Essentials: Understanding & Controlling Aggression

In this Huberman Lab Essentials episode, I explain the neural circuits that activate and control aggressive states and behaviors. I discuss how hormones, genes and environmental factors such as day length can shift our aggressive tendencies. I also share science-based tools for modulating aggression, including sunlight exposure, heat therapy and supplementation with ashwagandha or acetyl-L-carnitine. Read the episode show notes at hubermanlab.com. Thank you to our sponsors AG1: https://drinkag1.com/huberman LMNT: https://drinklmnt.com/huberman Eight Sleep: https://eightsleep.com/huberman Timestamps (00:00:00) Aggression, Types of Aggression (00:01:43) Context, Aggression vs Sadness (00:03:11) Hydraulic Pressure Model of Aggression (00:06:40) Sponsor: LMNT (00:08:12) Brain Areas for Aggression, Ventromedial Hypothalamus (00:15:26) Biting, Neural Circuits of Physical Aggression (00:17:52) Sponsor: Eight Sleep (00:19:09) Estrogen & Aggression, Testosterone & Competitiveness (00:22:37) Seasonality, Sunlight, Melatonin & Aggression (00:24:50) Cortisol, Serotonin & Aggression (00:26:35) Tool: Reduce Cortisol with Sunlight & Sauna; Ashwagandha (00:30:39) Sponsor: AG1 (00:31:58) Irritability, Aggression & Genetics; Seasonality (00:34:49) Tool: ADHD, Acetyl-L Carnitine & Aggressive Behavior Disclaimer & Disclosures Learn more about your ad choices. Visit megaphone.fm/adchoices

Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.

• 0:00Andrew D. Huberman?

  Welcome to Huberman Lab Essentials, where we revisit past episodes for the most potent and actionable science-based tools for mental health, physical health, and performance. I'm Andrew Huberman, and I'm a professor of neurobiology and ophthalmology at Stanford School of Medicine. Today, we are discussing aggression. I'm going to explain to you that there are several different types of aggression. For instance, reactive aggression versus proactive aggression, meaning sometimes people will be aggressive because they feel threatened or they are protecting those that they love who also feel threatened. There's also proactive aggression, where people go out of their way to deliberately try and harm others. And there is indirect aggression, which is aggression not involving physical violence.

• 0:45Andrew D. Huberman?

  For instance, shaming people and things of that sort. It turns out that there are different biological mechanisms underlying each of the different types of aggression. And today, I will define those for you. I'll talk about the neural circuits in the brain and body that mediate each of the different kinds of aggression, talk about some of the hormones and peptides and neurotransmitters involved. I promise to make it all accessible to you, even if you do not have any biology or science background. I'm certain that by the end of the episode, you will come away with a much more thorough understanding of what this thing that we call aggression really is. And when you see it in other people, I think it will make more sense to you. And when you observe it in yourself, or the impulse to engage in aggression, verbal or physical or otherwise, I hope that you'll understand it better as well.

• 1:29Andrew D. Huberman?

  And of course, the tools that I will describe should allow you to modulate and control aggressive tendencies or predispositions to aggressiveness, and just generally be able to engage with people in a more adaptive way overall. The context of aggression really matters. So there are instances where aggression is adaptive. For instance, a mother protecting her children. Of course, other forms of aggression, like unprovoked proactive aggression, somebody simply being violent to somebody else. Even when unprovoked, most of us cringe when we see that kind of behavior. It can even evoke aggression in people when they observe that kind of behavior. Many of you have probably heard the statement that I believe arises from pop psychology,

• 2:12Andrew D. Huberman?

  not from formal academic psychology, that aggression is just sadness. It's a form of sadness that's amplified and it shows up as aggression. But when we look at the underlying biology and the peer reviewed literature on this, nothing could be further from the truth. We have distinct circuits in the brain for aggression versus grief and mourning. Those are non-overlapping. Now, that doesn't mean that you can't be sad and aggressive, or in a state of mourning and aggressive at the same time. But the idea that sadness and aggression are one and the same thing is simply not true. And by understanding that, or perhaps by understanding that irritability and aggression are not the same thing, you'll be in a much better position to apply some of the tools

