[wybo]

There could still be reasons for picking such a sample.

1) In the far future, when most direct hardships are a thing of the past our lives probably resemble those of Harvard students more, than those of a random population sample in the 1930's

2) It is relatively easy to think of things that may make poor, disadvantaged, discriminated against, etc. people unhappy. And there is some data on this. While it was/is less clear for the advantaged.

3) Mentioned elsewhere, a study can gain discerning power if variables that vary a lot are kept constant. Similarly to how when doing a scientific experiment, one wants to control most variables besides the one one wants to test (simply makes the study more narrow, but deeper).

4) Practically speaking, a lot of psychological research is done on students, simply because they are within easy reach for scholars. Why Harvard students? Because that's a place where they would embark on something long-term like this 70 years ago.

5) More cynically, the scholars behind the study, and even after 70 years still the most powerful social demographic group, and probably a large portion of the most well-read, are in the sample they picked. So they sort of wanted to scratch their own itch first...

Finally, see the lovely series 7UP for a more random sample (allbeit much smaller, and British...)

http://www.imdb.com/title/tt0058578/