What is a "particular social group" and what is the test for establishing such a group?

We have defined a "particular social group" as a collection of people closely affiliated with each other, who are actuated by some common impulse or interest. Of central concern is the existence of a voluntary associational relationship among the purported members, which imparts some common characteristic that is fundamental to their identity as a member of that discrete social group. Perhaps a prototypical example of a "particular social group" would consist of the immediate members of a certain family, the family being a focus of fundamental affiliational concerns and common interests for most people.

Sanchez-Trujillo v. INS, 801 F.2d 1571, 1576 (9th Cir. 1986) (emphasis added).