• 2:56Andrew D. Huberman?

  that we will talk about in this episode in order to be able to reduce or eliminate, or if it's adaptive to you, to modulate aggression. And yes, there are cases where modulating your aggression, in some cases even amplifying aggression, can be adaptive. One of the names that's most associated with the formal study of aggression is none other than Conrad Lorenz. Conrad Lorenz studied so-called imprinting behaviors and fixed action pattern behaviors, patterns of behavior that could be evoked by a single stimulus. The idea that you can get a whole category of behaviors, like looking to somebody for comfort and only them, the idea that you could get a huge category of different behaviors

• 3:38Andrew D. Huberman?

  in a bunch of different contexts, triggered by just the presence of that person, is remarkable because what it suggested, and what turns out to be true, is that there are neural circuits, not just individual brain areas, but collections of brain areas that work together to engage a pattern of behaviors. And that's the first fundamental principle that we need to define today, that when we talk about aggression, we're talking about activation of neural circuits, not individual brain areas, but neural circuits that get played out in sequence, like keys on a piano. But that playing out in sequence means that aggression is a verb. It has a beginning, a middle, and an end. And it's a process. It's not an event. And as you'll see, that turns out to be very important in terms of thinking about how

• 4:23Andrew D. Huberman?

  one can halt aggression, prevent it from happening before it's initiated, or maybe even prolonging aggression if that's what's needed. Now Conrad Lorenz had no real knowledge of neural circuits. I mean, obviously, he knew there was this thing that we call a brain and a nervous system, and he knew that there were chemicals in the brain and hormones and things of that sort that were likely to play a role. But he really didn't take any measures to define what the neural circuits were. But he did think about what sorts of underlying processes could drive something like aggression. And he talked about one particular feature that's especially important, and that's this notion of a pressure.

• 5:03Andrew D. Huberman?

  The idea that, yes, certain hormones will bias somebody or an animal to be aggressive. Certain neurotransmitter states, and you'll learn what those are today, will bias somebody to be more or less aggressive. And yes, of course, there will be historical features based on their childhood, et cetera, et cetera. He understood that there will be a constellation of things that would drive people to be aggressive. And he described a so-called pressure, almost like a hydraulic pressure, just think about fluid pressure in a small container, being pushed, pushed, pushed until the can or the container is ready to explode, and how multiple features, multiple variables, could impinge on that and create that pressure. It turns out that's exactly the way the system works.

• 5:46Andrew D. Huberman?

  There is no single brain area that flips the switch for aggression, although we'll soon talk about a brain structure that generally houses the propensity and the output of aggression. This notion of a hydraulic pressure that can drive us toward aggressive behavior, or conversely, can be very low pressure and keep us in a state of non-reactivity, maybe even passivity or submissiveness, is a very important feature because it really captures the essence of how neural circuits work when we're talking about primitive behaviors generally. And you can start to notice this in yourself and in others. You can start to notice when you are veering toward aggression, or when someone is veering toward aggression, verbal or physical.

• 6:31Andrew D. Huberman?

  Now that veering is the buildup of this hydraulic pressure that Lorenz was referring to, and it really does have an underlying biological basis. I'd like to take a quick break and acknowledge one of our sponsors, Element. Element is an electrolyte drink that has everything you need and nothing you don't. That means the electrolytes, sodium, magnesium, and potassium in the correct amounts, but no sugar. Proper hydration is critical for optimal brain and body function. Even a slight degree of dehydration can diminish cognitive and physical performance. It's also important that you get adequate electrolytes. The electrolytes, sodium, magnesium, and potassium, are vital for functioning of all the cells in your body, especially your neurons or your nerve cells. Drinking Element dissolved in water makes it very easy to ensure that you're getting

• 7:15Andrew D. Huberman?

  adequate hydration and adequate electrolytes. To make sure that I'm getting proper amounts of hydration and electrolytes, I dissolve one packet of Element in about 16 to 32 ounces of water when I first wake up in the morning, and I drink that basically first thing in the morning. I'll also drink Element dissolved in water during any kind of physical exercise that I'm doing, especially on hot days when I'm sweating a lot and losing water and electrolytes. Element has a bunch of great tasting flavors. I love the raspberry, I love the citrus flavor. Right now, Element has a limited edition lemonade flavor that is absolutely delicious. I hate to say that I love one more than all the others, but this lemonade flavor is right up there with my favorite other one, which is raspberry or watermelon. Again, I can't pick just one flavor, I love them all.

