"Third, people vary, and two groups of scientists might end up with very different results if they do the same experiment on two different groups of volunteers.

"Third, people vary, and two groups of scientists might end up with very different results if they do the same experiment on two different groups of volunteers." This seems to me to be such an important key to understanding this. Considering even myself, I'm not sure I would behave the same way as a subject from one study to the next (pretending there was a way to blind myself). And when you look across regional and global cultural variation, it must have an effect on participants. The results are less headline-getting if they seem to be overly qualified (e.g. we find that midwestern white women with the middle name Susan have a positive psychological response to salsa dancing) but a deeper appreciation for the complexity and diversity of individual psychology would seem superior to the current state of affairs.