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| |     Standard of Proof: How Scientists Evaluate Claims of Cause and Effect by Robert Needlman, M.D., F.A.A.P. reviewed by Laura Jana, M.D., F.A.A.P. The diaper-infertility study raises a more general issue: How do scientists go about deciding that one thing (in this case, diapers) causes another thing (infertility)? Here are some comments that might be useful the next time you read about a new study. Before deciding that "A" causes "B", medical researchers ask several common-sense questions: (1) Is there an association between A and B? That is, when you see A (diapers), do you also see B (elevated scrotal temperatures)? If so, could chance alone account for the appearance of B? For example, if the researchers only measured the temperature of one baby's scrotum once with cloth and once with plastic diapers on, and the temperatures were not the same, anyone would say, "But that's just chance!" Statistical analysis tells researchers how many measurements they need to make on how many children so that chance is no longer a plausible explanation. (2) Are the results repeatable, especially by different researchers? The more the findings are repeated, the more confident everyone can be that they are real, and not due to some unrecognized flaw in the study. To date, the diaper study is one of a kind, so we can't be confident yet that it will pass this test. (3) Is there a plausible mechanism linking the proposed cause, and the observed effect? With the diaper story, there is: Higher testicular temperatures in infancy might interfere with early stages of sperm development, or with cells necessary for healthy sperm development, and these changes might only be evident years later when the immature cells present in the infant mature into full-fledged sperm. (4) Is there evidence that the proposed causal mechanism actually occurs? There has to be a chain of evidence, linking cause and effect. Each link needs to be forged by careful research. Most of the links in the diaper-infertility chain have yet to be forged. For example, no one has investigated whether higher temperatures in infant testes are associated with any change in their health or functioning. (5) Finally, is there a different explanation for the observed effect that might explain it as well, or better, than the one being proposed? In the diaper-infertility case, there are lots of these alternative hypotheses that could explain the observed fall in adult sperm counts. Increased exposure to environmental toxins (such as pesticides in food) is one popular explanation. This could also explain the increase in undescended testicles and penis malformations which, or course, wouldn't have anything to do with plastic diapers.
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