Matin Qaim of the University of Bonn in Germany and David Zilberman of the University of California, Berkeley looked at results from field trials conducted by the Maharashtra Hybrid Seed Company in India using genetically modified cotton carrying a gene from bacteria that resist pests.

"We use the example of Bacillus thuringiensis (Bt) cotton in India to suggest that currently existing GM crops can have significant yield effects that are most likely to occur in the developing world, especially in the tropics and subtropics," they wrote in their report, published in the journal Science.

"The yield gains are much higher than what has been reported for other countries where genetically modified crops were used mostly to replace and enhance chemical pest control," they said.

"Yield advantages of insect-resistant cotton in the United States and China, for instance, are less than 10 percent on average." But in India the results were dramatically better. "Over the 4-year period from 1998 to 2001, Bt hybrids showed an average advantage of 60 percent," they wrote.

Bt cotton carries a gene that helps the cotton repel three species of bollworm, all of them common in India.

The researchers examined field trials done at 395 farms in seven Indian states. In each, three adjacent plots were planted with cotton - one with a Bt cotton hybrid developed by Monsanto Corp., one with the same hybrid of cotton but lacking the Bt gene, and a third with a different cotton hybrid.

"On average, Bt hybrids were sprayed against bollworms three times less often than were non-Bt counterparts," they wrote. "Insecticide amounts on Bt plots were reduced by almost 70 percent, both in terms of commercial products and active ingredients."

"Most of these reductions occurred in highly hazardous chemicals such as organophosphanates, carbamates and synthetic pyrethroids."

This saved a good deal of money, they said. "Yet the more sizable benefits are due to yield advantages. Average yields of Bt hybrids exceeded those of non-Bt counterparts ... by 80 percent," they wrote.

They noted that bollworms are a worse problem in India than they are in other cotton-growing countries such as China or the United States.

Greg Jaffe, biotechnology project director at the U.S.-based Center for Science in the Public Interest, said he was impressed by the study.

"The results show that there are some real benefits to developing country farmers from Bt cotton," Jaffe, whose group has questioned the safety of bioengineered food, said in a telephone interview.

"Industry has constantly been touting the benefits of their products and it is good to see a scientific study that addresses that issue head-on. My view is you are going to need more research like this."