Behavior studies on hamsters have been done. When the hamster is about 25 days old, in the wild, they are booted out of the nest. For about two weeks they wander about looking for a nest that will take them in. If that hamster is put in a cage with an aggressive adult for an hour each day, it will grow up a bully and pick on animals smaller than itself. However, it will cower when around hamsters its own size.

These hamsters grow up to have lower levels of a chemical associated with aggression, in the hypothalamus, and they sprout more receptors in the hypothalamus for serotonin, the chemical that blocks vasopressin.

Scientists aren't exactly sure what to make of the chemical changes, but they are certain that at least for some hamsters the experience of intimidation by an adult during adolescence has permanent effects.

In adolescence, the brain appears to adapt to its environment and this appears to determine normal behavior. If the environment provokes risky behavior, then that behavior becomes normal to them.

One of the most frightening behaviors among teens is binge drinking. Studies have shown that alcohol exposure can have devastating effects on the development of the brain, and there is a fear the period of vulnerability could extend throughout childhood and adolescence.

Tests on the sensitivity of an adolescent brain to binge drinking was done by subjecting rats to large amounts of alcohol. After the rats sobered up, the researchers looked for brain damage and found more in the young rats than the adults. Brain damage was seen in the adolescent rats in the region of the brain associated with addiction.

The study of nicotine tells a similar story. The majority of smokers begin in their teen years, until recently no studies have been done how the adolescent brain responds to nicotine. When rats were injected with nicotine every day for over two weeks, all of the rats showed signs of addiction. Nicotine smoking in adBehavior olescence causes permanent behavioral problems as well, particularly for females. Even after two weeks without nicotine, female rats were not interested in moving about or raising their young. It may be because nicotine retards cell division in the hippocampus, the region of the brain that continues to grow into adulthood in females, but not in males.

It is thought the exposed rats become depressed because nicotine decreases the brains production of norepinephrin and dopamine, two chemicals that seem to be lower in depressed people. Smoking in early life increases a person's chance of suffering depression later in life. In this early stage of research, it appears that violence and drugs can alter a teenager's brain permanently.