In the world of big data scientists today have a real opportunity to push the limits scientific inquiry in ways that were never before possible. We have the collection methods and computing power available to analyze huge datasets and make observations in minutes that would have taken decades just a few years ago. However, many areas of science are not being strategic with this new power. Instead, many areas of science simply seem to be plugging variables into huge data sets and haphazardly looking for correlations and associations. Judea Pearl is critical of this approach to science in The Book of Why and uses the Genome-wide association study (GWAS) to demonstrate the shortcomings of this approach.
Pearl writes, “It is important to notice the word association in the term GWAS. This method does not prove causality; it only identifies genes associated with a certain disease in the given sample. It is a data-driven rather than hypothesis-driven method, and this presents problems for causal inference.”
In the 1950s and 1960s, Pearl explains, R. A. Fisher was skeptical that smoking caused cancer and argued that the correlation between smoking and cancer could have simply been the result of a hidden variable. He suggested it was possible for a gene to exist that both predisposed people to smoke and predisposed people to develop lung cancer. Pearl writes that such a smoking gene was indeed discovered in 2008 through the GWAS, but Pearl also notes that the existence of such a gene doesn’t actually provide us with any causal mechanism between people’s genes and smoking behavior or cancer development. The smoking gene was not discovered by a hypothesis driven method but rather by data driven methods. Researchers simply looked at massive genomic datasets to see if any genes correlated between people who smoke and people who develop lung cancer. The smoking gene stood out in that study.
Pearl continues to say that causal investigations have shown that the gene in question is important for nicotine receptors in lung cells, positing a causal pathway to smoking predispositions and the gene. However, causal studies also indicate that the gene increases your chance of developing lung cancer by less than doubling the chance of cancer. “This is serious business, no doubt, but it does not compare to the danger you face if you are a regular smoker,” writes Pearl. Smoking is associated with a 10 times increase in the risk of developing lung cancer, while the smoking gene only accounts for a less than double risk increase. The GWAS tells us that the gene is involved in cancer, but we can’t make any causal conclusions from just an association. We have to go deeper to understand its causality and to relate that to other factors that we can study. This helps us contextualize the information from the GWAS.
Much of science is still like the GWAS, looking for associations and hoping to be able to identify a causal pathway as was done with the smoking gene. In some cases these data driven methods can pay off by pointing the way for researchers to start looking for hypothesis driven methods, but we should recognize that data driven methods themselves don’t answer our questions and only represent correlations, not underlying causal structures. This is important because studies and findings based on just associations can be misleading. Discovering a smoking gene and not explaining the actual causal relationship or impact could harm people’s health, especially if they decided that they would surely develop cancer because they had the gene. Association studies ultimately can be misleading, misused, misunderstood, and dangerous, and that is part of why Pearl suggests a need to move beyond simple association studies.