Birth control and improved medical care could cause the human race to decline, according to a Soviet scientist writing in the latest edition of the science magazine Nature.

Alexey Kondrashov, from the Research Computer Center in Pushchino near Moscow, said improved medical care and birth control were interfering with Darwin's theory of natural selection and could cause a serious weakening of the human race.In his article, titled "Deleterious Mutations and the Evolution of Sexual Reproduction," Kondrashov said minor harmful genetic mutations were far more common than previously thought.

Everyone had at least one genetic defect and most people had several. But in modern life most of the defects had no real effect and some, such as bad eyesight, could be easily corrected.

However, without natural selection sorting out who survives to reproduce, bad mutations could begin to accumulate and contaminate the genetic structure, Kondrashov said.

"It is possible that the physiology of humans is designed to tolerate only natural contamination and even a relatively small rise would lead to drastic effects," he said in the article.

Eventually, these mutations would produce individuals unsuited to survive even under the favorable conditions modern mankind enjoys. The process, which would be virtually irreversible, has probably been hidden up to now by an improvement in living conditions, Kondrashov said.