To isolate the effect of the pesticide exposure on ADHD symptoms, the researchers controlled for a variety of health and demographic factors that could have skewed the results.

CNN: Study: ADHD linked to pesticide exposure

This has been done, but it is unsatisfactory because in a cross-country comparison many other things vary along with political institutions: it is impossible to isolate the effect of political arrangements on happiness.

ECONOMIST: Happiness is a warm vote

Tang and Thakuriah studied changes in ridership from 2002 to 2010 and adjusted for many other factors, including fare price, gasoline price, weather, and employment levels, to isolate the effect of Bus Tracker and Train Tracker.

FORBES: Online Bus Tracker Attracts Modest Number Of New Riders

Any decent economist would use standard econometrics to hold these other factors constant to isolate the ceteris paribus effect of taxes.

FORBES: Sorry New York Times, Tax Flight of the Rich Is Not a Myth