Results from the trial have shown that when parents got this training, their babies were less likely to be overweight at a year of age; the babies also slept better than those in the control group, in which parents got safety training rather than responsive parenting guidance. “Parents and grandparents are usually open to receiving this information,” Dr. Paul said, but it isn’t necessarily provided as part of standard well-baby care.

“Pediatricians in the newborn period tell parents to wake babies up every three or so hours to make sure they regain their birthweight,” he said. “I can’t tell you how often I see babies at two months, and no one has told them to stop doing that.”

A new study just showed that more than 10 percent of the world’s population is obese, with major public health and medical consequences. Among the many factors to consider is the science of how individual human beings eat and gain weight, right from the beginning.

Different babies may make different demands on their parents. “A lot of my research is on what is the infant is bringing to the table,” said Dr. Julie Lumeng, a professor of pediatrics at the University of Michigan. She emphasized that obesity is not well understood by scientists; many researchers believed that childhood obesity could be prevented by breast-feeding, or by changing strategies for introducing solid foods, but that has not been borne out in studies.

She hailed the responsive parenting intervention as a well-conducted trial that shed important light on feeding dynamics in early life, but argued for more research on the baby side of the equation. “Babies are born with different temperaments and I don’t think it’s crazy to say that some babies are voracious eaters and some are not and they require different kinds of parenting,” she said.

The Gemini study in Britain, which has been tracking 2,400 sets of twins born in Britain in 2007, offers useful insights on differences in appetite. The study design allows researchers to compare identical twins, who have the same genetic makeup, with nonidentical twins, who are more different genetically, but grow up in the same family environment at the same time.

The researchers used a Baby Eating Behavior Questionnaire, asking parents about four different aspects of infant appetite, the baby’s responsiveness to milk when it’s offered, the baby’s enjoyment of food, the baby’s satiety responsiveness (that is, how easily the baby fills up), and the baby’s slowness in feeding.

“We were actually quite surprised by how much variation there was in appetite,” said Dr. Clare Llewellyn, a lecturer in behavioral obesity research at University College London who leads the study. “We found that differences between babies in their appetite have a really important genetic component to them.” For slowness of feeding, for example, genetics explained 84 percent of the variation from baby to baby; for enjoyment of food, it was 53 percent. And among nonidentical same-sex twin pairs, babies with heartier appetites (higher food responsiveness or lower satiety responsiveness) gained weight faster than their twins.

“Some babies are born with a set of genes that make them more milk responsive or food responsive; some babies are born with a more avid appetite,” Dr. Llewellyn said. “And there are other babies whose mothers find feeding a constant battle.”

So even at the very beginning of life, with a helpless infant and a single food source, this can be a problematic interaction. “On the extreme ends of the spectrum it’s much more complicated,” Dr. Llewellyn said. “As a parent it’s about understanding what kind of eater your child is and feeding appropriately to their particular sort of appetite.”

And helping parents understand and respond to their babies should not mean that we are blaming them when some of those babies gain too much weight, or, for that matter, too little. “When medicine does not understand a problem, we blame it on personal responsibility,” Dr. Lumeng said. Some seem to think: “With obesity, we don’t understand it, so it must be due to incompetent mothering,”

The complexities of teasing out variations in infant appetite are just one piece of trying to understand the complicated biology and social politics of obesity, and how changes in food availability and the larger environment around us may affect people so differently.

“Even for the voracious eater, our goal is to help children,” Dr. Paul said. “We’re sometimes the no police; we say no to a lot of things but we don’t give alternatives — in our intervention, we try to give parents alternatives.”

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