updated 2/2/2009 7:37:41 PM ET 2009-02-03T00:37:41

Untangling the mystery of inherited versus acquired traits may be a step closer.

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Arguments have been long and contentious over how much people inherit and how much they are influenced by their environments.

Researchers led by Frances Rice and Anita Thapar of Britain's Cardiff University focused on reports that smoking by the mother during pregnancy increased the chance of low birth weight and anti-social behavior in children.

The researchers studied 533 children who were genetically related to the mother that carried them and 195 who resulted from egg donations and thus were not genetically related to the mother. The children were aged from 4 to 10 and had been conceived at clinics in the United Kingdom and United States.

"What we have been able to confirm is that cigarette smoke in pregnancy does lower birth weight regardless of whether the mother and child are genetically related or not," Thapar said.

However, that was not the case with anti-social behavior in children, such as temper tantrums, fighting, bullying and disobedience.

They found that smoking during pregnancy was associated with higher levels of anti-social behavior in children who were genetically related to their mothers, but not in children of unrelated mothers.

"It is now clear that offspring anti-social behavior is more dependent on inherited factors passed from mother to child, as our group of children with mothers who smoked during pregnancy with no direct genetic link showed no increased signs of anti-social behavior," Thapar said.

"This suggests that other influencing factors such as the mother's personality traits and other inherited characteristics are at play during the development of a baby."

Such findings can help guide efforts to improve children's health, she said. For example, having the mother quit smoking is clearly important in improving a child's birth weight.

But it may be better to spend money on parenting skills after birth than on arguing that quitting smoking could improve children's behavior, she added.

The results were reported in Tuesday's edition of the Proceedings of the National Academy of Sciences.

Thapar said the researchers are planning further studies on attention deficit-hyperactivity disorder, emotional symptoms, health outcomes and stress.

"The average reader needs to be careful and clear about what sorts of prenatal interventions are going to be helpful for what sorts of child health outcomes, so that public health money is spent in an effective fashion," she said in an interview via e-mail.

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