"I actually became pretty obsessed with social class, this major dimension of variation in the human race and certainly in American society," Farah said.
As sociological studies have corroborated, it seemed to Farah that child-rearing and children's early experience was very different depending on social class.
Poor children don't get as much exposure to language as their wealthier counterparts, research has found, and they tend to get more negative feedback. What they do hear is not as grammatically complex, with a narrower range of vocabulary. There is less understanding of how children develop and what they need for cognitive development, Farah said.
Stress is another huge factor in these disparities.
Parents of low socioeconomic status have uncertainty about having basic needs met, dangerous neighborhoods, crowding and other factors, causing stress for children and their parents. Stressed parents are less patient and affectionate, further stressing their children, according to Farah.
Farah wanted to investigate the huge differences she saw.
"We're so segregated by class, we don't even realize we're segregated because we don't even know what life is like just two miles north of here," she said.
How parenting matters
Farah and colleagues have conducted research suggesting that high-stress childhoods, which include less warm parenting, are correlated with changes in stress physiology and stress regulation.
One study, published in March in the journal PLOS One, involved African American adolescents who came from households of low socioeconomic status. At age 4, their parents' responsivity (warmth and supportiveness) was evaluated. Then, 11 to 14 years later, the same participants took a stress test: Giving a talk in front of an unfriendly audience.
Volunteers gave saliva samples so that researchers could analyze it for the stress hormone cortisol. Researchers found that cortisol reactivity was related to parental responsivity, and the less parental responsivity, the less of a normal stress response the volunteers had.
"You might say, 'Well, of course life is more stressful in lower socioeconomic strata,' " she said. "But the degree of magnitude of the stress that they live with is just unbelievable."
Such research points to the idea that stress leads to a stunting of brain development in children of low socioeconomic backgrounds. It is unknown whether that stunting can be reversed, but you shouldn't assume that it's unchangeable, Farah said.
In animal models, researchers have found that later enriching experiences can at least partially compensate for the effects of early life stress on the hippocampus -- a seahorse-shaped structure in the brain vital for memory and stress reactivity -- and other brain areas. It's not that the initial effects of the stress are reversed, though; it appears that different pathways are enabled to compensate.
"If you're interested in child policy and stuff, the important bottom line is: You never want to say, 'Oh, damaged goods, so there's nothing we can do now,' " she said.
Farah is quick to add that middle-class parents aren't perfect either. Eagerly watching children for every small advance in development, and showering children with praise, isn't necessarily helpful either.
"But I am also willing to make a value judgment: Smacking young children, saying a lot of negative things to them, not talking to them very much, is bad." She hits the table. "Let's just say it."
A study in progress
Across the Penn campus in the radiology department, Farah sits in a low chair while Brian Avants, assistant professor of radiology, explains their recent study, using a slide on a computer screen. Farah presented the study at the Society for Neuroscience meeting in November.
Researchers followed 53 children who came from low socioeconomic status from birth through adolescence. This is a relatively small number of participants, but it is typical for brain imaging studies.