Illustration: Polly Becker

Since the early 1980s, when the presence of women in the workforce began to grow dramatically, parents and social scientists alike have questioned—and disputed—the effects of placing children in daycare at an early age. A 2009 U.S. study, for example, coauthored by Eric Dearing, an associate professor of developmental and educational psychology at the Lynch School of Education, found that “higher quality” daycare for low-income infants and children (ages six months to four and a half years) boosted performance in math and reading at least into the fifth grade. That same year, a social scientist at Brigham Young University surveyed the academic literature and described a darker outcome: It seemed that the more hours a child spent in daycare, the more likely it was that he or she would exhibit antisocial and aggressive behavior. “It’s a real conundrum,” says Dearing, “because it would be great for children to have improvements in language and cognitive skills, but not if they are going to have higher levels of external problems like aggression.”

The United States is a difficult setting in which to parse daycare’s benefits and costs. Inconsistency prevails, from state to state—in standards of care and the training of providers—and also from family to family. With parents paying 90 percent of the expense of childcare in this country, nearly a third of children in daycare spend the years before school age in a cobbled together schedule involving multiple providers. In an attempt to reduce the statistical static and better gauge the effects that hours in daycare may have on children’s behavior, Dearing joined forces with a developmental psychologist, Henrik Zachrisson of the Norwegian Institute of Public Health (NIPH), to conduct research in a laboratory with more controls: Norway.

Norway provides universal, highly subsidized access to daycare for children starting at age 12 months (this following a year of parental leave with nearly full salary). In Norway, 97 percent of children ages three to five are enrolled in public daycare centers, where adult-to-child ratios, staff training, and curriculum are determined by the Ministry of Education.

Joined by researchers at Harvard Medical School and NIPH, Dearing and Zachrisson examined data from the Institute’s Mother and Child Cohort Study, which, beginning in 1999, asked mothers of children ages 18 months and 36 months to fill out extensive questionnaires pertaining to health and well-being. Working with a sample representing 75,271 children born before October 2007, Dearing et al. studied responses to a child-behavior checklist on which mothers identified as “not true,” “often true,” or “very true” descriptors such as “can’t sit still” and “defiant” and “hits others.”

The researchers looked for associations between behavioral problems and the number of hours in childcare. They looked for differences in behavior between siblings living under the same roof—17,910 of the study’s subjects, including 2,627 twins or triplets—and for variations in the behavior of individual children whose hours in childcare were increased. They also searched for links between behavior problems and cumulative time in care, to test the hypothesis that if the “dosage” of child care mattered, the cumulative effects would be larger. They published their findings in the online edition of Child Development on January 11, 2013.

The title of their report is “Little Evidence that Time in Child Care Causes Externalizing Problems During Early Childhood in Norway,” and the results are even more definitive than the heading indicates. The researchers found “zero association between time in childcare and antisocial behavior,” says Dearing. “The differences in . . . behavior problems between children in very few hours of care and those in 45 or more [hours] of care [per week] were variations within the normal range, rather than pathological manifestations,” the report states. They found no signs that differences in the number of daycare hours among siblings predicted behavior problems and no indication that changes in hours for an individual child affected behavior. “What makes this so powerful,” says Dearing, “is that we were looking at so many children that any correlation would be statistically significant.”

The authors note several limitations to their study. For instance, relying on mothers’ observations may have skewed the outcome (studies elsewhere suggest that teachers are freer with negative responses). The cutoff of data at age three prevented consideration of any latent long-term effects of daycare. And the 18-month starting age may be too far along to shed conclusive light on the experience of American infants, who often enter daycare at three months.

Twig Mowatt is a writer based in the Boston area.