Study leader Clancy Blair of New York University and colleagues at Pennsylvania State University and the University of North Carolina at Chapel Hill studied almost 1,300 children ages 7 months to 24 months from mostly low-income homes. They examined the household environment for safety, noise levels and the quality of parenting, the researchers said.
They also examined one indicator of stress -- levels of the stress hormone cortisol -- and administered a battery of three tests related to executive functions when the children were age 3.
The study, published in the journal Child Development, found children in lower-income homes received less positive parenting and had higher levels of cortisol in their first two years than children in slightly better-off homes. Cortisol was higher in African-American children than in white children and higher levels of cortisol were associated with lower levels of executive function abilities.
"In sum, early stresses in the lives of children living in poverty affect how these children develop executive functions that are important for school readiness," Blair said in a statement.