Decades ago, researchers identified that there was a higher risk of children developing leukaemia if they lived in the proximity of high voltage powerlines. With no obvious mechanism by which the low frequency electric and magnetic fields could induce childhood leukaemia, this finding was controversial. It was also seen as a good example of why correlation did not necessarily mean causation, with one suggested attempt to explain the correlation pointing out that families with good incomes were able to afford living elsewhere and perhaps the correlation was due to a secondary effect, such as the likely poorer nutrition available to lower income families living near such landscape visually dominating power lines.
Searching for the reasons behind this correlation now appears unnecessary, with a large UK based follow up study showing that the relative risk is not significant:
While not specifically relevant to understanding the causes of CLL, this does show the importance of being able to identify a specific causative factor behind correlations between exposure and cancer risk. I also wonder how many families living near high voltage power lines felt unnecessarily guilty when a child developed leukaemia.