• 7:56Andrew D. Huberman?

  If you'd like to try Element, you can go to drinklmnt.com/huberman, spelled drink L-M-N-T dot com slash huberman, to claim a free Element sample pack with a purchase of any Element drink mix. Again, that's drinklmnt.com/huberman to claim a free sample pack. Now, it was some years later that the first experiments came along, which really started to identify the brain areas and the biological so-called pressures that can induce aggressive behavior. And the person that really gets credit for this is a guy by the name of Walter Hess, who at that time was working on cats. And I know that when you say "working on cats," a lot of people will cringe, a lot of people have cats as pets, and certainly cats can be delightful, some people like

• 8:41Andrew D. Huberman?

  them more, some people like them less. Most people cringe at the idea of doing experiments on cats. In the time of Hess, very few laboratories worked on mice, most laboratories worked on cats or rats. So when I say he was working on cats, I realize that probably evokes some negative emotions in some of you, maybe even aggression in some of you. What we can do, however, is look at the data and make use of the data in terms of our understanding. What Hess did was, he had cats that were awake, and he was able to lower stimulating electrode into their brain. Now, keep in mind that the brain does not have any pain sensors, so after a small hole is made in the skull, electrodes are lowered into the brain. This is what's done commonly in human neurosurgery.

• 9:22Andrew D. Huberman?

  He was trying to identify brain regions that could generate entire categories of behavior. Eventually, his electrode landed in a site, and he provided electrical stimulation to the cat that caused this otherwise passive, purring, relaxing cat to suddenly go into an absolute rage when he stimulated this particular brain area. And the fact that when he turned off the stimulation of this particular brain area, the cat very quickly, within seconds, went back to being passive, calm kitty. And later experiments, done in mice but also in humans, confirm that indeed, stimulation of this brain area evoked not just behavioral aggression, but also subjective feelings

• 10:07Andrew D. Huberman?

  of aggression and anger. So what was this incredible brain area? The so-called VMH, or ventromedial hypothalamus. The ventromedial hypothalamus is a nucleus, meaning a small collection of neurons, only about 1,500 neurons on one side of your brain, and a matching 1,500 neurons on the other side of your brain. And that combined 3,000 neurons or so is sufficient to generate aggressive behavior of the sort that Hess observed in the cat. And, believe it or not, when you see somebody who's in an act of rage, or in an act of verbal aggression, or in an act of defensive aggression, protecting their family, or loved

• 10:52Andrew D. Huberman?

  ones, or country, et cetera, almost certainly, those neurons are engaged in that behavior. Experiments done by David Anderson's lab at Caltech were really the first to parse the fine circuitry and to really show that the ventromedial hypothalamus is both necessary and sufficient for aggressive behavior. What they did was, they identified where the ventromedial hypothalamus was in the mouse. That was pretty straightforward to do, was sort of known before they started these experiments. And then, they analyzed which genes...

• 11:24Andrew D. Huberman?

  Meaning which DNA, which of course becomes RNA and RNA becomes protein, which DNA and therefore which proteins are expressed in particular cells of the ventromedial hypothalamus. And it turns out that there's a particular category of neurons in the ventromedial hypothalamus that make an estrogen receptor. And it is those neurons in particular that are responsible for generating aggressive behavior. How did they know this? Well, they used a tool that's actually been described by a previous guest of this podcast. We had an episode with the psychiatrist and bioengineer and my colleague at Stanford School of Medicine, Carl Diceroff.

• 12:04Andrew D. Huberman?

