Represented by a conscious propensity to harm others against their will, aggressiveness is a complex behavior depending on which environmental conditions we have been living in, and the kind of features we have inherited from our ancestors. Humans tend to be an aggressive species.

Among mammals, members of the same species cause only 0.3 percent of deaths of their conspecifics (a member of the same species) [1]. Astonishingly, in Homo sapiens, the rate is nearly 7 times higher, around 2% (1 in 50)!

More than 1.3 million people worldwide die each year because of violence in all of its forms (self-directed, interpersonal and collective), accounting for 2.5% of global mortality. There are two critical conditions that endorse aggressive behavior: being fiercely territorial and living in social groups.

From the evolutionary perspective, aggression is usually described as adaptive. Struggle for resources like habitat, mates and food have had a key role in forming aggressive behavior in humans. Genetic variants that promote aggression have been more likely to be passed on to the next generation because they have increased the chances of survival. Indeed, among tribes of extremely violent hunter-gatherers, men who committed acts of homicide had more children, as they were more likely to survive and have more offspring [2]. This lethal legacy may be the reason we are here today.

Although there are several biological aspects related to aggression, their predictive value continues to be rather low. It is possible to inherit a predisposition to acting violently, but scientists also emphasize that modeling violence in the home environment is the most certain way of propagating aggressive behavior. Children learn to act violently through the simple observation of aggressive models. The way parents manage the inevitable conflicts that arise between themselves and their children is central to the learning of aggression. When parents are unable to stop the child from escalating the intensity of conflict, and when they at least intermittently reinforce the child’s coercive behavior, the child learns that escalation is a viable method of resolving conflict. When this conflict strategy is applied to interactions with siblings or peers, and if it is also reinforced in these contexts, this conflict escalation is likely to include acts of aggression [3].

In addition to being hereditary and learned through social modeling, there is one other crucial component to aggressive behavior: self-control. In humans, the urge to react aggressively stems from the ancient parts located deep in the brain.

The structure capable of controlling those impulses is evolutionally much newer and located just behind the forehead – the frontal lobes. Unfortunately, this “top-down” conscious control of violent impulses is slower to act in contrast with the circuits of eruptive violence deep in the brain. People convicted of murder had been found to have reduced activity in the prefrontal cortex and increased activity in deeper regions [4]. Although there are plenty of examples of people with prefrontal cortex damage who do not commit violent acts, these findings clearly demonstrate that the damage to the prefrontal cortex impairs decision making and increases impulsive behavior.

Early physical aggression needs to be dealt with care. Long-term studies of physical aggression clearly indicate that most children, adolescents and even adults eventually learn to use alternatives to physical violence [5].

Aggression is part of the normal behavioral repertoire of most, if not all, species; however, when expressed in humans in the wrong context, aggression leads to social maladjustment and crime [6]. By identifying mechanisms that predispose people to the risk of being violent – even if the risk is small – we may eventually be able to tailor prevention programs to those who need them most.

This post is adapted from an earlier blog on MiND the Gap/

References

[1] Gómez, J. M., Verdú, M., González-Megías, A., Méndez, M. (2016). The phylogenetic roots of human lethal violence. Nature 538(7624), 233–237.

[2] Denson, T. F., Dobson-Stone, C., Ronay, R., von Hippel, W., Schira, M. M. (2014). A functional polymorphism of the MAOA gene is associated with neural responses to induced anger control. J Cogn Neurosci 26(7), 1418–1427.

[3] Hodges, E.V.E., Card, N.A., Isaacs, J. (2003). Learning of Aggression in the Home and the Peer Group. In: Heitmeyer, W., Hagan, J. (eds) International Handbook of Violence Research. Springer, Dordrecht.

[4] Raine, A., Buchsbaum, M., LaCasse, L. (1997). Brain abnormalities in murders indicated by positron emission tomography, Biol Psychiatry 42(6), 495–508.

[5] Lacourse, E., Boivin, M., Brendgen, M., Petitclerc, A., Girard, A., Vitaro, F., Paquin, S., Ouellet-Morin, I., Dionne, G., Tremblay, R. E. (2014). A longitudinal twin study of physical aggression during early childhood: Evidence for a developmentally dynamic genome. Psychol Med 44(12):2617–2627.

6] Asherson, P., Cormand, B. (2016). The genetics of aggression: Where are we now? Am J Med Genet B Neuropsychiatr Genet 171(5), 559–561.

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A hot topic these days, that one can hear more and more information about is the microbiota-gut-brain axis, the bidirectional interaction between the intestinal microbiota and the central nervous system nowadays, this has become a hot topic. We are becoming increasingly aware that gut microbiota play a significant role in modulating brain functions, behavior and brain development. Pre- and probiotics can influence the microbiota composition, so the question arises, can we have an impact on our mental health by controlling nutrition and using probiotics?

Burokas and colleagues aimed to investigate this possibility in their study (2017), where the goal was to test whether chronic prebiotic treatment in mice modifies behavior across domains relevant to anxiety, depression, cognition, stress response, and social behavior.

In the first part of the study, the researchers fed mice with prebiotics for 10 weeks. They were administered the prebiotics fructo-oligosaccharides (FOS), galacto-oligosaccharides (GOS), a combination of both, or water. FOS and GOS are soluble fibers that are associated with the stimulation of beneficial bacteria such as bifidobacterium and lactobacillus.

Behavioral testing started from the third week including

  • the open field test (anxiety – amount of exploratory behavior in a new place),
  • novel object test (memory and learning – exploration time of a novel object in a familiar context), and
  • forced swimming test (depression-like behavior – amount of activity in the cylinder filled water).

Meanwhile, plasma corticosterone, gut microbiota composition, and cecal short-chain fatty acids were measured. Taken together, the authors found that the prebiotic FOS+GOS treatment exhibited both antidepressant and anxiolytic (anti-anxiety) effects. However, there were no major effects observed on cognition, nociception (response to pain stimulus), and sociability; with the exception of blunted aggressive behavior and more prosocial approaches.

In the second part, FOS+GOS or water-treated mice were exposed to chronic psychosocial stress. Behavior, immune, and microbiota parameters were assessed. Under stress, the microbiota composition of water-treated mice changed (decreased concentration of bifidobacterium and lactobacillus), which effect was reversed by treatment with prebiotics.

Furthermore, it was found that three weeks of chronic social stress significantly reduced social interaction, and increased stress indicators (basal corticosterone levels and stress-induced hyperthermia), whereas prebiotic administration protected from these effects.

After stimulation with a T-cell activator lectin (concanavalin A), the stressed, water-treated mice group presented increased levels of inflammatory cytokines (interleukin 6, tumor necrosis factor alpha), whereas in animals with prebiotics had these at normal levels.

Overall, these results suggest a beneficial role of prebiotic treatment in mice for stress-related behaviors and supporting the theory that modifying the intestinal microbiota via prebiotics represents a promising potential for supplement therapy in psychiatric disorders.

Watch YouTube Video:
https://youtu.be/E479yto8pyk

REFERENCES
Burokas, A., Arboleya, S., Moloney, R. D., Peterson, V. L., Murphy, K., Clarke, G., Stanton, C., Dinan, T. G., & Cryan, J. F. (2017). Targeting the Microbiota-Gut-Brain Axis: Prebiotics Have Anxiolytic and Antidepressant-like Effects and Reverse the Impact of Chronic Stress in Mice. Biological Psychiatry, 82(7), 472–487. https://doi.org/10.1016/j.biopsych.2016.12.031

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