Aggression and the Stress Response

This past Friday I came across a fascinating article by Stephanie van Goozen et al(1). They looked at the neurobiology of childhood antisocial behavior and they concluded that one of the major factors is, my favorite subject, stress.

First, some background. The stress response is made up of two major parts. On the one hand, you have the autonomic nervous system (ANS), which conscripts two hormones, epinephrine and norephinephrine (also known as adrenaline and noradrenaline), to do the bulk of the work. Epinephrine and norepinephrine are released from the adrenals and run around the body, increasing your heart rate, dilating your pupils and making your palms all sweaty. They also do some more subtle things, like increasing your metabolism (you'll need that energy to fight or flee) and suppressing the immune system (why worry about catching cold when there's, let's say, a giant bear in front of you). The ANS is responsible for acute stress response.

The other half of the stress response is the hypothalamic-pituitary-adrenal (HPA) axis. The HPA axis is a very clever self-regulatory network, the main product of which is cortisol. Cortisol is another hormone, and it also comes from the adrenal medulla. Like epinephrine and norepinephrine, it increases metabolism and diverts energy from lower-priority body functions like digestion and the immune system. However, it's effects are longer-lasting and chronic stress can cause all sorts of problems: muscle wastage, hypoglycemia, compromised immune system.


In any case, van Goozen et al. do a pretty thorough lit review and find that there's a marked correlation between ANS activation/cortisol levels and aggressive behavior(2). The catch? It's an inverse correlation. Most people would think that increased stress helps aggression, not hinders it. Of course, ANS activation/cortisol levels don't equal stress. They reflect a complex jumble of influences - the intensity of the stressor, the organism's perception of the stressor (a Vietnam Vet would likely have a much stronger reaction than me to Apocalypse Now), a person's upbringing and the amount of stress in their childhood, and even a number of factors seemingly unrelated to stress (cortisol levels are affected by what you eat and drink and how much sleep you got the night before). Which only makes the fact that ANS/cortisol levels can predict aggression that much more fascinating.

Why would people with a small stress response be more aggressive? There are two main theories. The first is stimulation-seeking theory, which claims that having low ANS/cortisol levels is aversive. As a college student in the midst of her senior thesis, I would probably claim the opposite, but the point is, stimulation-seeking theory suggests that aggressive behavior is an attempt to create stressful situations and therefore provoke ANS activation and cortisol release. The other theory suggests that aggression is caused by something else entirely. What they're saying is basically that when you have so many aggressive encounters, your body gets used to it. You don't find it so stressful - your cortisol levels stay low and your ANS response is blunted.

It's basically a question of the chicken or the egg. What came first, the aggressive behavior or the abnormally small response to stress? Studies have shown that early childhood stress - everything from exposure to smoking in utero to violence and neglect growing up - causes a blunted stress response. One particularly interesting study(3) involved Romanian orphans during the Ceausescu regime. Ceausecu, a communist, demanded that families bear more children than they could raise, and as a result a full 2% of the population ended up in the care of the state. These children had strikingly low cortisol levels. Animal studies have replicated these findings - rats taken away from their mothers show blunted cortisol and ANS responses. Rat studies also show a direct link between cortisol levels and aggression: rats with their adrenal glands removed become very aggressive, but injections of cortisol prevent that aggression.

This still leaves the question of why some people respond to early life stress by lowering cortisol levels and becoming aggressive, and others do not. Some people respond with raised cortisol levels (these people tend to develop mood and anxiety disorders). Obviously, some of this is genetic - the adopted children of antisocial parents are significantly more likely to be aggressive than the adopted children of healthy parents(4). Van Goozen et al. raise a fascinating idea. They point out that women are much more likely to develop mood and anxiety disorders than men. Conversely, men are much more likely to be antisocial than women.

Is it biology or society that's responsible for this difference? It can be difficult to disentangle the two. The authors also suggest that different types of stressors could result in different stress response patterns later on in life. While it is unclear what effects emotional neglect and sexual abuse have on the stress response, physical abuse may constitute the sort of extreme stress associated with low cortisol levels and blunted ANS response. Boys are more likely than girls to be physically abused(5), which may explain the higher rates of antisocial behavior for males.

The van Goozen article provokes many questions. Hopefully many others will be as compelled as I am to try and answer them.


1. van Goozen, S., Fairchild, G., Snoek, H. & Harold, G. (2007). The Evidence for a Neurobiological Model of Childhood Antisocial Behavior, Psychological Bulletin, 133, 149-82.
2. The correlation is surprisingly strong. One meta-analysis, looking at 5,868 children, found that heart rate, a product of ANS activation, was linked to antisocial behavior. The correlation was -.44, which means that nearly 20% of aggression could be explained/predicted by heart rate!
3. Carlson, M. & Earls, F. (1997). Psychological and neuroendocrinological sequelae of early social deprivation in institutionalized children in Romania. Annals of the New York Academy of Sciences, 807, 419-428.
4. Cadoret, R.J., Yates, W.R., Troughton, E., Woodworth, G. & Stewart, M.A. (1995). Genetic-environmental interaction in the genesis of aggressivity and conduct disorders. Archives of General Psychiatry, 52, 916-924.
5. Executive Summary of the Third National Incidence Study of Abuse and Neglect: http://www.childwelfare.gov/pubs/statsinfo/nis3.cfm#national (Published: July 4th, 2006)