  He and others have developed tools that allow people to control the activity of neurons, essentially by remote control, by shining light on those neurons. So in the context of an experiment on a mouse, which is what David's lab did, and these were the beautiful experiments of Dayu Lin, who's now in her own laboratory at New York University, put a little fiber optic cable down into the hypothalamus of the mouse. The mouse is able to move around in its cage freely moving, even though it has a little tether, this little wire, it's a very thin wire. And that little thin wire is actually a little what we call optrod. And the experimentalist, in this case Dayu, was able to stimulate the turning on of a little bit of blue light.

• 12:44Andrew D. Huberman?

  And that blue light activated only those estrogen receptor neurons in only the ventromedial hypothalamus. And the way she was able to do that is he had introduced a gene that had been developed by our friend Carl Diceroff that allows light to trigger electrical activity in those neurons. So if any of that is confusing, or if all of that is confusing, here's the experiment. There's a mouse in a cage, has a little wire coming out of its head. It doesn't notice, believe it or not. We know this because it's still eating and mating and doing all the things that mice like to do on a daily basis and sleeping, et cetera. And the mere pressing of a button will activate a little bit of light released at the end of that wire. That light activates particular neurons.

• 13:26Andrew D. Huberman?

  In this case, it's the estrogen receptor containing neurons in only the ventromedial hypothalamus. A large number of experiments were done, but the first experiment really was to put the male mouse in with a female mouse who's in the so-called receptive phase of estrus. That is, she will allow mating. And he starts mating with her. And they go through the standard repertoire of mating behaviors that you observe in mice. Mounting, thrusting, intromission, as it's called in the mouse sex world. Well, I guess, I don't know what the mice call it, but that's what the experimenters call it. And then afterwards, he will dismount. But about halfway through the behavior, Dayu turned on the light to stimulate these estrogen

• 14:10Andrew D. Huberman?

  receptor-contained neurons only in the male mouse. And what she observed was incredibly dramatic. The male mouse ceases from trying to mate with the female mouse and immediately tries to kill the female mouse. He starts attacking her. Then she turns off the light, the male stops, and goes back to trying to mate with the female mouse. I'm sure all of this was very confusing and disturbing to the female mouse. Nonetheless, that was the repertoire. These are such dramatic shifts in behavior triggered only by the activation of only this small set of neurons within the ventromedial hypothalamus. The shift in behavior is almost instantaneous. Occurs within seconds, if not milliseconds, thousandths of a second. The next experiment that she did was to put a male mouse with this stimulation with

• 14:54Andrew D. Huberman?

  light capability in its ventromedial hypothalamus into a cage alone, but with a rubber glove filled with air or water. Then she stimulates the activation of these ventromedial hypothalamus neurons, and the mouse immediately tries to kill the glove. It goes into a rage, attacking the glove as if it were another mouse or some other animate object, but of course, it's an inanimate object. It's just a rubber glove. She stops the stimulation, and the mouse immediately goes back to being completely calm, or at least not attacking. Again, we don't know what the mouse was feeling. Subsequent experiments done by Dayu Lin in her own laboratory and other laboratories have shown that the ventromedial hypothalamus is connected with a bunch of other brain areas.

• 15:34Andrew D. Huberman?

  One of them that I want to call out now is the so-called PAG, the periaqueductal gray nucleus. This is a large structure in the back of the brain that houses things like neurons that can create opioids. We all know of the opioid crisis, but these are neurons that can produce endogenous, means made by the body, chemicals that can cause pain relief. You could understand why that might occur in a circuit for aggression, right? Even if one is the aggressor, it's likely that they may incur some physical damage, and they'd want some pain relief.

• 16:09Andrew D. Huberman?

  The PAG also is connected to a number of neural circuits that eventually, through several processing stations, excuse me, arrive at things like the jaws. And in fact, stimulation of the ventricleal hypothalamus can evoke biting and aggressive biting behavior. Now, aggressive biting behavior is particularly interesting because in humans, and especially in human children, biting is something that while young children might do as a form of aggression, tends to disappear pretty early in childhood. And if it doesn't, it's often seen as a mark of pathology. I think there is general agreement in the psychology community, in the psychiatric community, that past a certain age, the using of one's teeth to impart aggression and damage

• 16:53Andrew D. Huberman?

