A new study published in Scientific Reports indicates that amyotrophic lateral sclerosis (ALS) and multiple sclerosis (MS) have an extremely high geographic association, even after controlling for race, gender, wealth, latitude, and access to neurological health care.

“The results of the study are surprising because previous studies have typically concluded there was no evidence for a mechanistic or genetic link between the two diseases,” explains study author Melissa Schilling, a professor at New York University’s Stern School of Business who specializes in analyzing large-scale datasets using econometrics.

The study also shows that the relationship between the two diseases has likely been overlooked until now because of a “Simpson’s Paradox”—a statistical phenomenon whereby a trend appears in different groups of data but disappears or reverses when the groups are combined.