Why does Hunger lead to the aggressive behavior?

Why does Hunger lead to the aggressive behavior?

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I have observed that frequently when people are hungry; they tend to get angry more easily on pointless issues. Does this mean that our fight or flight response is more active when a person is hungry? What is a possible reason for this? Is this phenomenon linked with our cell signaling pathways? If it is, then what would be the pathway that leads to the aggressive behavior?

To summarize the question:

When a person is hungry and they get angry, is it due to a cell signaling pathway? If so, what pathway?

Brain's main energy source is glucose. It uses about 20% of total glucose [1]. Brain hypoglycemia causes depressive-like behaviors in mice through adrenergic pathways [2].

When it comes to humans, here is a study that claims low glucose leads to increased aggression in married couples (see this too):

Self-control requires energy, part of which is provided by glucose. For 21 days, glucose levels were measured in 107 married couples. To measure aggressive impulses, each evening participants stuck between 0 and 51 pins into a voodoo doll that represented their spouse, depending how angry they were with their spouse… As expected, the lower the level of glucose in the blood, the greater number of pins participants stuck into the voodoo doll, and the higher intensity and longer duration of noise participants set for their spouse [3].

However, the conclusion is disputed:

Bushman et al.'s study does not demonstrate that fluctuations in blood glucose affect individuals' self-control abilities. As an important consequence, there is no reason to assume that giving couples a sugary “boost to their self-control energy” (p. 3) will reduce intimate partner violence. Because the glucose model of self-control lacks empirical foundation, it does not qualify as a framework for scientifically based intervention strategies [4].

What is sure, is that hypoglycemia activates sympathetic nervous system:

… the neurogenic symptoms of hypoglycemia are largely the result of sympathetic neural, rather than adrenomedullary, activation [5].

Hypoglycemia increases plasma levels of both epinephrine and norepinephrine. These catechols are released primarily from the adrenal medulla. However, it is well documented that hypoglycemic increases muscle sympathetic nerve activity, and that both alpha and beta adrenergic activity increase [6].

And this leads to behavioral changes (at least in animals):

Noradrenaline is involved in many different functions, which all are known to affect behaviour profoundly… Part of these effects may arise in indirect ways that are by no means specific to aggressive behaviour, however, they are functionally relevant to it. Other effects may affect brain mechanisms specifically involved in aggression. Hormonal catecholamines (adrenaline and noradrenaline) appear to be involved in metabolic preparations for the prospective fight; the sympathetic system ensures appropriate cardiovascular reaction, while the CNS noradrenergic system prepares the animal for the prospective fight… It appears that neurons bearing postsynaptic alpha2-adrenoceptors are responsible for the start and maintenance of aggression, while a situation-dependent fine-tuning is realised through neurons equipped with beta-adrenoceptors [7].


  1. Wikipedia contributors, "Human brain," Wikipedia, The Free Encyclopedia, (accessed July 6, 2014).
  2. Park MJ, Yoo SW, Choe BS, Dantzer R, Freund GG. Acute hypoglycemia causes depressive-like behaviors in mice. Metab. Clin. Exp. 2012 Feb;61(2):229-36. doi: 10.1016/j.metabol.2011.06.013. PubMed PMID: 21820138.
  3. Bushman BJ, Dewall CN, Pond RS, Hanus MD. Low glucose relates to greater aggression in married couples. Proc. Natl. Acad. Sci. U.S.A. 2014 Apr 29;111(17):6254-7. doi: 10.1073/pnas.1400619111. PubMed PMID: 24733932.
  4. Lange F and Kurzban R (2014) Sugar levels relate to aggression in couples without supporting the glucose model of self-control. Front. Psychol. 5:572. doi: 10.3389/fpsyg.2014.00572
  5. DeRosa MA, Cryer PE. Hypoglycemia and the sympathoadrenal system: neurogenic symptoms are largely the result of sympathetic neural, rather than adrenomedullary, activation. Am. J. Physiol. Endocrinol. Metab. 2004 Jul;287(1):E32-41. doi: 10.1152/ajpendo.00539.2003. PubMed PMID: 14970007.
  6. Hoffman RP. Sympathetic mechanisms of hypoglycemic counterregulation. Curr Diabetes Rev. 2007 Aug;3(3):185-93. PubMed PMID: 18220670.
  7. Haller J, Makara GB, Kruk MR. Catecholaminergic involvement in the control of aggression: hormones, the peripheral sympathetic, and central noradrenergic systems. Neurosci Biobehav Rev. 1998;22(1):85-97. PubMed PMID: 9491941.

