The biological story behind some of therapy's most popular ideas is under serious scientific fire. Here is what the debate is actually about, and what it means for therapists and clients.

Back in 2019, I was working on a question that might sound distant from neuroscience: whether integrating Dual Process Theories with Compassion Focused Therapy could give therapists a sharper framework for understanding how thinking actually works in the room with a client. Dual Process Theories propose that we think in two distinct modes, one fast and instinctive, one slow and deliberate. The question was whether bringing that insight into Compassion Focused Therapy could make us better at understanding what happens in the therapy room.

That work taught me something I keep returning to. The biological stories we tell in psychotherapy, the narratives we borrow from neuroscience and evolutionary biology to explain why people feel and behave the way they do, need to be handled with real honesty about what they can and cannot claim. A compelling story is not the same thing as a well-supported one. And in therapy, the stories we tell patients about their own minds and bodies carry weight that goes well beyond the lecture hall.

You've probably encountered polyvagal theory, even if you've never heard it called that. It's the framework behind phrases like "nervous system regulation," "ventral vagal state," and "co-regulation" that have quietly taken over the language of therapy, trauma work, yoga studios, and parenting books.

Maybe your therapist has talked about helping you find your "window of tolerance," that zone where you feel activated enough to engage but calm enough to think. Maybe you've heard that slow, deep breathing activates your "rest and digest" system. Maybe someone explained your tendency to go completely blank under pressure as your nervous system going into "freeze mode," an ancient survival response you inherited, essentially, from your reptilian ancestors.

All of that language, to a large extent, flows from polyvagal theory.

Now, in a rare and unusually blunt piece of scientific publishing, 39 internationally recognized experts, physiologists, neuroscientists, and evolutionary biologists from institutions like Oxford, Harvard, UCLA, and Mayo Clinic, have signed their names to a joint paper declaring that the biological foundations of this theory are "untenable." That's a strong word in science. It essentially means: this cannot be defended.

And Stephen Porges, the psychologist who created the theory three decades ago, has fired back, saying the critics have fundamentally misunderstood what he was ever claiming in the first place.

This isn't just a fight between academics in journals that nobody reads. It has real implications for how therapists work, what they tell their clients, and whether the biological story underpinning some of the most popular therapeutic approaches today is actually accurate.

So what does polyvagal theory actually say?

Let's start from the beginning, because this theory is often explained with a lot of technical language that can make it feel more solid and settled than it may actually be.

The vagus nerve is a long, wandering nerve. "Vagus" is Latin for wandering, which tells you something about its personality. It runs from your brainstem all the way down through your throat, heart, lungs, and gut. It's the main pathway of what's called the parasympathetic nervous system. If the sympathetic nervous system is your accelerator, the system that revs you up when you sense danger, the parasympathetic is your brake. It calms you down, helps you digest, and allows you to rest and connect with others.

Stephen Porges proposed that this nerve didn't just evolve once, fully formed. It evolved in stages, across hundreds of millions of years of vertebrate evolution, and that we humans, as mammals, carry all those evolutionary layers inside us, stacked on top of each other like geological strata. He called this polyvagal theory. "Poly" means many, referring to the idea that there isn't just one vagal system but several, each with a different evolutionary age and a different job.

In his framework, we have three layers of response, arranged in a kind of hierarchy.

The social engagement system is the newest layer, unique to mammals according to Porges. When you feel genuinely safe, not just intellectually safe but safe in your body and in your gut, this system comes online. It helps you make easy eye contact, soften your voice, listen well, and connect with the people around you. It is mediated by a specific part of the vagus nerve originating from a brainstem region called the Nucleus Ambiguus. Think of this as your "open for connection" mode. It is, in his view, what makes human social life possible.

The fight-or-flight system is older. When you sense threat, your sympathetic nervous system fires up. Heart rate increases, muscles tense, breathing shallows, attention narrows to the source of danger. You mobilize. This is the system most people are familiar with from everyday stress.

The freeze or shutdown system is the oldest of all, a last resort Porges argues was inherited from our pre-mammalian ancestors. When a situation feels utterly inescapable or life-threatening, a different part of the vagus nerve, originating from a brainstem region called the Dorsal Motor Nucleus, triggers a collapse response: dissociation, emotional numbness, going completely blank, or in extreme cases, fainting. Like a possum playing dead. This is what Porges calls "dorsal vagal shutdown," and it features heavily in how polyvagal theory explains trauma responses.

Porges also proposed a way to measure how well your social engagement system is functioning: something called respiratory sinus arrhythmia, or RSA. This is the natural rhythm by which your heart rate gently rises when you breathe in and falls when you breathe out, something that happens in every healthy person, all the time, without you noticing. Higher RSA, in his framework, signals stronger "vagal tone," meaning your nervous system is more flexible, more socially engaged, and better at regulating your emotions.

It is, taken as a whole, a beautiful and intuitive story. Your nervous system as an evolutionary layer cake. Your anxiety, your freeze responses, your difficulty connecting with others, all explained through the lens of ancient biology, and all potentially measurable through something as simple as your breathing pattern.

