Americans are rude. I say this not to preach, which is neither my right nor my intention, but as a scientist, a developmental neuroscientist. My concern about American rudeness relates to my scientific research and knowledge about the development of the human brain. My conclusion comes from a recent trip to Japan, and from a reminder of times past, the death of actress Barbara Billingsley, who died Oct. 16, 2010.

Billingsley portrayed June Cleaver, the sympathetic and iconic, nurturing mother on the popular 1950s sitcom "Leave It to Beaver." Remember her signature line? "Ward, I'm worried about the Beaver." She confided her concern earnestly to her husband whenever their young son seemed the slightest bit distressed. The latest scientific research backs up with detailed molecular and cellular mechanisms what June Cleaver (and we) always knew intuitively, that through adolescence, the human brain is molded by the social environment in which a child is reared. A disrespectful, stressful social environment is a neurotoxin for the brain and psyche, and the scars are permanent.

One can debate how accurately television entertainment reflects reality, but there is no doubt that it represents the ideals of the time. Commercial art and entertainment always reflect and reinforce a society's values, as the public buy it (literally) because they value it. There is no doubt that American society has changed dramatically with respect to manners and social discourse in a generation. The "Leave It to Beaver" model of American polite society in the 1950s and early 1960s is gone. Those black-and-white sitcoms have been supplanted today by garish reality television programs that showcase domestic and social interactions driven by narcissism, factionalism, competition and selfishness.

The contrast between the brash, comparatively disrespectful behavior of Americans today and the courtesy, formal manners, civil discourse, polite behavior and respect for others regardless of social status that is evident in Japanese society is striking. The contrast hits an American like a splash of cold water upon disembarking the airplane in Japan, because it clashes so starkly with our behavior. For an American, Japanese manners and courtesy must be experienced.

American children today are raised in an environment that is far more hostile than the environment that nurtured today's adults. Children today are exposed to behaviors, profane language, hostilities and stress from which we adults, raised a generation ago, were carefully shielded. When I was a boy, there were no metal detectors at the entrance to my school. The idea was inconceivable, and there was indeed no need for them. Not so today. I wonder: how does this different environment affect brain development?

First it is helpful to consider, from a biological perspective, what "rudeness" is, so that we can consider what is lost when formal polite behaviors are cast away. People (and animals) living together in large numbers must develop strict formalized behaviors governing interactions between all individuals in the group, or there will be strife and chaos. In the natural world, as in the civilized world, it is stressful for individuals (people or animals) to interact with strangers, and also with other members of a working group and family members. As the size of the group increases, so do the number of interactions between individuals, thus raising the level of stress if not controlled by formal, stereotyped behavior, which in human society is called "manners." The formal "Yes, Sir, Yes, Ma'am," is not a showy embellishment in the military; strict respect and formal polite discourse are the hub of the wheel in any effective and cohesive social structure. True, many chafe under a system of behavior that is overly rigid, as do many young Japanese, but my point is that these polite and formalized behaviors reduce stress in a stressful situation that arises from being an individual in a complex society. Stress is a neurotoxin, especially during development of a child's brain.

Studies have shown that children exposed to serious psychological trauma during childhood are at risk of suffering increased psychiatric disorders, including depression, anger, hostility, drug abuse, suicidal ideation, loneliness and even psychosis as adults. Using modern brain imaging, the physical damage to these children's brain development can be seen as clearly as a bone fracture on an X-ray. Early-childhood sexual abuse, physical abuse and witnessing domestic violence undermine the normal wiring of brain circuits, especially those circuits connecting the left and right sides of the brain through a massive bundle of connections called the corpus callosum. Impairment in integrating information between right and left hemispheres is associated with increased risk of craving, drug abuse and dependence, and a weakened ability to make moral judgments. (See my post "Of Two Minds on Morality" for new research on the corpus callosum and the ability to make moral judgments.)

A series of studies by a group of psychiatrists and brain imaging scientists lead by Martin Teicher, of Harvard Medical School, shows that even hostile words in the form of verbal abuse can cause these brain changes and enduring psychiatric risks for young adults. In a study published in 2006, the researchers showed that parental verbal abuse was more strongly associated with these detrimental effects on brain development than was parental physical abuse. In a new study published in the July issue of the American Journal of Psychiatry, they report that exposure to verbal abuse from peers is associated with elevated psychiatric symptoms and corpus callosum abnormalities. The main causes are stress hormones, changes in inhibitory neurotransmitters, and environmental experience affecting the formation of myelin electrical insulation on nerve fibers. The most sensitive period for verbal abuse from peers in impairing brain development was exposure during the middle school years. Why? Because this is the period of life when these connections are developing in the human brain, and wiring of the human brain is greatly influenced by environmental experience.

Unlike the brains of most animals, which are cast at birth, the human brain develops largely after we are born. The brain of a human infant is so feeble that human babies are helpless. Human infants cannot walk, visual perception is rudimentary, and cognitive abilities, likes and dislikes, talents and skills, and the ability to communicate by speech or through reading and writing do not develop fully until the completion of adolescence. Our brains are the product of the environment in which we are nurtured through the first two decades of life. Whether you are Mormon or Muslim or speak Spanish or French depends primarily on where you were born and raised. Our experience during childhood and adolescence determines the wiring of our brain so powerfully that even processing of sensory information is determined by our childhood environment. Whether or not we can hear eight notes in a musical scale or 12, or whether we find symmetry in art beautiful or boring, or whether we can hear the difference in sound of the English letter "R" vs. "L", depends entirely upon whether our brains wired up during childhood in Western culture or Asian culture. The neural circuitry underlying those sensory perceptions is directed by what we experienced in early life, and these circuits cannot be rewired easily in the adult brain.