This is a short review about the issue and not complete. This and the earlier answer are not proofs of the link between the two proceses.

To investigate this better, I think it would be much easier first to narrow the case to

  • testosterone and energy homeostasis (Embryology; or better formulated research case)
  • catabolism and anger
  • anger and aggressive behaviour.

Then, think about specific cases. Think genes which are associated with aggressive behaviour and violence in catabolism (criminal studies).


  • Satiety
  • Appetite
  • Hunger

I was not sure in which specific area you are particularly interested. At this stage, we cannot provide a proof between hunger and aggressive behaviour. It depends so much on the individual (life style; genomics) what is the end result. Here follows little general pieces of information:

Some cases

  • InsR/FoxO1 Signaling Curtails Hypothalamic POMC Neuron Number [1]: it's possible that the hormonal and nutrient milieu contributes to alterations in POMC neuron development.
  • Early-Life Exposure to Testosterone Programs the Hypothalamic Melanocortin System [3].
  • Study about testosterone and basal metabolic rate [2].


  • hypothalamus lateral nucleus
  • hypothalamus tuberal medial perifornical nucleus [4]
  • hypothalamus arcuate nucleus (appetite and energy expenditure - POMC-CART; upregulative NPY AGRP)


  • androgenic (NR3C4) (adipose tissue; more in visceral) (no link)

where I found no receptor existing directly between those two events - appetite and testosterone secretion by comparing the NCBI gene databases between those two processes. It is much more easier to show the thing first in the embryologic studies and then by using the gene found in bigger studies. At the moment, more work is needed in these studies in Embryology.

My initial clause is based on some of my notes in Embryology. There are researches who are trying to show this link between studies in embryology. The problem is at the moment in the development of hypothalamus and something else. I will update this post when I remember about the situation better and when finding the right things from my notes.


  4. John E. Hall. Guyton and Hall Textbook of Medical Physiology. 12Th edition.

Is Being ‘Hangry’ Really a Thing ― or Just an Excuse?

It’s one thing to crave a big meal or a bottomless bag of your favorite snack. It’s another to be so famished that you are irritable and overreact to minor annoyances. That’s the difference between being hungry and being “hangry,” a clever combo of “hungry” and “angry.”

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Whether or not you’ve experienced it, you probably know someone who has. But is it really a physiological phenomenon — or just a grown-up version of crying for your bottle?

The biology of being hangry

“There is a physiological reason why some people get angry when they’re hungry,” says gastroenterologist Christine Lee, MD.

“When you haven’t eaten for a while, the level of sugar (glucose) in your blood decreases,” she explains. When your blood sugar gets too low, it triggers a cascade of hormones, including cortisol (a stress hormone) and adrenaline (the fight-or-flight hormone). These hormones are released into your bloodstream to raise and rebalance your blood sugar.

So why am I so hangry?

“The release of cortisol can cause aggression in some people,” says Dr. Lee. “Also, low blood sugar may interfere with higher brain functions, such as those that help us control impulses and regulate our primitive drives and behavior.”

So, there truly is a medical explanation for being hangry. It’s a biochemical reaction due to low blood sugar — not the same thing as being crabby when you’re tired, sick or otherwise feeling out of sorts.

Other consequences of getting too hungry

Why do some people get hangry and others just hungry?

“People who struggle with controlling their anger or who have impulse-control issues may be more susceptible to becoming hangry,” says Dr. Lee. “However, it is unclear if there is an association between having regular hanger and having a personality trait disorder.”

“Hunger comes with various negative consequences, not just anger,” she says. If hunger doesn’t make you angry, it might cause one of these reactions instead:

  • Fatigue.
  • Sleepiness.
  • Difficulty concentrating.
  • Poor coordination.
  • Susceptibility to making mistakes.

When hanger is a problem

“Becoming so hungry that you get hangry isn’t necessarily a health concern,” says Dr. Lee. “If you’re otherwise healthy, an occasional bout of extreme hunger isn’t a problem.”

However, people who have other health concerns should take steps to prevent hanger. That includes those who are on multiple medications, those who have medical conditions, and those who are underweight or malnourished.

“People who have metabolic stressors, such as diabetes, pancreatic or liver disorders, and adrenal insufficiency syndromes, are particularly at risk for complications or adverse effects of low blood sugar due to inadequate counter-regulatory response,” says Dr. Lee.

