Researchers studying 1,300 mostly low-income children looked at demographic characteristics, household environment, parenting quality, and cortisol levels when the children were 7-24 months old and executive functions when the children were 3. They found that 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, and that higher levels of cortisol were associated with lower levels of executive function abilities.