  on others is a particularly primitive and troubling, or at least for the observer, the person that experiences, a pretty disturbing event. Dayu's lab has shown that activation of the ventricleal hypothalamus triggers a downstream circuit in the periaqueductal gray, which then triggers a whole other set of circuits of fixed action patterns. Here we are, back to Lorenz again, with fixed action patterns, including swinging of the limbs, right, punching. This wouldn't necessarily be controlled punching, but also biting behavior. So it's remarkable to me, at least, that we have circuits in our brain that can evoke violent use of things like our mouth, or violent use of things like our limbs, that

• 17:34Andrew D. Huberman?

  of course could be used for things like singing, or kissing, or eating, or, you know, gesticulating in any kind of polite or impolite way. The point here is that neural circuits, not individual brain areas, evoke the constellation of behaviors that we call aggression. I'd like to take a quick break and acknowledge our sponsor, Eight Sleep. Eight Sleep makes smart mattress covers with cooling, heating, and sleep tracking capacity. One of the best ways to ensure you get a great night's sleep is to make sure that the temperature of your sleeping environment is correct. And that's because in order to fall asleep and stay deeply asleep, your body temperature actually has to drop by about one to three degrees. And in order to wake up feeling refreshed and energized, your body temperature actually has to increase by about one to three degrees.

• 18:19Andrew D. Huberman?

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• 19:02Andrew D. Huberman?

  and the UAE. Again, that's eightsleep.com/huberman to save up to $350. Now, many of you are probably puzzled, or at least should be, because I've been talking about this highly specialized brain area, the ventromedial hypothalamus, and this highly specialized subcategory of neurons in the ventromedial hypothalamus, these neurons that make estrogen receptor. And yet, the activation of those cells triggers dramatic and immediate aggression, both in males and in females, and both against males and against females. So what's going on here? Most of us think about estrogen, and we don't immediately think of aggression. Most of us hear testosterone, and we might think about aggression. To make

• 19:44Andrew D. Huberman?

  a long story short, and to dispel a still, unfortunately, very common myth, testosterone does not increase aggressiveness. Testosterone increases proactivity and the willingness to lean into effort in competitive scenarios. If people are given testosterone, or if you look at people who have different levels, excuse me, of testosterone endogenously that they naturally make, what you'll find is that testosterone tends to increase competitiveness, but not just in aggressive scenarios. So if somebody is already aggressive, giving them testosterone will have the tendency to make them more aggressive. If somebody,

• 20:26Andrew D. Huberman?

  however, is very benevolent and altruistic, giving them testosterone will make them more benevolent and altruistic, at least up to a point. Turns out there's evidence that in certain contexts, estrogen can make people more aggressive. So what's going on here? Well, what's going on is that testosterone can be converted into estrogen through a process called aromatization. There's an enzyme called aromatase. Anytime you have a word that ends in A S E, at least if it's in the context of biology, it's almost always, not always, but almost always, an enzyme. So the aromatase enzyme converts testosterone into estrogen, and it is actually testosterone aromatized, converted into estrogen, and then binding

• 21:10Andrew D. Huberman?

  to these estrogen-containing neurons in the ventromedial hypothalamus that triggers aggression. I want to repeat that. It is not testosterone itself that triggers aggression. It is testosterone aromatized into estrogen within the brain and binding to these estrogen-receptor-containing neurons in the ventromedial hypothalamus that evokes aggression, and dramatic aggression at that. Now, this effect of estrogen-causing aggression in the brain is very robust, so much so that if you take a mouse that lacks the aromatase enzyme, or a human that lacks the aromatase enzyme, and they do exist, then there is a reduction in overall aggression despite high levels of testosterone. It doesn't matter how much you increase testosterone,

• 21:55Andrew D. Huberman?

  or any of its other derivatives, you do not observe this aggression. This runs counter to everything that we know and think about the role of testosterone. So the next time somebody says testosterone makes people aggressive, you can say, "Ah, no, actually, it's estrogen that makes people aggressive, and animals aggressive for that matter." Now, of course, it is the case that because males have relatively less estrogen circulating in their brain and body than females, right, because they have testes, not ovaries, that testosterone is required in the first place in order to be converted into estrogen to activate this aggressive circuit involving these estrogen-receptor-containing neurons in the ventromedial hypothalamus. So we've established that it's not testosterone, but