If you are prone to getting hangry, take these steps to control or prevent it:

  • Eat several small meals throughout the day, or make sure breakfast, lunch and dinner are fulfilling and nutritious.
  • Avoid junk foods, which can cause another sugar crash — after they first incite a sugar rush. Nutrient-rich, high-fiber foods are best and keep you feeling fuller longer.
  • Have healthy snacks on hand — a few handy snacks inside your purse, car or desk can offer peace of mind if you’re worried about hanger rearing its ugly head while you’re away from home.
  • Exercise regularly.
  • Get plenty of sleep.
  • Stay hydrated.

Your body will thank you. And your family and friends might too.

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Hunger can lead to anger, but it's more complicated than a drop in blood sugar, study says

Credit: CC0 Public Domain

What makes someone go from simply being hungry to full-on "hangry"? More than just a simple drop in blood sugar, this combination of hunger and anger may be a complicated emotional response involving an interplay of biology, personality and environmental cues, according to research published by the American Psychological Association.

"We all know that hunger can sometimes affect our emotions and perceptions of the world around us, but it's only recently that the expression hangry, meaning bad-tempered or irritable because of hunger, was accepted by the Oxford Dictionary," said lead author Jennifer MacCormack, MA, a doctoral student in the department of psychology and neurocience at the University of North Carolina at Chapel Hill. "The purpose of our research is to better understand the psychological mechanisms of hunger-induced emotional states—in this case, how someone becomes hangry."

The research was published in the journal Emotion.

When someone is hungry, there are two key things that determine if that hunger will contribute to negative emotions or not, according to MacCormack: Context and self-awareness.

"You don't just become hungry and start lashing out at the universe," said assistant professor Kristen Lindquist, Ph.D., the study's co-author. "We've all felt hungry, recognized the unpleasantness as hunger, had a sandwich and felt better. We find that feeling hangry happens when you feel unpleasantness due to hunger but interpret those feelings as strong emotions about other people or the situation you're in."

The researchers first conducted two online experiments involving more than 400 individuals from the United States. Depending on the experiment, participants were shown an image designed to induce positive, neutral or negative feelings. They were then shown an ambiguous image, a Chinese pictograph, and asked to rate the pictograph on a seven-point scale from pleasant to unpleasant. Participants were also asked to report how hungry they felt.

The researchers found that the hungrier participants were more likely to rate ambiguous Chinese pictographs as negative, but only after first being primed with a negative image. There was no effect for neutral or positive images. "The idea here is that the negative images provided a context for people to interpret their hunger feelings as meaning the pictographs were unpleasant," said MacCormack. "So there seems to be something special about unpleasant situations that makes people draw on their hunger feelings more than, say, in pleasant or neutral situations."

It's not just environmental cues that can affect whether someone goes from hungry to hangry, according to MacCormack. People's level of emotional awareness also matters. People who are more aware that their hunger is manifesting as an emotion are less likely to become hangry.

In a laboratory experiment involving more than 200 university students, the researchers asked the participants either to fast or eat beforehand. After some of the students were asked to complete a writing exercise designed to direct their focus on their emotions, all participants were asked to participate in a scenario designed to evoke negative emotions. Students were asked to complete a tedious exercise on a computer that, unbeknownst to them, was programmed to crash just before it could be completed. One of the researchers then came into the room and blamed the student for the computer crash.

Participants were then asked to fill out questionnaires on their emotions and their perception of the quality of the experiment. The researchers found that hungry individuals reported greater unpleasant emotions like feeling stressed and hateful when they were not explicitly focused on their own emotions. These individuals also thought that the researcher conducting the experiment was more judgmental or harsh. Participants who spent time thinking about their emotions, even when hungry, did not report these shifts in emotions or social perceptions.

"A well-known commercial once said, 'You're not you when you're hungry,' but our data hint that by simply taking a step back from the present situation and recognizing how you're feeling, you can still be you even when hungry," MacCormack said.

This research emphasizes the mind-body connection, according to MacCormack. "Our bodies play a powerful role in shaping our moment-to-moment experiences, perceptions and behaviors—whether we are hungry versus full, tired versus rested or sick versus healthy," she said. "This means that it's important to take care of our bodies, to pay attention to those bodily signals and not discount them, because they matter not just for our long term mental health, but also for the day-to-day quality of our psychological experiences, social relationships and work performance."

Although this study focused on hunger, MacCormack believes these results may extend to other bodily states that induce negative emotion, such as fatigue or inflammation, but that further research needs to be done to confirm this.

