Many longitudinal studies have shown that anger during adolescence is normal, but that most adolescents express their anger in ways.

Many longitudinal studies have shown that INCREASED  anger during adolescence is normal, but that most adolescents express their anger in ACCEPTABLE ways.

The increase in aggressiveness couldn't be controlled by the teenager because it's influenced by our bodily hormones. But the way we express that anger could be controlled. On example is if we expressing it through arts or physical activities rather than vandalism / violence.

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