The findings suggest that in a given grade, students born at the end of the calendar year may be at a distinct disadvantage. Those perceived as having academic or behavioral problems may in fact be lagging simply as a result of being forced to compete with classmates almost a full year older than them. For a child as young as 5, a span of one year can account for 20 percent of the child’s age, potentially making him or her appear significantly less mature than older classmates.

The new study found that the lower the grade, the greater the disparity. For children in the fourth grade, the researchers found that those in the youngest third of their class had an 80 to 90 percent increased risk of scoring in the lowest decile on standardized tests. They were also 50 percent more likely than the oldest third of their classmates to be prescribed stimulants for A.D.H.D. The differences diminished somewhat over time, the researchers found, but continued at least through the seventh grade.

The new study, published in the journal Pediatrics, used data from Iceland, where health and academic measures are tracked nationally and stimulant prescription rates are high and on par with rates in the United States. Previous studies carried out there and in other countries have shown similar patterns, even among college students.

Rachel Klein of the Child Study Center at New York University Langone Medical Center and her colleagues studied the potentially long-term effects of ADHD among men who were diagnosed as kids. In their 33-year follow-up study, Klein and her team looked at 135 middle-aged men with childhood ADHD who were referred to the study by their teachers when they were between six to 12 years old.

The researchers compared this group to 136 men without ADHD and found that men with ADHD struggled more in occupational, educational, economic and social arenas later in life compared to men without the diagnosis.

At the 33-year follow-up, when the men were in their forties, those with childhood-diagnosed ADHD without conduct disorders had about 2.5 years fewer years of education compared to the other men; only 3.7% had higher degrees compared to nearly 30% of the control group.

The majority (84%) were holding jobs, but at significantly lower positions than peers without ADHD and were therefore at a financial disadvantage. On average, the researchers say, the ADHD group earned $40,000 less in salary than their unaffected counterparts.

Socially, men with ADHD also struggled with higher divorce rates, more antisocial personality disorders and substance abuse. On the positive side, however, they did not have higher rates of mood and anxiety disorders, like depression.

On the flip side, those children whose mothers consumed the most fish while pregnant were the least likely to exhibit fidgety, distracted, and impulsive behaviors in class, according to the study of 604 children published Monday in the Archives of Pediatrics and Adolescent Medicine.

At first blush, this finding appears contradictory because most of the mercury we consume comes from fish. “It seems a little paradoxical,” said study co-author Dr. Susan Korrick, an assistant professor of medicine at Brigham and Women’s. “But fish consumption doesn’t necessarily correlate with mercury levels since you could eat a high amount of fish that are low in mercury.”

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Fatty kinds of fish such as salmon, tuna, and sardines have higher levels of omega-3 fatty acids, which are thought to be crucial for cognitive function. All types of fish have a host of nutrients such as vitamin D, B-12, and iodine, which could play a role in brain development as well.

While government agencies have advised pregnant women to limit their fish intake to no more than two six-ounce servings a week, Korrick said they might want to aim for three or four servings of low-mercury fish such as salmon, canned light tuna, haddock, cod, and shrimp. (Albacore tuna has more mercury, so consumption should be limited to six ounces a week.)

The key is for pregnant women to avoid fish known to have high mercury levels, including swordfish, shark, tilefish, and king mackerel, Korrick added.

In the pediatric office today, parents often bring up spoiling, as that mother did last week, in reference to young babies, sleep and feeding. It’s as if the later, more confusing questions about how to respond to a child’s demands crystallize in those early months when the new baby cries and the parents worry.

The official pediatric line — I said some version of this to that mother last week — is that you can’t spoil babies by taking good care of them. But even that doesn’t turn out to be simple.

“It’s important to be there and to be responsive and responsible, but it also doesn’t mean that you have to be totally at the whim of the baby,” said Dr. Pamela High, a professor of pediatrics at Brown University and medical director of the Fussy Baby Clinic at the Brown Center for the Study of Children. “You’re teaching them patterns and routine and regularity.”

Parents can meet a baby’s needs while still allowing her a chance to learn to settle down and sleep without being held. In a randomized study on babies with colic that was published this year by Dr. High’s group, when parents got help with issues of feeding, sleep, routine and their own mental health, those colicky babies cried less and slept more.

As children get older, setting limits and establishing family routines and expectations gets more complicated. But it’s still a question of balancing immediate gratification and larger life lessons.

It’s also an area where we still feel comfortable and righteous blaming and judging other parents — and ourselves.

Problematic childhood behaviors once attributed to incompetent or destructive parenting are now understood to be hard-wired, set by genetics, reflecting neurological differences. We don’t blame bad parenting for autism now, or A.D.H.D. But “spoiled” evokes traits and behaviors for which we’re often quick to hold parents responsible.

As Roald Dahl put it in 1964 in “Charlie and the Chocolate Factory,” “A girl can’t spoil herself, you know.”

Overall, prescriptions for kids ages 0-17 dropped seven percent during that time period, while prescription drugs dispensed to adults rose 22 percent, it said.

“Children are experiencing fewer serious medical problems than perhaps they had in the past,” said Victor Fornari, director of child and adolescent psychiatry at North Shore-Long Island Jewish Health System in New York.

The report tracked the number of prescriptions dispensed for the youths, not the number of patients, and was based on two major US commercial prescription databases.

A key rise was seen in stimulant medications for ADHD, which the US Centers for Disease Control and Prevention describes as one of the most common neurobehavioral conditions of childhood, affecting about five million children.