Gender Differences in Aggression

Given what we know about the tendency toward self-enhancement and a desire for status, you will not be surprised to learn that there is a universal tendency for men to be more violent than women (Archer & Coyne, 2005 Crick & Nelson, 2002). In comparison to women and girls, who use more nonphysical and relational aggression such as shouting, insulting, spreading rumors, and excluding others from activities, men and boys prefer more physical and violent aggression—behaviors such as hitting, pushing, tripping, and kicking (Österman et al., 1998).

Strong gender differences in aggression have been found in virtually every culture that has been studied. Worldwide, about 99% of rapes are committed by men, as are about 90% of robberies, assaults, and murders (Graham & Wells, 2001). Among children, boys show higher rates of physical aggression than girls do (Loeber & Hay, 1997), and even infants differ, such that infant boys tend to show more anger and poorer emotional regulation in comparison to infant girls. These findings will probably not surprise you because aggression, as we have seen, is due in large part to desires to gain status in the eyes of others, and (on average) men are more concerned about this than are women.

Although these gender differences exist, they do not mean that men and women are completely different, or that women are never aggressive. Both men and women respond to insults and provocation with aggression. In fact, the differences between men and women are smaller after they have been frustrated, insulted, or threatened (Bettencourt & Miller, 1996). And men and women seem to use similar amounts of verbal aggression (Graham & Wells, 2001).

Gender differences in violent aggression are likely caused in part by hormones. Testosterone, which exists at higher levels in boys and men, plays a significant role in aggression, and this is in part responsible for these differences. And the observed gender differences in aggression are almost certainly due, in part, to evolutionary factors. During human evolution, women primarily stayed near the home, taking care of children and doing the cooking, whereas men engaged in more aggressive behaviors, such as defense, hunting, and fighting. Thus men probably learned to aggress, in part, because successfully fulfilling their duties required them to be aggressive. In addition, there is an evolutionary tendency for males to be more competitive with each other in order to gain status. Men who have high social status are more attractive to women, and having status allows them to attract the most desirable, attractive, and healthy mates (Buss & Shackelford, 1997).

But gender differences are not entirely determined by biology and evolution many of these differences are the result of social learning. Imagine for a moment that 10-year-old Jean comes home from school and tells her father that she got in a big fight at school. How do you think he would respond to her? Now, imagine that her twin brother, Jake, comes home and reports the same thing. I think you can imagine that the father’s response would be different in this case. Boys are more likely to be reinforced for being aggressive than are girls. Aggressive boys are often the most popular children in elementary schools (Rodkin, Farmer, Pearl, & Van Acker, 2000) because they can use their aggressiveness to gain and maintain social status. On the other hand, girls who successfully use nonphysical or relational aggression may also gain social benefits.

Eagly and her colleagues have proposed that gender differences in aggression stem primarily from social norms and expectations about the appropriate roles of men and women (Eagly, 1987 Eagly & Wood, 1991). Eagly notes that in many nations, women are expected to have more highly developed other-oriented attributes, such as friendliness and emotional expressivity, and that when women do aggress, they use aggression as a means of expressing anger and reducing stress. Men, on the other hand, are socialized to value more self-oriented attributes, such as independence and assertiveness, and they are more likely to use aggression to attain social or material rewards (Campbell, Muncer, & Gorman, 1993). One meta-analysis found that participants were more likely to indicate that men, rather than women, would and should engage in the most aggressive behaviors (Eagly & Steffen, 1986).

Serotonin levels affect the brain's response to anger

Fluctuations of serotonin levels in the brain, which often occur when someone hasn't eaten or is stressed, affects brain regions that enable people to regulate anger, new research from the University of Cambridge has shown.

Although reduced serotonin levels have previously been implicated in aggression, this is the first study which has shown how this chemical helps regulate behaviour in the brain as well as why some individuals may be more prone to aggression. The research findings were published September 15, in the journal Biological Psychiatry.

For the study, healthy volunteers' serotonin levels were altered by manipulating their diet. On the serotonin depletion day, they were given a mixture of amino acids that lacked tryptophan, the building block for serotonin. On the placebo day, they were given the same mixture but with a normal amount of tryptophan.

The researchers then scanned the volunteers' brains using functional magnetic resonance imaging (fMRI) as they viewed faces with angry, sad, and neutral expressions. Using the fMRI, they were able to measure how different brain regions reacted and communicated with one another when the volunteers viewed angry faces, as opposed to sad or neutral faces.

The research revealed that low brain serotonin made communications between specific brain regions of the emotional limbic system of the brain (a structure called the amygdala) and the frontal lobes weaker compared to those present under normal levels of serotonin. The findings suggest that when serotonin levels are low, it may be more difficult for the prefrontal cortex to control emotional responses to anger that are generated within the amygdala.

