Monday, December 15, 2014

The Causal Arrow Problem

Science Daily reports researchers have found different microbiota in the guts of people with and without Parkinson's Disease. Certain common bacteria are missing in the gut of those with the disease. Other entero bacteria are more common in those with movement problems.

The question is one we always confront in correlational studies: in which direction does the causal arrow point? Does A cause B, B cause A, or in fact does some third factor C cause them both? 

In this example we ask: does having different gut microbiota cause Parkinson's or does Parkinson's cause different gut microbiota to flourish? Or is some as yet unimagined third factor the cause of both outcomes? We echo what every research report ritually intones, more research is needed.