The Book of Why: The New Science of Cause and Effect

by Judea Pearl

Several studies had already shown that the babies of smoking mothers weighed less at birth on average than the babies of nonsmokers, and it was natural to suppose that this would translate to poorer survival. Indeed, a nationwide study of low-birth-weight infants (defined as those who weigh less than 5.5 pounds at birth) had shown that their death rate was more than twenty times higher than that of normal-birth-weight infants. Thus, epidemiologists posited a chain of causes and effects: Smoking Low Birth Weight Mortality. What Yerushalmy found in the data was unexpected even to him. It was true that the babies of smokers were lighter on average than the babies of nonsmokers (by seven ounces). However, the low-birth-weight babies of smoking mothers had a better survival rate than those of nonsmokers. It was as if the mother’s smoking actually had a protective effect. If Fisher had discovered something like this, he probably would have loudly proclaimed it as one of the benefits of smoking. Yerushalmy, to his credit, did not. He wrote, much more cautiously, “These paradoxical findings raise doubts and argue against the proposition that cigarette smoking acts as an exogenous factor which interferes with intrauterine development of the fetus.” In short, there is no causal path from Smoking to Mortality. Modern epidemiologists believe that Yerushalmy was wrong. Most believe that smoking does increase neonatal mortality—for example, because it interferes with oxygen transfer acr...