Using a personality questionnaire, they also determined which individuals have a natural tendency to behave aggressively. In these individuals, the communications between the amygdala and the prefrontal cortex was even weaker following serotonin depletion. 'Weak' communications means that it is more difficult for the prefrontal cortex to control the feelings of anger that are generated within the amygdala when the levels of serotonin are low. As a result, those individuals who might be predisposed to aggression were the most sensitive to changes in serotonin depletion.

Dr Molly Crockett, co-first author who worked on the research while a PhD student at Cambridge's Behavioural and Clinical Neuroscience Institute (and currently based at the University of Zurich) said: "We've known for decades that serotonin plays a key role in aggression, but it's only very recently that we've had the technology to look into the brain and examine just how serotonin helps us regulate our emotional impulses. By combining a long tradition in behavioral research with new technology, we were finally able to uncover a mechanism for how serotonin might influence aggression."

Dr Luca Passamonti, co-first author who worked on the research while a visiting scientist at the Cognition and Brain Sciences Unit of the Medical Research Council in Cambridge (and currently based at the Consiglio Nazionale delle Ricerche (CNR), Unità di Ricerca Neuroimmagini, Catanzaro), said: "Although these results came from healthy volunteers, they are also relevant for a broad range of psychiatric disorders in which violence is a common problem. For example, these results may help to explain the brain mechanisms of a psychiatric disorder known as intermittent explosive disorder (IED). Individuals with IED typically show intense, extreme and uncontrollable outbursts of violence which may be triggered by cues of provocation such as a facial expression of anger.

"We are hopeful that our research will lead to improved diagnostics as well as better treatments for this and other conditions."


Almost 40 people witnessed the murder of Kitty Genovese, but it took almost 20 minutes for someone to call to police. Most psychologists feel that the bystander effect came into play. Because there were many witnesses, each felt that someone else would act. Additionally, due to the number of witnesses, a diffusion of responsibility occurred. Thus, each witness felt less responsible for acting to save Genovese.

Juniper, a college student, is walking to her next class, which begins in five minutes. She knows that the professor is very irritated by lateness and may ask her to leave the classroom if she is late. While she is rushing to the class, she notices a man lying on the sidewalk. The man is clutching his throat and turning blue. When she notices this, Juniper's palms begin to sweat and she feels agitated. Juniper notices that there are many other students in the area who have noticed the man.

Which of the following factors is likely to discourage Juniper from performing an altruistic act?

Some ant colonies consist of a queen, who performs all reproductive duties for the colony, and worker ants, who perform all the labor that the colony needs to gather food, expand and defend territory, and survive. The worker ants are also able to—but do not—contribute reproductively to the colony. In a colony like this, the worker ants will give their lives to defend the queen.

Why is the worker ant considered altruistic?

Please select the best answer from the choices provided

Please select the best answer from the choices provided

Juniper, a college student, is walking to her next class, which begins in five minutes. She knows that the professor is very irritated by lateness and may ask her to leave the classroom if she is late. While she is rushing to the class, she notices a man lying on the sidewalk. The man is clutching his throat and turning blue. When she notices this, Juniper's palms begin to sweat and she feels agitated. Juniper notices that there are many other students in the area who have noticed the man.

Which of the following factors is likely to encourage Juniper to perform an altruistic act?

An adult vervent monkey sees a jaguar approaching. He cries out loudly, warning all the other vervent monkeys in the area of danger.

Why is this behavior considered altruistic?

Please select the best answer from the choices provided

Please select the best answer from the choices provided

Please select the best answer from the choices provided

Please select the best answer from the choices provided

Kenneth has just received his annual job performance review. Due to his hard work, Kenneth's salary was raised. Later in the day, Kenneth sees someone he doesn't know changing his car tire on the side of the road and pulls over to help.

Which of the following factors is most likely responsible for Kenneth's altruistic behavior?

Please select the best answer from the choices provided

Please select the best answer from the choices provided

Some ant colonies consist of a queen, who performs all reproductive duties for the colony, and worker ants, who perform all the labor that the colony needs to gather food, expand and defend territory, and survive. The worker ants are also able to—but do not—contribute reproductively to the colony. In a colony like this, the worker ants will give their lives to defend the queen.

What is the most likely explanation for the altruistic behavior of the worker ants?

Please select the best answer from the choices provided

Paul watches a lot of violent television. When he goes to school, he is usually anxious because he believes that the other students may attack him. His constant agitation prevents him from concentrating in class.

Which of the following effects is most likely responsible for Paul's behavior?