• 22:40Andrew D. Huberman?

  testosterone converted into estrogen that activates these circuits for aggression. Nonetheless, it's still surprising, right? I mean, most of us don't think about estrogen as the hormone that stimulates aggression. It turns out it's all contextual. There are beautiful data showing that whether or not estrogen stimulates aggression can be powerfully modulated by whether or not days are short or days are long. In other words, whether or not there's a lot of sunshine or not. Day length... Is converted into hormonal signals and chemical signals and the primary hormonal and chemical signals involve melatonin and dopamine and also the stress hormones. So to make a very long story short, in the long days where we get a lot of sunlight,

• 23:22Andrew D. Huberman?

  both in our eyes and on our skin, melatonin levels are reduced. Melatonin is a hormone that tends to produce states of sleepiness and quiescence. It also tends to activate pathways that tend to reduce things like breeding and sexual behavior. In long days, dopamine is increased. Dopamine is a molecule associated with feelings of well-being and motivation and the desire to seek out all sorts of things. And in long days, provided we're getting enough sunlight on our skin and to our eyes, the stress hormones, especially cortisol and some of the other stress hormones are reduced in levels. If estrogen levels are increased experimentally, under long day conditions, it does not evoke aggression.

• 24:06Andrew D. Huberman?

  However, in short days, if estrogen is increased, there's a heightened predisposition for aggression. And that makes perfect sense if you think about what short days do to the biology of your brain and body. The melatonin signal goes up. There's more melatonin circulating for more of each 24-hour cycle. Stress hormones are circulating more. Why? Short days tend to be associated with winter. In winter, we are bombarded with more bacteria and viruses because bacteria and viruses actually survive better in cold than they do in heat. So shorter days are conducive to aggression, not because days are short per se, but because stress hormone levels are higher and because dopamine levels are lower.

• 24:50Andrew D. Huberman?

  Now here's where all of this starts to converge on a very clear biological picture, a very clear psychological picture, and indeed, a very clear set of tools that we can think about and use. Under conditions where cortisol is high, where the stress hormone is elevated, and under conditions where the neuromodulator serotonin is reduced, there is a greater propensity for estrogen to trigger aggression. For males who make a lot of testosterone relative to estrogen, You have to swap in your mind this idea that if testosterone is high, that means that estrogen is low, because if testosterone is high, there is going to be some aromatization, that conversion of testosterone to estrogen. So anytime you hear that testosterone is high, you should think testosterone is high

• 25:34Andrew D. Huberman?

  in the body and perhaps estrogen is low in the body, but that means that there's going to be heightened levels of estrogen in the brain and therefore increased propensity for aggression. In females who generally make less testosterone relative to estrogen, there is sufficient estrogen already present to trigger aggression. So both males and females are primed for aggression, but that's riding on a context. And that context of whether or not you get a tendency for aggression or not, depends on whether or not cortisol is high or low. And I'm telling you that if cortisol is relatively higher in any individual, there's going to be a tilt, an increase in that hydraulic pressure that Lorenz talked about toward aggression.

• 26:18Andrew D. Huberman?

  And if serotonin, the neuromodulator that is associated with feelings of well-being and sometimes even of slight passivity, but certainly of well-being, if serotonin is low, there's also going to be a further shift towards an aggressive tendency. So if we return to Lorenz's hydraulic pressure model of aggression and other internal states, we realize that external stimuli, things that we hear, things that we see, for instance, someone saying something upsetting or seeing somebody do something that we don't like to others or to us, as well as our internal state, our subjective feelings of well-being,

• 27:00Andrew D. Huberman?

  but also our stress level, our feelings of whether or not we have enough resources and are content with what we have. All of that is converging on this thing that we call internal state and creating this pressure of either to be more aggressive or less aggressive. Now we have some major players feeding into that final pathway, that question of whether or not, will we hit the other person? Will we say the thing that is considered aggressive? Will we not say it? Again, there are many things funneling into that question and dictating whether or not the answer is absolutely, I'll fight back, or I'm going to attack them even unprovoked. We really can boil them down to just a few common elements. And I'm telling you that those elements are whether or not cortisol levels are relatively lower.

