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Humans Most Violent When Only 2 Years Old




An excellent article by Erin Anderssen and Anne McIlroy in the Canadian Globe And Mail summarizes research on child development and human violence. They report that Richard Tremblay has found that 2 year old babies are more physically aggressive than teenagers or adults but fortunately too uncoordinated to do much damage to others.

Are human beings born pure, as Rousseau argued, and tainted by the world around them? Or do babies arrive bad, as St. Augustine wrote, and learn, for their own good, how to behave in society?

Richard Tremblay, an affable researcher at the University of Montreal who is considered one of the world leaders in aggression studies, sides with St. Augustine, whom he is fond of quoting.

Dr. Tremblay has thousands of research subjects, many studied over decades, to back him up: Aggressive behaviour, except in the rarest circumstances, is not acquired from life experience. It is a remnant of our evolutionary struggle to survive, a force we learn, with time and careful teaching, to master. And as if by some ideal plan, human beings are at their worst when they are at their weakest.

St. Augustine was obviously much closer to the truth.

What Dr. Tremblay and his colleagues around the world have now demonstrated is that the ability to feel rage exists the moment human beings take their first breaths. A four-month-old infant can show anger. And as they gain more control over their arms and legs, their mothers report increasing incidents of kicking and biting: They can also act in anger.

By the second year, aggressive behaviour peaks in temper tantrums, with slapping and pushing; according to Dr. Tremblay's work, a typical two-year-old, playing with others over the course of an hour, will commit one act of physical aggression for every four social interactions.

With teenagers, he says, researchers talk in terms of years or months or weeks between aggressive acts -- never hours -- though the incidents, obviously, are more severe. By their third birthdays, children have the motor skills to perform any of the acts of aggression an adult can. But at just that age, aggression begins to drop.

For almost everyone, it continues to drop for the rest of their lives. By Dr. Tremblay's calculation, only in about 5 per cent of men does the rate of aggression remain relatively stable into early adulthood. They are the most dangerous group to society.

The article tries to put what I consider to be an excessive environmental spin on the reason for the decline in physical aggression as children age. The fact that babies simultaneously become more physically coordinated and less violent at the same time strikes me as too much of a coincidence to be the result of teaching and discipline. Likely there is a genetically programmed stage of mental development that builds inhibiting neural circuits to control the physical outbursts.

The article vaguely refers to a study on the genetic and environmental factors that cause children to grow up to be antisocial. That is probably a reference to a New Zealand twins study which showed that a combination of a genetic variant for low level of expression of Mono-Amine Oxidase A (MAOA) and childhood abuse produces much higher rates of adolescent and adult criminal violence. However, contrary to Anderssen and McIlroy the study did not show that both the environmental and genetic factors studied had to be present to result in violence. It is just that those two factors made children far more prone to grow up to be violent. Some children who were not abused still grew up to be violent. Similarly, some children who expressed MAOA at a high level still grew up to be violent as well. There may be still more as yet unknown genetic variations, nutritional factors, toxins, social environmental factors, and other factors which contribute to higher probability of violent behavior.

The article also refers to Adrian Raine's work using positron emission tomography (PET) scans that showed differences in the glucose consumption rates in the brains of murderers.

For the Biological Psychiatry study, Dr. Raine directed scientists at USC and the University of California at Irvine as they used positron emission tomography (PET) to scan the brains of 41 murderers who had pleaded not guilty by reason of insanity. The scientists also scanned the brains of 41 control subjects matched for known mental disorders and for age and gender. Mental disorders among the subjects included schizophrenia, organic brain damage and a history of head injury.

PET scans measure the uptake of blood sugar (glucose) in various brain areas during the performance of simple, repetitive tasks. (Glucose is the basic fuel that powers most cell functions. The amount used is directly related to the amount of cell activity.)

On average, the murderers showed significantly lower rates of glucose uptake in three areas of the brain -- the prefrontal cortex, the corpus callosum and the posterior parietal cortex. Their rates were 4, 18 and 4 percentage points lower, respectively, than the rates measured in control subjects performing the same tasks.

When the researchers compared the brain's two hemispheres for glucose uptake rates, they found that murderers consistently showed weaker activity in the amygdala and the hippocampus of the brain's left -- or more rational -- hemisphere. These glucose uptake rates were each 4 percentage points lower than the rates measured in control subjects performing the same tasks.

But the murderers showed stronger activity in the thalamus, the amygdala, and the hippocampus of the right -- or more emotional -- hemisphere. These glucose uptake rates were 6, 6 and 3 percentage points higher, respectively, than the rates measured in control subjects performing the same tasks.

Raine has also shown that that psychopaths have distinct differences in the shapes of some parts of their brains and that violent criminals have less brain gray matter.

These discoveries add up to suggest that there are limits to how much violent tendencies can be reduced using environmental changes and different methods of teaching and disciplining children. It seems unlikely that methods of teaching and socialization can fully compensate for less grey matter in those who commit violent acts as adults or the larger corpus callosums and asymmetrical hypothalamuses found in psychopaths.

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