Instead, almost everybody starts out with some measure of violent and aggressive tendencies that systematically decline as we get older. What we learn and/or grow out of, in greater or lesser degrees, is how to control violent and aggressive tendencies that come naturally. These tendencies are akin to the Christian notion of original sin, which can be overcome in life, despite the fact that everyone is born with it.
Until recently, most research on aggression has focused on adolescents and adults. A minority of longitudinal studies using large samples of elementary school-aged children has provided important information on the development of physical aggression. One significant and unexpected finding in these longitudinal studies was that the vast majority of children reduced the frequency of their physical aggression from the time they began school until the end of high school. The same phenomenon applied equally to both girls and boys, although girls systematically showed lower frequencies of physical aggression than did boys. This phenomenon was observed during the 1980s and 1990s in Canada, New Zealand, and the United States, where the homicide rate was rising.
This decline in the frequency of physical aggression with age was unexpected from a social learning of aggression perspective, since the children are exposed to more and more models of physical aggression as they grew older. Longitudinal studies have also shown that it is extremely unlikely that an adolescent who has not been highly physically aggressive in the past will suddenly manifest significant problems with physical aggression. . .
[T]he frequency of physical aggression use increases during the first 30 to 42 months after birth and then decreases steadily. Fewer girls than boys reached the highest frequency levels, and girls tended to reduce the frequency of their aggression earlier in life.
Further, longitudinal studies up to adolescence show that preschool is a sensitive period for learning to regulate physical aggression. Indeed, the minority of elementary school children (5% to 10%) who continue to show high levels of physical aggression remain at greatest risk of engaging in physically violent behaviour during adolescence.
Interestingly, while the frequency of physical aggression was found to decrease from the third or fourth year after birth, the frequency of indirect aggression (making disparaging remarks about another person behind his or her back) increases substantially from four to seven years of age, and girls tended to use this form of aggression more frequently than did boys.
The main risk factors for women to have children with serious physical aggression problems are the following: a low education level, a history of behaviour problems, first delivery at a young age, smoking during pregnancy, and low income. A study of a large sample of twins also points to genetic effects on individual differences in frequency of physical aggression at 19 months of age. . . .
In 1972, Donald Hebb, a father of modern psychology, noted that children did not need to learn how to have a temper tantrum. In his 1979 book on social development, Robert Cairns reminded human development students that the most aggressive animals were those that had been isolated from the time they were born. Indeed, infants appear to use physical aggression spontaneously to achieve their goals when angry. Following the pioneering work of Charles Darwin, Michael Lewis and his colleagues showed that angry reactions could be observed as early as two months after birth. Children also seem to resort spontaneously to play-fighting. Thus, rather than learning to use physical aggression from their environment, human children learn not to use physical aggression through various forms of interaction with their environment.
From: Tremblay RE. Development of physical aggression from early childhood to adulthood. Rev ed. In: Tremblay RE, Barr RG, Peters RDeV, Boivin M, eds. Encyclopedia on Early Childhood Development [online]. Montreal, Quebec: Centre of Excellence for Early Childhood Development; 2008:1-7. Available at: http://www.child-encyclopedia.com/documents/TremblayANGxp_rev.pdf. Accessed January 23, 2009.
The research on longitudinal patterns in physical violence and aggression mirror longitudinal studies of academic development.
The road that leads to academic failure, physical violence, and aggression by high school aged children and young adults is already well established in most cases by elementary school.
Dropout rates and disparities in academic achievement in high school are substantially in place by the time kids are taking third grade CSAPs. Violent and aggressive behavior that will lead to school discipline, and run ins with the law, for teens and young adults have also narrowed to a fairly well defined and small majority of kids at risk in elementary school. And, we know from other research that the distinct quality of lack of conscience associated with the emerging psychological classification of "psychopathy" also manifests in early childhood.
(In contrast, schizophrenia, bipolar disorder, substance abuse problems and a number of common severe mental health issues often manifest clinically for the first time only in adolescence and young adulthood.)
From a policy perspective, the research favors early identification of academically and behaviorally "at risk" kids and intensive intervention as early as possible, in problems which will be deeply problematic for society later in life if they are not successfully addressed. It also suggests that education focused on teaching kids to find alternatives to aggressive behavior and managing anger may be appropriate on a much wider basis with younger people, as opposed to the narrow rehabilitative setting where it is currently mandated through the criminal justice system.
While less definitive, it would appear to me that (1) this research tends to disfavor the notion that harm is done by stigmatizing individuals who will live up to negative expectations of them (since the research tends to indicate that failing to act in the interest of avoiding this kind of stigma doesn't help), and that (2) regulation of violent media is unlikely to have beneficial effects.
This research appears to strengthen some liberal and some conservative ideas about criminal justice.
On one hand, it supports the politically liberal tendency to favor leniency towards young offenders charged with violent crimes. A young offender who is a late bloomer has a much greater likelihood of growing out of violent and aggressive tendencies than an offender who remains that way at a greater age.
On the other hand, it supports that politically conservative notion that, by the time one is dealing with adult offenders, a significant portion of people who offend are "criminals" who are a high risk of reoffending because of who they are, rather than simply products of their momentary circumstances. The judge, lawyers and jurors in a courtroom probably wouldn't have resorted to violence or aggression had they been walking in the shoes of the defendant.