• 27:41Andrew D. Huberman?

  Or relatively higher. Again, relatively higher is going to tend to make people more reactive. Why? Because reactivity is really a function of the autonomic nervous system, which is sort of like a seesaw that oscillates between the so-called sympathetic arm of the autonomic nervous system, which tends to put us into a state of readiness through the release of adrenaline, cortisol and adrenaline, when they're circulating in the brain and body, make us more likely to move and to react and to speak. It's actually what will induce a kind of low level tremor, which is an anticipatory tremor, to be able to move more quickly, right? A body in motion is more easily set into further motion, that is. And in terms of keeping cortisol in a range that's healthy and doesn't bias someone toward

• 28:23Andrew D. Huberman?

  high levels of aggression and irritability, that's again going to be set by a number of larger modulators or contextual cues. And I've talked about some of those on the podcast, but I'll just briefly recap them now. Obviously, getting sunlight in your eyes early in the day and as much sunlight as you safely can in your eyes throughout the day is going to be important. Again, because of this effect of estrogen in long days not increasing aggression. However, in shorter days, estrogen increases aggression because of the increase in cortisol observed in short days. Another way to reduce cortisol was discussed in our episode on heat and the use of sauna and heat, but also hot baths. It turns out that hot baths and sauna can be very beneficial for reducing cortisol.

• 29:08Andrew D. Huberman?

  All the details on that are included in the episode on heat and its timestamp. So you can go directly to that if you want to learn about the temperatures and the various durations, but to just give a synopsis of that, a 20 minute sauna at anywhere from 80 to 100 degrees Celsius is going to be beneficial for reducing cortisol. If you don't have access to a sauna, you could do a hot bath. And of course, some of you may be interested in exploring the supplementation route. And for reductions in cortisol, really the chief player there is ashwagandha, which is

• 29:40Andrew D. Huberman?

  Known to decrease cortisol fairly potently, I should just warn you that if you're going to use ashwagandha in order to reduce cortisol, first of all, check with your doctor or healthcare provider before adding or subtracting anything from your supplementation or health regimen. Of course, I don't just say that to protect us, I say that to protect you. You are responsible for your health, what you take and what you don't take. Chronic supplementation with ashwagandha can have some not so great effects of disruption of other hormone pathways and neurotransmitter pathways. So, the limit seems to be about two weeks of regular use before you'd want to take a break of about two weeks. So, ashwagandha, again, a very potent inhibitor of cortisol, but with some other effects as well. Don't use it chronically for longer than two weeks. But if your goal is to reduce cortisol, let's say you're going through

• 30:24Andrew D. Huberman?

  a period of increased irritability and aggressive tendency, maybe you're also not getting as much light as you would like, and perhaps also if there are other circumstantial things leading you towards more aggressiveness and your goal is to reduce aggressiveness, that can be potentially helpful. As many of you know, I've been taking AG1 for nearly 15 years now. I discovered it way back in 2012, long before I had a podcast, and I've been taking it every day since. AG1 is, to my knowledge, the highest quality and most comprehensive of the foundational nutritional supplements on the market. It combines vitamins, minerals, prebiotics, probiotics, and adaptogens into a single scoop that's easy to drink and tastes great. It's designed to support things like gut health, immune health, and overall energy, and it does so by helping

• 31:09Andrew D. Huberman?

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• 31:53Andrew D. Huberman?

  to get a week's supply of AGZ and a bottle of D3K2 with your subscription. In light of all this stuff about cortisol and estrogen and day length, I should mention that there are, in fact, some people who have a genetic predisposition to be more irritable and aggressive. There is a genetic variant present in certain people that adjusts their estrogen receptor sensitivity. And that estrogen receptor sensitivity can result in increased levels of aggression, sometimes dramatic increases. However, and also very interestingly, photo period, meaning day length, is a strong modulator

• 32:37Andrew D. Huberman?

