In their study of 2,232 5-year-old twins, half of whom had low birth weight, researchers found a significant interaction between children's birth weight and maternal warmth in predicting mothers' and teachers' ratings of ADHD, says lead researcher Terrie Moffitt, PhD, a professor of psychology at King's College in London and the University of Wisconsin-Madison.

The study stemmed from observations of lingering problems with hyperactivity and intellectual deficits among Romanian orphans who were adopted into English families. Psychiatrist Sir Michael Rutter, MD, of the Institute of Psychiatry in London, had previously argued that deprivation of caregiver warmth in the orphanage might be a key factor in that finding. Moffitt and her team sought to investigate his claim.

"We also reasoned from other studies that warmth is a good indicator of a parent's overall investment in child-rearing, so it might be a factor that could prevent hyperactivity and intellectual deficits in at-risk children, such as those with low birth weight," she adds.

In the current study, maternal warmth was coded from mothers' audiotaped answers to open-ended questions about their feelings for their children, she says. Then, both teachers and parents were asked to rate the children's ADHD symptoms. Each child also took an IQ test. (The study found no significant indication that maternal warmth affected IQ.)

The researchers coded a mother's warmth on a six-point scale, based on tone of voice, spontaneity, sympathy and empathy toward the child. They indicated "high warmth" and "moderately high warmth" when mothers expressed definite warmth, enthusiasm, interest in and enjoyment of the child, exemplified by comments like "she is a delight; she is so happy; I love taking her out; she is my ray of sunshine." They coded "some warmth" when mothers showed a detached and rather clinical approach, with little or no warmth of tone but moderate understanding, sympathy and concern. "Very little warmth" showed up when there was only a slight amount of understanding, sympathy, concern or enthusiasm about or interest in the child.

Within the sample, 20 percent of twins had mothers who expressed low warmth, 37 percent had mothers who expressed moderate warmth and 43 percent had mothers who expressed high warmth. The results of twins with the same mother, but varying degrees of warmth expressed toward them were particularly useful to the researchers, Moffitt says.

Low-birth-weight children who had more warm, loving relationships with their mothers were less likely to be described as having ADHD symptoms by parents and teachers, a correlational finding that may suggest that high levels of warmth protect some children from poor behavioral outcomes, Moffitt says. Moreover, low levels of warmth appeared to exacerbate the behavioral problems associated with low birth weight.

"Some researchers have argued lately that what parents do has little effect on their children," Moffitt says. "Some have said that parents' actions only matter if the acts are very extreme, such as child abuse. This paper provides one small bit of initial evidence to the contrary. A simple natural parental inclination to be warm and affectionate toward children did matter for children's outcomes in this study."

The findings suggest emphasizing warmth might be a useful addition to parent education curricula, she says.

"Parent training programs have been proven to be effective, but in addition to emphasizing monitoring, control and consistent discipline, they might wish to encourage parents to express affection too," Moffitt says.