The scenes are too common for comfort: A mother grabs her daughter's arm roughly on the bus. A father at a Wawa growls coarsely into his son's ear.
Not legally defined as child abuse, it's known as harsh or authoritarian parenting. Regardless of race or income level, mothers and fathers everywhere are capable of it.
But low-income 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 from it 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 condemning a child to a life hampered by heart disease, among other maladies.
"This is an incredibly important public-health issue," said Joan Luby, professor of psychiatry at the Washington University School of Medicine in St. Louis. After studying 145 children over 12 years, she authored an article about the effect of poverty on children's brains in the journal JAMA Pediatrics in October.
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 becomes hyperactive, gets into fights, has 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.
If toxic hormones are released constantly, children will suffer elevated blood pressure and sugar levels, as well as accelerated heart rates, making a person with toxic stress just as likely to develop heart disease as someone with high cholesterol, said social worker Marcy Witherspoon, an expert in Philadelphia on child welfare and brain development.
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."
Dave suffered from toxic stress after a poor childhood in which, he said, "my over-disciplining, yelling, cursing mother ripped me apart."
Dave, 31, a security supervisor at a Philadelphia business, requested that his last name not be used because he feared losing his job.
A single father who lives in Frankford with his two children, ages 4 and 18 months, Dave was a participant in a parenting workshop run by Witherspoon.
"My mother would always tell me I'd never amount to anything," Dave said. "I was depressed."
Fearful that he'd pass along toxic stress like a malevolent legacy to his children, Dave said, "there are only two ways you come out of a situation like mine: You're either a victim and let the bad experiences control you, or you become better in spite of it."
Dave believes that, in part by writing poetry and by drawing, he exorcised demons and forestalled bequeathing toxic stress to his kids.
"I'm proud that even when I'm angry with them, I don't demean them," Dave said. "I don't say they're bad, only that they're doing something bad. I control myself."
Not all poor children suffer toxic stress.
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. showed that more than 33 percent of Philadelphia adults experienced emotional abuse during childhood. High poverty correlated with high levels of abuse.
Parents don't have to be harsh to do damage. A poor mother distracted by lack of food can become incapable of engaging her children, which can also cause toxic stress, said Bruce McEwen, a neuroscientist at Rockefeller University in New York.
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.
"As long as one adult in a child's life has his back, believes in him, creates hope for the future, that's a positive."
Parents are taught to stop and give their children attention for the things they do correctly, Witherspoon said.
They're made to recognize that they perpetuate toxic stress because they themselves were victims of it. That knowledge can stop the cycle.
Parents need advocacy and support, Witherspoon said. That way they stop taking their stresses out on their kids.
"To help," Witherspoon said, "I say to parents: 'Life sounds really hard. Tell me about it.' "