hypothetically “invest” small amounts of money with other people. As they lay inside the machine, some of them held an ice pack for a few seconds; others held a pack heated to a toasty 105.8 degrees F. The scientists observed clear differences in activation within the insula, depending not only on the decisions the players made in the game but also on the temperature of the pack they held. In addition, they noted that participants primed with cold were less willing to invest. (Practical tip: if you want to ask your boss for a raise, bring a hot cup of coffee first!)
In 2013 another study lent further credence to the idea that physical and psychological temperatures run on the same thermostat, located in the insula. Two psychologists—Naomi Eisenberger of the University of California, Los Angeles, and Tristen K. Inagaki, now at the University of Pittsburgh—placed participants in fMRI scanners while holding either a warm pack or a room-temperature ball. Next they asked the volunteers to read messages from their close friends and family. Some of the messages were emotionally neutral, such as “You have curly hair.” Others were heartwarming, such as “I love you more than anything in the world.” The researchers found that whether the subjects read tender notes or held heated packs, the activity in their insula looked similar. In addition, the volunteers themselves reported that they actually felt physically warmer after they read the emotional messages.
The Opioid Link
Inagaki, Eisenberger and another colleague at U.C.L.A., Michael R. Irwin, have further explored the ties between warmth, social bonding and trust by manipulating the opioid system in the brain, which controls pain, rewards and addictive behaviors. In fact, the insula is packed with opioid receptors of the type that play a role in drug addiction. And previous research has shown that opioids such as morphine and heroin can increase body temperature, which may be why people taking these drugs sometimes describe the experience as being “wrapped in a blanket” or feeling a “warmness inside.”
In 2015 Inagaki and his co-workers gave 31 volunteers a four-day course of naltrexone, a medication commonly administered to help recovering alcoholics and drug users quit. Naltrexone blocks opioid receptors in the brain and prevents addictive substances from having their desired effect. The scientists discovered that blocking these same receptors also makes people feel less socially connected—an effect recovering addicts should be warned about. Specifically, they found that while the study volunteers received a placebo drug and held a warm pack, they described feeling closer to loved ones when prompted to think about them. Those feelings became less intense when the same participants took naltrexone and held a warm pack. “We think that there is really no reason for why this should happen,” Inagaki says, “unless physical warmth and social warmth are using the same mechanisms.”
Animal studies have revealed that other substances—such as oxytocin, the so-called cuddle hormone, and serotonin—are involved in regulating both physical and psychological warmth. “They are all part of this network that we think drives us toward rewarding outcomes and social connections,” Inagaki adds. Scientists have long known that serotonin plays a key role in social behavior—with abnormally low levels associated with social anxiety. Newer evidence indicates that physical temperature may affect serotonin production. In 2011 neuroscientist Christopher Lowry and his colleagues at Boulder raised two groups of rats—one incubated at a sizzling 98.6 degrees F for a brief period and another kept at room temperature. Later the scientists removed the animals' brains for examination. They discovered that the hot surroundings had activated more serotonin-producing neurons in the brain stem.
Researchers have also found that mice genetically engineered to lack oxytocin receptors have trouble regulating body temperature—and that warm temperatures prompt oxytocin's release, much in the same way that a touch or a hug can. Skin, IJzerman notes, is another key element in temperature control. In 2012, in collaboration with his colleagues at Purdue University and the University of Milano-Bicocca in Italy, he asked 41 student volunteers to play an interactive computer game, in which online players (preprogrammed by the researchers) actively shunned some of the players in the lab. While they played, the researchers monitored their skin temperature. Among the students who were excluded, skin temperature dropped by an average of about 0.68 degree F. The finding may help explain why people experiencing rejection literally feel a chill in the air and tend to perceive a room's temperature as lower. Of interest, the scientists also found that if they asked socially excluded students to hold a hot cup of tea for only 30 seconds, those students described feeling less hurt than others who did not hold the cup.
The big question, of course, is why? Why are physical and psychological temperatures linked in the first place? There are two theories, which are not necessarily mutually exclusive. “One notion is that from birth we've learned that warmth signals the presence of loved ones, so one experience brings to mind the other one,” Inagaki says. “The second theory is that it's part of our innate system.”
For years researchers have explained the connection by way of the first theory, but recent neurobiological evidence gives more weight to the second idea that we have evolved this way. “For all warm-blooded animals, temperature regulation is very metabolically expensive and also required for survival,” IJzerman points out. “But it becomes cheaper when there are others to help us regulate our temperature.”
Indeed, animal research has revealed that kleptothermy—or stealing warmth from others, much as huddled emperor penguins do in Antarctica—saves metabolic resources. One 2014 study estimated that in a species of Chilean rodents, sharing a cage with just a few other animals lowered an individual's basal metabolic rate by up to 40 percent. Similarly, a 2015 study of vervet monkeys showed that friendly grooming not only helps these animals with tangles and pests, it also renders their pelts better insulated against the cold.
If we can save precious energy and feel warmer among others, it makes sense that we would also feel more socially included and trusting when primed with physical warmth. “Throughout evolutionary time, if you needed somebody else to cuddle with, you needed to know how reliable they were,” IJzerman explains, “so temperature expectation became involved as a ‘sociometer’ to assess how we think of other people. Despite modern conveniences like central heating, thermoregulation has remained important for how we understand our relationships, which is why in English we refer to emotionally responsive people as ‘warm’ and emotionally unresponsive as ‘cold.’”
Lowry has hopes of exploiting this innate connection to treat depression. Serotonin appears to be involved both in the disorder and, as noted above, in temperature. In addition, depressed people often have a raised body temperature and unusual temperature perception. In 2013 Lowry and his colleagues reported the results of a novel experiment: They administered a single session of whole-body heating with infrared lamps to 16 severely depressed adults, all of whom were hospitalized in a private clinic in Switzerland. “Infrared radiation doesn't penetrate the body very effectively,” he says, “so what we are really doing is heating the skin.” In fact, the lamps boosted skin temperature by several degrees F. Some participants reported that it was the hottest they had ever felt.
The lamps also produced impressive changes in the participants' moods. Compared with a control group, who lay under a nonheating lamp, those who baked under infrared radiation scored on average more than six points lower on the Hamilton Depression Rating Scale, a classic scale used to gauge depressive symptoms. This change was present six weeks later and significant enough to shift some patients from severe to moderate depression. Though promising, heat-lamp therapy needs to be validated by additional and bigger studies.
IJzerman speculates that some comforts of modern society, such as readily available hot showers, may be interfering with how we relate to others. “In the Middle Ages, people would sleep with about five people to one bed because they needed to warm one another up,” he says, “but we don't do that anymore—we have central heating.”
Newer technologies may continue this trend of separating warmth from social contact. IJzerman mentions a product under development called Wristify, a bracelet that can cool or heat your body whenever you want. “It could make us even less dependent on others,” he notes, “and, perhaps, profoundly alter our interpersonal relationships.” To counter such trends, we would do well to practice the time-honored traditions of offering friends a warm embrace and a steaming cup of tea.