But then, for many studies about (e.g.) Eating, Behavior, Psychology (etc.): Even for well defined populations (lets say, "religious male adults in Texas") there would be so many incidental variables that I would have strong doubts that results are easily reproducible. On the other hand, studies with "well defined" but very narrow subpopulations (e.g. "religious male adults in Texas that work in printing offices and eat granola every morning") are not easily transferrable, which makes it difficult to generalize results (thus, results are less significant).