PHILADELPHIA – The scenes are too common for comfort: A mother grabs her daughter's arm roughly on the bus. A father at a convenience store growls coarsely into his son's ear.

Not legally defined as child abuse, it's referred to as harsh or authoritarian parenting. Mothers and fathers everywhere are capable of it.

But parents who struggle with stresses from overwhelming issues such as hunger, or lack of a job or adequate housing seem to engage in harsh parenting more often, researchers have concluded.

And children in poverty suffer in ways science is just beginning to understand.

Harsh parenting unleashes so-called toxic stress in children, researchers say, changing the structure and functionality of their brains, heightening chances for negative behavior, and potentially making them more prone to heart disease, among other maladies.

Think of harsh parenting as an agent as destructive as lead poisoning, said Daniel Taylor, a pediatrician at St. Christopher's Hospital for Children in North Philadelphia.

Such parenting, often involving "quick 'do-as-I-say' orders from Mom or Dad without the buffering effect of a loving, supportive attitude," causes the release in children of stress hormones such as cortisol that are toxic to developing brains, Taylor said.

One possible consequence is damage to a child's amygdala, the part of the brain that regulates emotion. The child can become hyperactive, get into fights, have attention deficits and cannot be calm, Taylor said.

Toxic stress also damages the hippocampus, a part of the brain that affects memory, he said, so such children may have trouble remembering things, which impairs reading ability and test performance.

Taylor and others extrapolate that poor neighborhoods likely hold countless families suffering from compromised brain development, generation after generation.

"If a child's developing brain was being damaged by high lead levels, landlords would be sued, houses repaired," Taylor said. "If a child's brain was being damaged by mercury in the water, the system would be changed.

"Who is going to pay, who is responsible for ensuring our children are not affected by the toxin of child poverty in America?

"We all are, and we'll pay the price of neglecting to build strong children."

Some parents do a good job of protecting their children's brains by being engaged and attentive, said Maria McColgan, medical director of the Child Protection Program at St. Christopher's.

Conversely, middle-class people are quite capable of delivering toxic stress to their children, a 1998 California study showed.

More recently, a Philadelphia study prepared in September for the Institute for Safe Families by the Public Health Management Corp. found that more than 33 percent of Philadelphia adults experienced emotional abuse during childhood. High poverty correlated with high levels of abuse.

For good mental health, parents and children need to engage in "serve and return" mode, meaning that a child says something or makes a sound and a parent always responds, akin to serving and returning a tennis ball. The dynamic supports development of language and emotions, McEwen said.

But, he added, if a parent stops responding, "it's a form of emotional abuse."

Despite the pain of toxic stress, there is some good news: "You can heal from it," Witherspoon said.