A relationship between prostate cancer risk and having no sons would point to a mutation in the Y chromosome, which determines the male sex of a child. But there are complexities to such a relationship, Harlap noted.
"If the effect is due to Y chromosomes, they are quite specific to ethnic groups," she said. "Israeli Jews are different from Danes and Swedes."
The complexity of the issue is also illustrated by the different findings of the two Scandinavian studies about fatherhood and risk. The new Danish report finds that "among fathers, a significant trend was observed of gradually reduced prostate cancer with increasing number of children" -- in other words, fatherhood was linked to an increased risk for prostate cancer, but fathering more children begins to bring that risk down again. In contrast, the earlier study found "no further change in risk associated with fathering of more than two children."
Dr. Otis Brawley, chief medical officer of the American Cancer Society, was less than impressed by the Copenhagen findings, however. Despite the large numbers, the result was "just barely statistically significant," Brawley said, and could be the result of pure chance.
In a study of this kind, he said, "you occasionally get something that is statistically significant but is not really significant biologically."
In any case, men shouldn't make decisions on fatherhood based on the study results, Brawley stressed. "I would never suggest to men that their wives not get pregnant so they don't get prostate cancer," he said. "Lack of fatherhood is not a strong preventive of prostate cancer."
Any relationship that does exist is probably very weak, Brawley said. "If there is a real correlation, I would like to know what the true cause is," he said. "I suspect we will never know."
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