Terrible twos who stay terrible

These findings have been replicated in multiple large studies by several researchers on several continents.

Tremblay learned that with adolescents,aggressive acts can be counted in incidents per month; with toddlers,“you count the number per hour”.

In most children,though,this is as bad as it gets. The rate of violence peaks at 24 months,declines steadily through adolescence and plunges in early adulthood. But as Tremblay and Daniel S Nagin,a criminologist at Carnegie Mellon University,found in a pivotal 1999 study,a troublesome few do not follow this pattern.

The study tracked behaviour in 1,037 mostly disadvantaged Quebec schoolboys from kindergarten through age 18. The boys fell into four distinct trajectories of physical aggression.

The most peaceable 20 per cent,showed little aggression at any age; two larger groups showed moderate and high rates of aggression as preschoolers. In these three groups violence fell through adolescence. A fourth group,about 5 per cent,peaked during toddlerhood and declined more slowly.

As they moved into late adolescence and young adulthood,their aggression grew ever more dangerous. At age 17 they were four times as physically aggressive as the moderate group and committed 14 times as many criminal infractions.

(These numbers are all for boys and young men; girls’ physical aggression declines at sharply lower levels.) In 2006,Tremblay and Nagin published a larger study tracking 1,000 Canadians between ages 2 and 11. The research echoed the 1999 study. A third of the children were peaceable throughout; about half used physical aggression as toddlers,but rarely as preadolescents; and about a sixth remained physically aggressive as 11-year-olds.

The results were surprising. At first glance,they seemed at odds with one of criminology’s oldest tenets — the age-crime curve,first graphed in 1831 by the Belgian statistician Adolphe Quetelet.

Mining French crime records,Quetelet found that arrest rates soared in the midteens before falling in the 20s. His famous curve was later replicated in studies of criminal records going back to the 16th century. By contrast,the Tremblay-Nagin findings suggested that violent behavior peaked much earlier than the teen years.

To Tremblay,the findings suggest cause for optimism: that humans more readily learn civility than they do cruelty.

We start as toddlers. We learn through conditioning,as we heed requests not to hit others but to use our words. We learn self-control. Beginning in our third year,we learn social strategies like bargaining and charm.

But what of the relative few who remain physically aggressive? If it’s possible to spot this cohort as early as kindergarten,why can’t we bend their arc? Here,says Tremblay,“the entire field is stumped”