When Does Correlation Become Causation?

What’s the difference between correlation and causation? The answer to this question is of the utmost importance to scientific researchers, who must use their understanding of this distinction to make conclusions about experimental data. The dictionary definitions of correlation and causation are fairly straight-forward. Correlation is when two or more things occur at the same time, but there’s no proof that a change in one variable would cause a change in the other variable. Causation is when two things occur at the same time, yet there is proof that a change in one would cause a change in the other. Most scientists testing a hypothesis about the relationship between two variables therefore seek to prove causation.

The distinction between causation and correlation has been particularly relevant to discussions about toxic elements like mercury. Mercury has long been associated with adverse conditions like neurodevelopmental disorders. As our exposure to mercury has increased over the course of the last century, so too have cases of autism, autoimmune disorders, and anxiety and depression. Despite this correlation, additional scientific research was needed to prove that the relationship between mercury exposure and adverse health events was not coincidental.

Recent scientific research demonstrates that causation is in fact at play. Scientists Mark Geier and David Geier have long studied the relationship between mercury and adverse health outcomes. While the mercury-containing compound Thimerosal, a preservative used in vaccines, was the primary focus of their earlier research, their recent publication on mercury dental amalgams also clearly demonstrates a causal relationship between mercury and adverse outcomes. According to the study, mercury exposure via dental amalgams increases an individual’s risk for illness, fatigue, depression, anxiety, and most alarmingly, suicide. Adverse effects also include muscle discomfort, frequent infections, GI disturbances, impaired memory, and sore throats. Through their own research and extensive reviews of other scientific findings, the Geiers find that the toxicity resulting from mercury amalgam exposure is broad and varied, and can be delayed in certain individuals.

While mercury exposure affects different individuals in different ways, there remains proof of a strong correlation because findings from these studies consistently meet the Bradford Hill criteria for epidemiology, which includes strength, specificity, consistency, temporality, biological gradient, plausibility, coherence, experiment, and analogy. In other words, while a mercury dental amalgam won’t cause every individual aching lymph glands, the relationship between mercury and adverse health events meets enough Bradford-Hill criteria to reflect a causal dynamic.

Unfortunately, others seem to insist that the connection between mercury and the increase in adverse health events is simply corollary, rather than causal. Dental amalgams containing mercury continue to be offered as a treatment option to dental patients in the United States, and given that over 180 million Americans possess these amalgams, this hesitation to recognize a causal relationship ultimately comes at the expense of human health.