So you are saying that almost all studies in social sciences (e.g. psychology) are invalid because almost all of them require participants to voluntarily sign up for the studies.. Also most psychology studies are done with college students, hardly a very representative sample of the population...
This is not new information to social science researchers. Responsible research will attempt to correct for these biases and/or simply acknowledge them upfront and not generalize their result further than the demographics included in the study.
The second part is true. The first part is partially negated through trickery; what is actually being studied is almost never what the respondents believe is being studied.