  of whether or not that aggressiveness turns up or not. Whether or not that person with the particular gene variant is more aggressive or not depends on how long the day is and how long the night is. One particular study that I like that references this is Trainer et al. The title of the study is "Photo Period Reverses the Effects of Estrogens on Male Aggression Via Genomic and Non-Genomic Pathways." This was a paper published in the Proceedings of the National Academy of Sciences. It really points to the fact that rarely is it the case that just one gene will cause somebody to be hyper-aggressive. Almost always, there is going to be an interplay between genetics and environment, and as environment changes, such as day length changes and the length of night changes, so too will

• 33:21Andrew D. Huberman?

  the tendency for people with a given genetic variant to be more aggressive or not. Now, of course, in the absence of detailed genetic testing for this particular estrogen receptor variant, most people, I'm guessing you, are probably not walking around knowing that you have this gene or not. Regardless, I think it's important to pay attention to how you feel at different times of year, depending on whether or not it's summer, whether or not it's winter, whether or not you're getting sufficient sunlight, meaning viewing sufficient sunlight or not, whether or not you're getting sufficient sunlight exposure to your skin or not, whether or not you're indoors all the time. Generally, those things correlate with season, but not always. You can go through long bouts of hard work in the summer months when days are long, but

• 34:04Andrew D. Huberman?

  you're indoors a lot and getting a lot of fluorescent light exposure late in the evening, and perhaps that's when you're feeling more aggressive. So we have to be careful about drawing a one-to-one relationship between any biological feature and certainly psychological or behavioral feature like aggressiveness. But it's, I believe, helpful to know that these genetic biases exist. How they play out, again, they shift our biology in a general thematic direction. They don't change one thing. They change a variety of things that bias us toward or away from certain psychological and behavioral outcomes. And the various things that we can do in order to offset them. We described those earlier in terms of trying to keep cortisol low by getting sufficient sunlight, regardless of time of year and regardless of whether or not you happen to have this

• 34:47Andrew D. Huberman?

  particular genetic variant. I want to share with you a study that's focused on kids, but that has important ramifications for adults as well. There are many kids out there that suffer from so-called attention deficit hyperactivity disorder, ADHD. There are also many adults, we are finding, that are suffering from ADHD. In any event, the study I'm about to share with you explored how a particular pattern of supplementation in kids with ADHD was able to reduce aggressive episodes and impulsivity and increase self-regulation. And the title of the study is efficacy of carnitine in the treatment of children with attention deficit hyperactivity disorder. Even though they put carnitine in the title,

• 35:27Andrew D. Huberman?

  that what they focused on was whether or not acetyl L-carnitine supplementation could somehow adjust the behavioral tendency of these kids with ADHD. And to make a long story short, indeed, it did. There was a very significant effect of acetyl carnitine supplementation on improving some of the symptomology of ADHD. This was a randomized, double-blind, placebo-controlled, double crossover study. They showed significant reductions in their so-called total problem score. The total problem score is a well-established measure of behavioral problems in kids with ADHD and, I should say, adults with ADHD. Reductions in attentional problems overall, reductions in delinquency, and, most important for sake of today's discussion, significant reductions

• 36:11Andrew D. Huberman?

  in aggressive behavior. They were able to confirm the shifts in L-carnitine within the bloodstream of these kids. That is, they were able to correlate the physiology with the psychological changes. So studies such as this, I think, are useful because they point to the fact that

• 36:28Andrew D. Huberman?

  Very seldom, if ever, will there be one supplement, or one nutritional change, or even one behavioral change that's going to completely shift an individual from being aggressive and impulsive, but rather that by combining different behavioral regimens, by paying attention to things like time of year, and work conditions, and school conditions, and overall levels of stress, and likely, therefore, levels of cortisol, et cetera, that you can use behaviors, diet, and supplementation as a way to shift that overall internal milieu from one of providing a lot of internal hydraulic pressure, as it's been called throughout the episode, toward aggressive impulsivity, and relax some of that hydraulic pressure, and reduce aggressive tendencies.

• 37:13Andrew D. Huberman?

  Thank you for joining me for our discussion about the biology, psychology, and actionable tools around aggression. And as always, thank you for your interest in science.

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