We came across a threesome of studies this month that provide a deeper understanding of why some children achieve less than others.

First, the relationship of stress to learning. While it's easy to see that children who are poor live with a lot of stress, researchers at Cornell University discovered a disturbing link of stress with reduced memory capacity. Studying nearly 200 teenagers in New York state, they found that those teenagers who had spent their lives in poverty could hold only 8.5 items on average in their memories, compared to the average of 9.4 among middle class teens. They lay the blame on stress, including the ability of stress to suppress new brain cell growth and shrink areas of the brain associated with working memory.

Next, on the need for physical activity. University of Illinois researchers found that physical activity may help students pay attention, something that most of us figured out long ago -- except the many wrong-headed school officials who eliminate recess to make more room for instructional time. Their study of 20 nine-year-olds showed that a period of exercise before a task led to better performance and higher concentration on the task than did a period of rest before a similar activity. Their findings also indicate that the positive effects of physical activity wane quickly, meaning that teachers should look for ways to integrate movement with classroom lessons.

Last, of course we learn that poor children are less apt to get this beneficial physical activity. Medical researchers at the Albert Einstein School of Medicine studied the effect of recess on better classroom behavior and increased attentiveness, using a large data set of 15,000 8- and 9-year-old children. They found the poorer the children, the more likely they were on average to attend a school where there was little or no recess. Almost half of the children from poor families weren't getting recess, compared to only 18 percent of children whose parents earned over $200,000.