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In honor of Valentine's Day, we're highlighting our interview with Dr. Hans Rocha IJzerman about the impact that temperature can have on our interpersonal relationships. Hans just released his book, Heartwarming: How Our Inner Thermostat Made Us Human, that explores the topics covered in this piece even more, and we encourage you to check it out!

Smiling man in blazer with yellow bow tie.

Hans Rocha IJzerman is an Associate Professor at Université Grenoble Alpes. He holds a PhD in Psychology and has done groundbreaking work in the world of social thermoregulation — the study of how temperature interacts with our social relationships. We spoke to him over video call about his work and what he thinks about the potential of temperature to create social connections.

Tell me a little bit about who you are, your background and your work.

I'm originally from the Netherlands, and in 2017 I moved to France with my now wife. She's from Brazil, and my daughter will be three months in a couple of days. My original plan was actually to become a professional basketball player. I would have never, ever been good enough to be an NBA player, but I think I was sufficiently good to play in Europe. That's the reason why I did my undergrad in the United States, where I started studying psychology. I got injured playing basketball, so I had to choose an alternative career. I did my master’s and PhD degrees in the Netherlands before moving to France, where I now work as an associate professor in psychology.

The NBA’s loss is our gain. So then, how did you end up getting into the temperature psychology field?

When I was a master’s student in psychology between 2004-2006, I went to the University of Illinois to do my thesis with Dov Cohen. He proposed studying the role of the body in how we think about moral values. From there, I learned about conceptual metaphor theory, which is a theory that tries to explain how the body is involved in abstract thought.

When I returned back to Amsterdam, my adviser had heard about studies conducted in the United States on warmth and how we perceive other people. These original studies suggested that holding something warm makes a person judge other people as friendlier. While there are some questions about the findings of that study now, it did set me on a path to try and understand the role temperature had in social behavior. I was thinking about temperature from the linguistic perspective, how they use it to structure their abstract thoughts about relationships.

When I started reading more about this, it made little sense to me; but a few years later when I visited Jim Coan at the University of Virginia, something clicked. He has a theory that being with other people is our “social baseline.” His work investigates how other people can help us regulate threat. Jim’s idea is that being by ourselves is threatening, particularly if we find ourselves in stressful situations. Core to the theory is that we outsource our stress regulation to other people.

Because of Jim, I started thinking about temperature in very much the same way: Other humans can help us regulate temperature. Unlike something like oxygen intake, we have to regulate temperature all the time. Metabolically, it’s very costly, and we constantly have to adapt. So if temperatures drop down, this should be costly, metabolically speaking. An assumption behind the research we do is thus that when temperatures go down, other people can help share the load that temperature regulation imposes upon us.

To be fair, we don’t have a lot of strong evidence for these assumptions, but we do have a number of cues that point into that direction. For example, if the temperature outside goes down or the temperature in the room goes down, we tend to think of people that we love. In other work, we measured core body temperature and a number of variables that can help us understand people’s interpersonal relationships. What we found, through a technique called machine learning, is that the diversity of people’s social networks seems to buffer people from the cold. People farther away from the equator tend to have more diverse social networks (i.e., they participate in various social groups), and for some reason, this buffers against the cold. Why exactly, we don’t know yet. The number of people in their social networks do not seem to play a role.

Your work has been described as the study of Social Thermoregulation. Can you explain a little more about what that is?

So to me, social thermoregulation in humans is the idea that modern interpersonal relationships are reliant on an (evolutionarily speaking) earlier mechanism of body temperature regulation. More intuitively stated: the way that humans engage in the types of relationships we engage in is rooted in our "penguin bodies.” We have very similar basic physiological systems, even though on top of that, we should expect to find many differences between humans and penguins.

One of the reasons we started thinking about it in this way is because in preparation of the study on diversity and social networks and core body temperature, a study on vervet monkeys by Richard McFarland and colleagues caught our attention. Vervet monkeys are a species that is often studied for their similarity to humans. McFarland and colleagues followed them and observed their core body temperatures. What he found is that when environmental temperatures drop, their minimum core body temperature is higher if they have a larger social network. That study was the impetus for our own study on core body temperature that I mentioned before. Indeed, we find this in humans for more diverse social contexts: if we have more diverse social roles in our network, we seem to be protected better against the cold. We don’t know why, but we know that there is a reliable difference between vervet monkeys and humans in relation to how social networks protect against the cold.