Please select the best answer from the choices provided

Please select the best answer from the choices provided

Please select the best answer from the choices provided

Please select the best answer from the choices provided

Please select the best answer from the choices provided

Please select the best answer from the choices provided

Kathy is a big fan of horror movies. Although she was initially too afraid to watch them alone, she soon found that she could not get enough of them. Kathy began to watch television shows that were more violent as well.

Which of the following effects is most likely responsible for Kathy's behavior?

Please select the best answer from the choices provided

Please select the best answer from the choices provided

Please select the best answer from the choices provided

Please select the best answer from the choices provided

Please select the best answer from the choices provided

Please select the best answer from the choices provided

Frank is living in a third floor apartment. He has four neighbors on the third floor:

Judy, who lives next door, Henry, who lives two doors down, Polly, who lives three doors down, and Julian, who lives four doors down.

In accordance with the effects of proximity, which of the following is the most likely outcome of this living arrangement?

The Socioeconomic Distribution of Hunger

The claim that people of lower SEP in developed countries might be hungry initially meets with a credibility problem: underweight is very rare in such populations whilst rates of obesity are high, so it seems hard to argue that food is short. Indeed, in developed countries, lower SEP predicts increased probability of obesity, at least for women (Sobal and Stunkard, 1989 McLaren, 2007). However, fat reserves are built up when caloric intake exceeds metabolic requirements averaged over extended periods of time. There is thus no contradiction between overall over-nutrition and having many brief instances of hunger. Indeed, one explanation for overall over-consumption of calories by the poor is as a response to their experience of irregularity in the food supply (Dietz, 1995 Townsend et al., 2001 Nettle et al., 2017).

One way it is possible to put on weight whilst experiencing frequent hunger is by eating less satiating but higher-calorie meals. As we move from higher to lower SEP, diets are composed of progressively less whole grains, vegetables, fruit, and lean meat, and a greater proportion of fats and particularly refined sugars (Drewnowski and Specter, 2004). The consumption of sugar-sweetened beverages, a substantial source of calories in contemporary populations, is strongly socially patterned (Han and Powell, 2013). To a considerable extent, the shift from low- to high-energy density of foods with lower SEP is driven by cost: refined sugars provide many more calories per dollar than fruit or vegetables (Drewnowski and Specter, 2004). However, although energy-dense foods fulfill caloric requirements at low financial cost, they are less satiating than those higher in protein or fiber and lower in sugars (Bornet et al., 2007): that is, hunger returns sooner after eating them.

In addition to lower-SEP meals being less satiating, they may be less regular: studies have found that young people’s omission of breakfast (Hoyland et al., 2012) and of family evening meals (Neumark-Sztainer et al., 2003) is more common in low-SEP groups. It is therefore a reasonable contention that people of lower SEP in developed countries tend to be more often hungry that those of higher SEP, even in the absence of lower total calorie intake or body masses. To demonstrate this unequivocally, one would need to use methods such as experience sampling (Csikszentmihalyi and Larson, 1987) that pinpoint subjective hunger states in time over the course of the participants’ regular lives. Such evidence appears to be lacking at present. However, there is abundant survey evidence based on more global self-reports, to which we now turn.

The prevalence and importance of hunger within populations that are affluent overall began to be appreciated in the United States in the 1980s and 1990s (Kendall et al., 1995 Kleinman et al., 1998). Realizing that a burden of hunger in such populations, if one existed, would not be detectable in body masses, researchers developed two key self-report constructs relating to hunger: food insufficiency (Kleinman et al., 1998), and food insecurity (Kendall et al., 1995 Gundersen et al., 2011).

The concept of food insufficiency was developed particularly in the context of children, and essentially attempts to estimate the incidence of hunger within the life of the child and his or her family. It is assessed using eight questions to parents that specifically probe the occurrence of temporary food shortfall due to resource constraints (i.e., shortfalls due to religious abstinence or other causes are excluded). An example question is 𠇍o your children ever say they are hungry because there is not enough food in the house?”. The responses are most often used to form a discrete classification of children as ‘hungry,’ 𠆊t risk from hunger’ and ‘not hungry.’ The terms here do not refer to instantaneous states, but the incidence of hunger states over time they might perhaps be better understood as 𠆏requently hungry due to constrained resources,’ ‘occasionally hungry due to constrained resources,’ and ‘never hungry due to constrained resources.’ The social distribution of food insufficiency was extensively studied in the Community Child Hunger Identification Project (CCHIP), a linked series of 18 community studies in different US cities (see Kleinman et al., 1998). The CCHIP showed that food insufficiency was surprisingly prevalent: 8% of children under 12 were classified as ‘hungry’ with another 21% classified as 𠆊t risk from hunger.’ However, the social distribution was very uneven: in the lowest-income families, the proportion classified as ‘hungry’ rose to 21%, and 𠆊t risk from hunger’ to 50%. Thus, most children from low-income families in the US were classified as either hungry or at risk from hunger.

