Tolerance: overcoming fear to understand others. (Keep the cool in school: a scholastic campaign against violence and verbal abuse).

FOR 99 PERCENT of human history, we lived in hunter-gatherer clans of 40 to 50 members. Our major predators were other humans--people killing people. If we were out hunting and saw an unfamiliar face--with unfamiliar dress, language, and behavior--the person was as likely perceived to be a threat as a potential ally.

Our brain is the product of thousands of generations of evolution; it has evolved specialized systems for a hunter-gatherer world permeated, with threat. And since the ultimate job of the brain is to keep us alive, we have a neurobiological (physiological link between brain and body) tendency to view new things with a prejudice--a negative bias. Our brain categorizes "unfamiliar" people, information, ideas, etc., as potentially threatening until proven otherwise.

So when we encounter new people, new languages, and new cultures, our brain actually activates our fear response. And when fearful, we're less capable of mature, abstract reasoning. We become more reactive and categorical in our thinking.

What does this have to do with tolerance? And what does this tell us about our fear of differences? One main lesson: We need to learn to overcome the neurobiological tendency to be threatened by unfamiliar things we don't initially understand. Fortunately, the tendency of the brain for bias is not an absolute. We can learn to accept and understand different views, cultures, and values.

Once we learn that differences make other people stimulating and capable, it makes us more comfortable with our world. But if we're fearful of new things and the diversity of people, we will be left behind. The world is constantly changing--and the more capable we become of tolerating and, ultimately, enjoying diversity in the world, the richer our own world and lives become

BRAIN WORK: The ultimate "job" of the human brain is to keep you alive. To do this, it has dozens of neural systems across all brain regions--spanning from your brain stem to your cortex--involved in reading and responding to potential threats. New situations--good or bad--activate stress-regulating neural systems in your brain. If you're in a safe and familiar setting, the feelings are perceived as "excitement." If you're in an unfamiliar and potentially threatening situation, the same sensations will be perceived as frightening.

This is the fifth of six articles by Dr. Bruce Perry on "Core Strengths" that can help prevent teen violence. Dr. Perry is the Provincial Medical Director of Children's Mental Health in Alberta, Canada.