BOSTON -- A carefully designed study of adopted children has provided some of the strongest evidence to date that intelligence is determined by a combination of heredity and environment, researchers said last week. The French study, published Thursday in the journal Nature, found that children who were either born to, or raised by, parents of high socioeconomic status had IQs 12 to 15 points higher than children born to, or raised by, parents of low status. By comparing children adopted across socioeconomic lines, the study found that biological and environmental factors have independent effects on intelligence. For years, the "nature vs. nurture" debate has divided scientists favoring either biological or environmental explanations for individual differences in intelligence and behavior. In an accompanying commentary, Matt McGue, a University of Minnesota behavioral geneticist, said the French adoption study, "by clearly showing that the IQ of children is influenced by both their biological background and the circumstances of their rearing, should help behavioral scientists move beyond tired controversies and begin to address the real issues surrounding the mechanisms of genetic and environmental effects." Sandra Scarr, University of Virginia professor and a leading researcher into IQ and adoptees, said the study appears to provide concrete evidence of the effects of biology and upbringing. "It really is nature and nurture," she said. Pierre Roubertoux, director of the University of Paris genetics lab where researchers Christiane Capron and Michel Duyme did their study, said the research is the first to show that children born to high-status parents but adopted by low-status parents have lower IQs than similar children adopted by high-status parents. That is one of the strongest pieces of evidence for an environmental effect, he said. "This has never been shown before," Roubertoux said in a telephone interview from France. Capron and Duyme were on vacation this week and could not be reached. While the study looked at only 38 adopted children, researchers in the field said its design allowed it to pick up a larger environmental effect on IQ than earlier studies. The children's average age was 14. The study could not make clear, however, whether the biological influence on IQ is due to heredity or other biological factors. Another explanation might be that high-status natural parents may be able to provide better prenatal care, giving their children a good developmental start. High-status parents in the study had about 15 years of schooling and worked as doctors, professors or executives, while low-status parents had about six years of schooling and worked as farmers or unskilled laborers. The researchers did not know the IQ of the parents, but earlier studies have shown correlations between higher educational and occupational status and IQ. Capron and Duyme found that adopted children who had been born to high-status parents had an average IQ nearly 12 points higher than children born to low-status parents, regardless of the status of the adoptive parents. This showed that biology plays a role in intelligence. But the study indicates that upbringing plays an important role as well. The researchers found that children adopted by high-status families had an average IQ more than 15 points higher than children adopted by low-status parents -- regardless of the status of their birth parents. In some cases, upbringing may even outweigh the influence of biology, the study indicated. The University of Minnesota's McGue said in a phone interview Tuesday: "The most unique outcome of this study is the large effect they saw, the large environmental effect. Other studies don't tend to get that large an effect." The University of Virginia's Scarr, who has done some of the previous adoption and IQ studies, said the reason earlier U.S. studies did not show a similar effect may be that U.S. adoption agencies usually do not place children with low-status families. She warned that the specific differences in IQ scores found in the French study cannot be generalized to the population at large, since the study looked only at children born or adopted at the extremes of the socioeconomic spectrum. McGue said it is not clear what biological or environmental mechanisms cause socioeconomic status to affect children's IQ.