If they were selected that way on purpose, they're probably just trying to control for confounding differences to try and make it easier to distinguish the sign

If they were selected that way on purpose, they're probably just trying to control for confounding differences to try and make it easier to distinguish the signal from the noise in the statistics. Same reason that "there were no major differences in sex, age or years of education between meditation practitioners and control subjects." Edit: They say "handedness was measured by Annett Hand Preference Questionnaire" which suggests that this wasn't deliberate.