Food insecurity is defined as the state where the ability to acquire adequate and safe food is limited or uncertain (Kendall et al., 1995). It is routinely assessed in US social and nutritional surveys (Gundersen et al., 2011), and increasingly measured in Latin America and to a lesser extent in other regions too (Nettle et al., 2017). Although food insecurity is not exactly synonymous with hunger, high food insecurity does imply frequent hunger. Indeed, many of the questions in the standard 18-item USDA food insecurity questionnaire (reproduced in Gundersen et al., 2011) in fact address the experience of hunger: for example, “In the last 12 months, did you or other adults in the household ever cut the size of your meals or skip meals because there wasn’t enough money for food?” and “In the last 12 months, were you ever hungry, but didn’t eat, because you couldn’t afford enough food?”. Again, the responses are used to categorize respondents and their households. Two key categories for our purposes are (the slightly confusingly named) 𠇏ood insecurity,” which means more than one affirmative response to a food insecurity question, and “very low food security,” which means more than 6 affirmative responses (8 for households with children), and necessarily entails reporting that some household members went hungry at least some times within the last year because of lack of resources. (All households categorized as very low food security by this typology are also food insecure).

The 2008� US prevalence of food insecurity was estimated at around 16%, with around 6% for very low food security (Gundersen et al., 2011). The rate is, however, strongly related to income: of households whose income is half the poverty line, around 40% are classified as food insecure and around 20% as very low food security. This compares to less than 6 and 2% for affluent households. The strong dependence of food insecurity on income is unsurprising, since the construct specifically probes the inability to secure food due to scarce resources. What is important for present purposes is that a large proportion of low-income households report experiencing food insecurity. The implication is that a substantial fraction of people from such households experience an excess of hunger due to their SEP, at least some of the time.

To summarize this section, the available evidence shows clearly that within very affluent populations, individuals of lower SEP eat less satiating diets do so on more irregular schedules and a very sizable proportion, at least in the USA, report experiences such as food insufficiency and food insecurity that imply an increased frequency of hunger. Thus, the claim that people of lower SEP are more likely to be hungry at any given time𠅊 necessary assumption of the hunger hypothesis𠅊ppears reasonable. However, the hunger hypothesis can be expressed in at least two subtly different versions, each of which makes slightly different predictions. These are the subject of the next section.

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“Hunger gates incoming food-related sensory information into physiological responses, psychological states, and behavioral propensities that evolved for the function of reuniting our bodies with nutrients.”

Or not. Either way, it’s fun to imagine that guy, probably wearing a flowing robe or something, hunched over his computer while he scrolls through Google Scholar with one hand and grips a massive, stinky durian with the other.

*Yes, as a matter of fact, I am aware that coffee, collards, and cocoa are bitter, along with many other things that are nice to eat and drink. There may be an adaptive reason why we’re attracted to bitterness in some cases—and it turns out those cases are the exceptions that prove the rule. But that’s a story for another day.

Hunger Affects Behavior and Changes Pathways in the Brain

Scientists at the Max Planck Institute have shown in a new study that hunger modifies behavior and changes pathways in the brain, revealing that hunger affects decision making and perception of risk in fruit flies.

Hungry people are often difficult to deal with. A good meal can affect more than our mood, it can also influence our willingness to take risks. This phenomenon is also apparent across a very diverse range of species in the animal kingdom. Experiments conducted on the fruit fly, Drosophila, by scientists at the Max Planck Institute of Neurobiology in Martinsried have shown that hunger not only modifies behavior, but also changes pathways in the brain.

Animal behavior is radically affected by the availability and amount of food. Studies prove that the willingness of many animals to take risks increases or declines depending on whether the animal is hungry or full. For example, a predator only hunts more dangerous prey when it is close to starvation. This behavior has also been documented in humans in recent years: one study showed that hungry subjects took significantly more financial risks than their sated colleagues.

Also the fruit fly, Drosophila, changes its behavior depending on its nutritional state. The animals usually perceive even low quantities of carbon dioxide to be a sign of danger and opt to take flight. However, rotting fruit and plants – the flies’ main sources of food – also release carbon dioxide. Neurobiologists in Martinsried have now discovered how the brain deals with this constant conflict in deciding between a hazardous substance and a potential food source taking advantage of the fly as a great genetic model organism for circuit neuroscience.

