When the Boy Scouts admitted girls to Cub Scouting in 2017, the argument proceeded as if the question had never been tried; at least eight countries had already merged or opened their programs. The episode reads the results. Britain grew for years yet cannot place over 100,000 children for lack of adult volunteers, while Canada's and Australia's declines began long before girls were admitted. Almost no research tests whether coeducation matters.
There is essentially no research on whether coeducation helps or hurts membership, as the one empirical retention study ignores gender. Per-capita comparisons across countries rest on unpublished arithmetic and incomparable counts, and several country dates are disputed.