Why are children and teenagers capable of great ...

Hey Uri,
Some ideas:

• With children, I think the issue is they don't fully understand the consequences, and therefore lack in empathy. In other words, if one can comprehend the full weight and impact of one's actions on another, one is bound to feel empathic to the other – understand what they feel. I think that children cannot grasp that, and therefore are prone to be careless with the pain they cause others.
• With Teenagers, I would guess it has something to do with the whole "self-definition" issue, both in the sense of deviating from the accepted grownups norm and in the sense of conforming to your peers.
• Another explanation I can think of, which is valid for both children and teenagers, is that both groups are essentially powerless. Sure, many parents feel that their kids are bossing them around and getting what they want, but on the larger scheme of things, under a certain age you have no power to decide what your life will look like, because other people, namely, grownups, decide it all for you, on almost every level. Perhaps being cruel to other creatures, with even less power, is means for empowerment.

hey, there is a theory that springs to mind, which concerns social identity theory. Children and teenagers are at an age where identity is been established, and belonging to a group is extremely important to the individuals self esteem. They often do things for approval or to discriminate people that do not belong in their social circle, seemingly uncaring at how cruel this can be.

To illustrate this there is a study conducted by Tajfel where he divided boys aged 14 and 15 into two groups, and asked them to preform a task where they were asked to give money as rewards to both their own group and another  group. All showed signs of outer group discrimination. Heres a link to a pretty good website which describes the study in depth:

It would take a textbook on psychology to properly answer your question, but here goes:

1) Nice guys don't always win the evolutionary race.

Neanderthals were apparently nice, intelligent folks who stayed home and hunted or cultivated food from their local area and made toys and stuff in their spare time for their families to enjoy. They were supposedly peaceful intelligent folks who kept to themselves.

They were wiped out by mean pack creatures who would rather get together in gangs and bully and steal. We evolved from the meanies. Eventually they had to get partly civilized to prevent extinction via civil war but we still have much of the bully in our genes.

 Game theory and simian behavior can be used to predict what situations are likely to promote cooperation or bullying. E.g. If an environment requires cooperation to secure life's essentials (especially safety from competing groups) loyal cooperative groups will form. If no major external menace exists persecution of people different from the norm will increase or civil war will erupt. This is especially so if resources are seen to be scarce, but even in high prosperity the greedy may still seek more than what is needed.

2) Violence is learned.

The prevailing culture being presented to our youth is one glorifying violence. E.g. Gangster Rap, War heroes

Permissive parenting and general permissiveness of society do not encourage moral courage or caring for others.

Political and legal system is based on idea of winners and losers, where any gains for others must mean you loose out. This promotes competition rather than cooperative policies. E.g. feeding the poor might have a 1% drop in profits for the rich; or decreasing the war budget might loose contracts which your home town is getting jobs from.

If the USA used all its resources for a coordinated plan to improve the standard of living for all, it would not be expensive to reduce poverty related crime, or provide basic health and employment for all citizens.

Sadly however the common belief may be true; that such cooperative methods go against human nature. Competitive greed is the preferred basis of modern capitalism.

Even if we do not need to have a society based on greed, hate and fear, it is unlikely that the momentum of global commercialism and the corruption evil in the global society is likely to be easily turned around.

 3) Needs

E.g. Maslows "Needs hierarchy".

Youths (and people in general) firstly need basic survival security, then they look for social belonging and identity, and later look for self expression and finally helping others and intellectual and spiritual pondering. 

If theses and other needs of youth are not provided by civilized society they will be met by gangs or popular culture such as media violence.