Exceptionally bright 12- and 13-year-olds are more likely than others to be left-handed, near-sighted or asthmatic, a Johns Hopkins University study shows.

"We have identified some biological correlates of intelligence," neurobiologist Camilla Benbow told Science News.

Benbow's research does not show that these biological traits cause children to do well on intelligence tests. But for unknown reasons, children who scored well on the Scholastic Aptitude Test often had those characteristics.

Researchers gave the SAT, which is a college entry test, to 100,000 children between 12 and 13. They selected the 292 youngsters who scored at least 700 out of 800 on the math portion of the test and 165 who scored at least 630 on the verbal part.

More than 20 percent of these top-scoring youths were left-handed -- twice the percentage of left-handed people that age. In addition, they were twice as likely to have asthma and four times as likely to be near-sighted as their less academically successful counterparts.

The study also confirmed previous research that boys did better in math than girls, even when their attitudes about math were the same. For every 13 boys who scored 700 or better on the math portion of the SAT, there was only one girl who scored that high.