Children who are under emotional or physical stress score 13 percent lower on intelligence tests than youngsters who are functioning virtually worry-free, two researchers say.

Bernard Brown and Lilian Rosenbaum of Georgetown University created a stress index using a sample of 4,000 7-year-old children.

''What we found was that the stress index showed the I.Q. of the children decreased 13 percent from low stress to high stress,'' Mr. Brown said Sunday in an interview after addressing the American Association for the Advancement of Science annual meeting. The I.Q. scores dropped from 104 with no stress to 91 with high stress, he said.

The researchers did not test children before and after they were under stress but compared children who were under stress to those who were not.