The critics say: the story is too beautiful. And in key places, too wrong.

The four main problems the critics identify

1. That breathing-linked heart rhythm doesn't measure what polyvagal theory says it does

RSA, that gentle rise and fall in your heart rate as you breathe, plays a starring role in polyvagal theory. Porges treats it as a direct window into vagal activity from the brain to the heart: a biological readout of your social engagement system. Measure someone's RSA, and you are measuring the health of their ventral vagal pathway.

But the 39 critics argue this claim is fundamentally mistaken. RSA is shaped by a huge range of factors that have nothing to do with how much vagal signal your brain is sending to your heart. How fast you are breathing matters enormously. How deeply. Your blood pressure. Your age. The level of carbon dioxide in your blood. Even your sympathetic nervous system, the stress accelerator, influences RSA.

Think of it this way. Imagine trying to figure out how loudly a musician is playing by measuring how much the candle on top of the piano flickers. The flame does respond to the music, because sound waves move air and air moves the flame. But it also responds to the air conditioning, people walking past, and whether someone left a window open. You cannot confidently say "the flame flickered a lot, therefore the musician played loudly." Too many other things are going on.

RSA is like that candle. It tells you something, but it is a much noisier and more indirect signal than polyvagal theory assumes. And you cannot build a reliable theory of psychological states on a measurement that unreliable.

2. The brain regions don't behave the way polyvagal theory describes

Here is where the neuroscience gets particularly direct. Polyvagal theory draws a sharp, meaningful line between two brainstem regions. The Nucleus Ambiguus is linked to social safety and healthy heart regulation. The Dorsal Motor Nucleus is linked to primitive shutdown and freeze. In polyvagal theory's story, one region is the hero and one is the villain. The problem is that the evidence doesn't cast them that way.

Across dozens of studies in rats, cats, dogs, sheep, and other mammals, it is consistently the Nucleus Ambiguus, not the Dorsal Motor Nucleus, that drives heart rate changes, both small and large. The Dorsal Motor Nucleus, in most mammals, has a surprisingly modest effect on the heart.

In one recent experiment using optogenetics, a precise technique where scientists use light to switch specific neurons on and off like flicking individual switches in a fuse box rather than cutting power to the whole building, strong stimulation of the Dorsal Motor Nucleus in rats produced no significant change in heart rate at all. Another study in mice found some heart slowing from that region's stimulation, but also found this actually reduced anxiety-like behavior. That is the exact opposite of what polyvagal theory would predict.

And what about "dorsal vagal shutdown," the collapse response that polyvagal theory says explains dissociation and freeze in trauma survivors? The critics reviewed the human evidence and found that emotional freezing and dissociation in people with PTSD do not typically produce the massive heart slowing the theory describes. Studies measuring heart rate during actual freeze and dissociative episodes find either no change, or very small decreases. The dramatic vagal shutdown, in humans, appears not to be happening the way the theory says it should.

3. The evolutionary story about nerve fibers doesn't hold up

One of polyvagal theory's central claims is evolutionary: that mammals developed a special kind of faster nerve fiber called myelinated fibers. They are called myelinated because they are wrapped in a fatty sheath called myelin that speeds up signal transmission, the way rubber insulation on a wire prevents signal loss. Polyvagal theory argues this biological innovation gave mammals the capacity for rapid, flexible social engagement. Reptiles and other non-mammals, the theory says, lack these fibers, which is why their autonomic responses are slower, cruder, and less socially oriented.

The critics document, drawing on nearly five decades of comparative research across species, that this is simply not true. Myelinated cardiac vagal fibers have been found in fish, amphibians, reptiles, and birds. The catshark, a primitive fish that doesn't even have sympathetic innervation of the heart, actually shows some of the most tightly synchronized heart-rate-to-breathing coordination ever recorded in any vertebrate, including mammals. These faster fibers, and the rapid responses they enable, are ancient. They did not appear with mammals, and they are not what makes us socially capable.

4. Reptiles are considerably more social than polyvagal theory implies

This may be the most surprising part of the critique for anyone outside behavioral biology. Polyvagal theory essentially portrays reptiles as socially limited creatures, primarily defensive, reactive, and incapable of the kind of flexible, nuanced social behavior that mammals show, because they lack the mammalian vagal upgrade.

Several of the 39 authors have spent their careers studying exactly this question, and they strongly disagree. Depending on the species, reptiles show long-term pair bonding, extended parental care, communal nesting, cooperative hunting, social learning, and what appears to be genuine social stress buffering, meaning the ability to calm down in the presence of a familiar companion. Some crocodilians show behaviors that careful observers describe as resembling play.

And then there are insects. Bees, ants, and termites have no vagus nerve whatsoever, yet build some of the most elaborate social structures on Earth. Whatever the vagus nerve contributes to social life, it is clearly not the prerequisite for sociality that polyvagal theory implies.

What Porges says in his defense

Porges's published response is sharp and raises a genuinely interesting point. His main argument: the critics have carefully dismantled a version of his theory that is not actually his theory.

