According to rosenthal and rosnow, when compared to nonvolunteers, volunteers tend to be: highly educated. of a lower socioeconomic status. less in need of approval. more unsociable.

I believe the correct answer is: highly educated.

According to the research "The volunteer subject revisited" by Ralph L. Rosnow and Robert Rosenthal, when compared to non-volunteers, volunteers tend to be highly educated. Alongside higher education, volunteers also tend to come from a higher social class and be more social.