One question I get frequently is whether it's just warm is positive and cold is negative. That’s not the case. When you think about temperature, it's important to keep the principle of homeostasis in mind. Temperature needs to be regulated all the time, and people regulate to stay within what is called the “Thermoneutral Zone” — you don't want it to be too warm or you don't want it to be too cold. It means that any deviation from this comfortable zone could benefit from up or down regulation to get back to the comfortable range. There is, however, a remarkable asymmetry between the two of them: When temperatures go up, it is almost immediately dangerous, so we typically regulate this by ourselves. But when temperatures go down, it's not immediately dangerous; we have time to regulate, so we can rely on other people for help.

Another point that is important to illuminate is that temperature is far from the only reason why people engage in relationships. It is thus not just that colder or warmer climates make you more social. But if other people can help you warm up when it is cold, it should, hypothetically, make you feel better.

So do you think it works the other way around? Can you use temperature to create social connection?

Broadly speaking, we have done various studies that relate temperature to interpersonal relationships and a meta-analysis we have conducted on studies between 2008 and 2017 shows ~90 published and unpublished reports on the topic. We ourselves have done studies where we manipulate temperature and where we see that other people feel psychologically closer to each other. One of the first ones was in 2009. That study on warmth and feeling closer to one another is one that I am now less certain about, as a lot of developments on the robustness of our science have taken place since then and this study was, what we call now, underpowered. (Another study from the same article we were able to replicate successfully, however, so at least we know that warmth leads to seeing things more relationally in perception.)

We have tried to move on now to more complex studies to start mapping the relationship between temperature and interpersonal connection in a more coherent way, so that we formulate a better theory on social thermoregulation. Some of the more recent studies that we've been doing now are with couples where we measure their peripheral temperature. We then try to apply computer techniques to see if there is any relationship of “co-thermoregulation” in couples at all. For example, we want to know whether, if one partner’s peripheral temperature drops, whether the other partner responds with temperature going up and what kind of behaviors are associated with that within the relationship.

Ideally, if we are able to find reliable relationships, we think that we can apply this within relationship therapy. The idea is that we hope to distinguish lower and higher quality relationships in terms of their social thermoregulatory patterns. That also means, hypothetically, that we should be able to manipulate temperature to help “lower relationship quality” couples look more like “higher relationship quality” couples (at least a tiny bit). For that, we imagine using the Embr Wave to manipulate temperature to see whether we can make people be more responsive to their partner.

Admittedly, this is still more a concept than a well-worked out theory. We still have a lot of work to do. Temperature of course depends on various factors, such as body weight, height, gender, daily temperature, and what situation people find themselves in, emotionally speaking. We do have some preliminary findings that this idea may work, but we are still in the process of replicating these findings. I have tried to summarize the findings surrounding social thermoregulation — and some of the uncertainty about it — in my book that will come out in February.

Does having a physical reliance on someone increase the sense of intimacy one might have with someone else?

For social thermoregulation, we have some reliable evidence that intimacy and social thermoregulation are related. For example, when we ask the question whether people want to warm up with a partner when they feel cold, it relates to questions about opening up to others emotionally. In addition, that same question also relates to whether people report being healthier. Both of these are still correlations, meaning that we are still uncertain about the causal relationship between the two. We also find that the same set of questions (warming up with a loved one when feeling cold) relates to whether people have larger and more diverse social networks. So there seems to be something about warming up with other people and their sense of intimacy with others.

Even though many of these findings are still correlations (e.g., warming up with a partner feels good and leads to greater intimacy), they do seem to have some truth to them, intuitively. 

But I also hypothesize that there will be enormous variation between people. Some people may not always be thrilled to receive warmth from someone else, and they want to have more control over when this happens. That’s why I really like the feature that you built in so that users have to consent to receiving warmth. Particularly for those who may sometimes not be waiting for warmth, it can be a way to build up intimacy with their partner more gradually and help them open up to their partner. 

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