Polyvagal theory, he says, was never meant to be a precise, millimeter-by-millimeter map of which brainstem region controls which behavior. It is a systems-level framework, a way of thinking about how different autonomic states shape psychological experience and social behavior, not a literal wiring diagram of neural circuits. By demanding that kind of hard mechanistic precision from what is fundamentally an organizing framework, the critics are applying the wrong measuring stick and then declaring the theory wrong for failing a test it was never designed to pass.

He also argues that RSA was never claimed to be a perfect direct measure of vagal outflow, only a useful indicator within certain contexts, and that critics have been recycling the same objections for two decades without properly engaging with his responses to them.

This is a reasonable point up to a limit. Many valuable frameworks in psychology are not meant as literal mechanistic descriptions. They are useful maps, organizing metaphors that help clinicians and researchers think more clearly, even if the map is not identical to the territory it describes.

The harder question is: how far can you stretch that defense before the framework stops making any testable predictions at all? A theory that cannot be shown to be wrong about anything specific is not really a scientific theory. It is a story. And stories, however useful in the clinic, need to be clearly labeled as such.

What this means if you are a therapist or a client

Let's be clear about what this debate does and does not mean in practice, because it is easy to overcorrect in either direction.

It does not mean that:

• Somatic therapy, the kind that works with body sensation and physical experience, doesn't work
• Breathwork is useless or misguided
• The idea that trauma lives in the body and needs to be processed through the body is wrong
• Co-regulation, the way a calm and attuned therapist can genuinely help settle a dysregulated client, doesn't happen
• Feeling safe is unimportant for healing

All of those ideas have their own foundations in attachment research, developmental psychology, trauma studies, and decades of clinical experience. They stand independently of whether the vagus nerve biology is correct. They were true before polyvagal theory, and they remain true regardless of this debate.

What the debate does challenge is the specific biological story used to explain why those things work. When a therapist tells a client "your dorsal vagus is shutting you down right now" or "we're building your vagal tone," they are using a framework whose accuracy is now seriously in question.

Why does that matter? Because the explanatory stories we tell in therapy are not neutral. They shape how clients understand themselves and their bodies. A client who believes their dissociation is caused by an ancient reptilian shutdown system firing in their brainstem may relate to their experience very differently, and make very different meaning of it, than a client who understands the same experience as a learned protective response shaped by their personal history. Those are not equivalent framings, and the difference can matter clinically in ways that are hard to see in the moment.

The critics make this point plainly: presenting an inaccurate account of neurophysiology as settled scientific fact to healthcare providers, patients, and families may be harmful. Not because the therapies built on it are necessarily harmful, but because false certainty about mechanisms can distort both treatment decisions and self-understanding in ways that are difficult to detect and correct.

The bigger picture, and why this feels personal

This is where I want to return to my own work, not to position myself in the debate, but because I think it illuminates something important about the broader challenge facing psychotherapy.

When I explored whether integrating Dual Process Theories with Compassion Focused Therapy could sharpen our understanding of how thinking works in the therapy room, one of the central tensions I kept encountering was exactly this: how do we borrow usefully from neuroscience and evolutionary biology without overclaiming? How do we let scientific frameworks genuinely inform clinical thinking without letting a particular biological story become so load-bearing for the whole therapeutic structure that it can no longer be questioned or revised?

Polyvagal theory may be, in this sense, a cautionary example of what happens when a compelling biological narrative becomes so deeply embedded in a clinical framework that the two can no longer be separated, even when the biology starts to crack under scrutiny. The framework becomes the identity. And at that point, challenging the science feels, to its advocates, like an attack on the therapy itself. That is a dangerous place for any field to be.

The psychological insights that polyvagal theory brought into mainstream therapy are real and valuable: the centrality of felt safety in healing, the role of the body in emotional regulation, the importance of the therapeutic relationship as a genuinely co-regulating force. Those ideas deserve to survive this debate. But they deserve better and more honest scientific grounding than polyvagal theory's particular version of vagal neuroscience currently provides.

Science is supposed to work like this. A theory makes specific claims. Specialists examine those claims against the evidence. Disagreements get published. The field updates. What makes this episode unusual is the scale. Thirty-nine experts co-signing a single critique is remarkable in any field, and the bluntness of the conclusion is equally striking. "Untenable" is not a word scientists reach for lightly.

Porges has spent three decades genuinely changing how therapists think about the body and about safety. That contribution is real and should not be dismissed. But the field of psychotherapy, which has sometimes embraced polyvagal theory with considerably more certainty than the evidence warranted, would do well to sit with this debate seriously. And to remember that good therapy can rest on good psychology, even when the neuroscience turns out to be more complicated, and more contested, than the story we were told.

The critique "Why the Polyvagal Theory Is Untenable" was published in Clinical Neuropsychiatry (February 2026), co-authored by Paul Grossman and 38 colleagues. Porges's response, "When a Critique Becomes Untenable," appeared in the same issue.