A study of twins raised in different households shows that genes exert the strongest influence on intelligence, according to researchers from the University of Minnesota.

The researchers found that 70 percent of the influence on IQ scores was hereditary and about 30 percent was a result of environment.The scientists based their conclusions on 50 pairs of twins who were separated as infants, reared apart, and reunited as adults.

"They do indicate that, in the current environments of the broad middle class, in industrialized societies, two-thirds of the observed variance of IQ can be traced to genetic variation," the study said.

The research contradicts some earlier studies of twins that attributed about half of the similarity in intelligence tests to genetics and half to environmental effects.

Some researchers, however, suggest that the number of twins in the current study is too small to make sweeping conclusions.

The study's lead author, Thomas Bouchard, also warned that the findings cannot be applied to all people, since none of those studied were raised by poor or illiterate parents.

The report was published last week in the journal Science.