In various experiments, the scientists presented the flies with environments containing carbon dioxide or a mix of carbon dioxide and the smell of food. It emerged that hungry flies overcame their aversion to carbon dioxide significantly faster than fed flies – if there was a smell of food in the environment at the same time. Facing the prospect of food, hungry animals are therefore significantly more willing to take risks than sated flies. But how does the brain manage to decide between these options?

Avoiding carbon dioxide is an innate behavior and should therefore be generated outside the mushroom body in the fly’s brain: previously, the nerve cells in the mushroom body were linked only with learning and behavior patterns that are based on learned associations. However, when the scientists temporarily disabled these nerve cells, hungry flies no longer showed any reaction whatsoever to carbon dioxide. The behavior of fed flies, on the other hand, remained the same: they avoided the carbon dioxide.

In further studies, the researchers identified a projection neuron which transports the carbon dioxide information to the mushroom body. This nerve cell is crucial in triggering a flight response in hungry, but not in fed animals. “In fed flies, nerve cells outside the mushroom body are enough for flies to flee from the carbon dioxide. In hungry animals, however, the nerve cells are in the mushroom body and the projection neuron, which carries the carbon dioxide information there, is essential for the flight response. If mushroom body or projection neuron activity is blocked, only hungry flies are no longer concerned about the carbon dioxide,” explains Ilona Grunwald-Kadow, who headed the study.

The results show that the innate flight response to carbon dioxide in fruit flies is controlled by two parallel neural circuits, depending on how satiated the animals are. “If the fly is hungry, it will no longer rely on the ‘direct line’ but will use brain centers to gauge internal and external signals and reach a balanced decision,” explains Grunwald-Kadow. “It is fascinating to see the extent to which metabolic processes and hunger affect the processing systems in the brain,” she adds.

The nature of animal aggression

Aggression sometimes occurs when parents defend their young from attack by members of their own species. Female mice, for example, defend their pups against hostile neighbours, while male stickleback fish defend eggs and fry against cannibalistic attack. More frequently, however, animals fight over resources such as food and shelter—e.g., vultures fight over access to carcasses, and hermit crabs fight over empty shells. Another important resource over which fighting commonly occurs is potential mates. In this case the biology of gamete production has an influence on aggressive behaviour: because a female’s eggs are larger, are fewer in number, and require more energy to produce than a male’s sperm, competition among males over females is usually more frequent and intense than competition among females over males. As a result, the most spectacular fights among animals, whether they are crickets, salmon, tree frogs, chaffinches, or stags, occur between males over fertile females.

Aggression may be focused on a specific area, such as a defended territory from which rivals are vigorously excluded. A notable example is shown by mudskippers, intertidal fish that defend small territories where they browse on microscopic plants. The fish build mud walls around the borders of their territories, and at low tide water is retained within the walls (incidentally permitting the human observer to visualize the mosaic of territories in a colony of these fish). Territorial behaviour is also shown by rag worms and fiddler crabs when they defend their burrows, by male dragonflies and sticklebacks defending breeding grounds, by male tree frogs, sage grouse, and Uganda kob defending high-quality sites for courting and mating, and by spiders, reef fish, and hyenas when they defend feeding areas.

A common feature of aggression in most species is that fights tend to start with relatively harmless displays or postures. For example, aggressive interactions between two red deer stags begin with an exchange of deep roars followed by a display of “parallel walking,” in which the stags strut side by side assessing their relative size. The aggression may then escalate to direct attacks during which the stags charge at each other, stabbing and wrestling with their antlers. Most confrontations are resolved early while displaying, but many others continue to the point of intense and dangerous fighting.

Contrary to previous assumptions, injury and death during animal fights are not uncommon. In species where animals live in established groups, however, overt fighting is often replaced by a set of relationships in which a subordinate individual consistently defers to a dominant one. Wolf packs, for example, are known for their clear hierarchical relationships. When two group members meet, the dominant animal adopts an upright stance, with raised ears and tail, while the subordinate flattens its body to the ground with the ears against the head and the tail lowered, a submissive posture that serves to protect it from attack. In a number of bird species, variations in plumage act as “badges of status,” especially in large winter flocks. The black throat patch or bib of the house sparrow and the dark chest stripe of the great tit are signals of status dominant individuals have more-conspicuous bibs or stripes than do subordinates and thus have preferential access to food.


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