Final answer:
Purposive, solidary, and material incentives encourage interest group membership.
Step-by-step explanation:
There are three types of incentives that encourage interest group membership: purposive incentives, solidary incentives, and material incentives. Purposive incentives motivate people to join a group because they want to help the group achieve its goals. For example, after the 9/11 attacks, membership in the ACLU increased as people were concerned about the government's impact on civil liberties.
Solidary incentives attract people to join groups for the pleasure of interacting with like-minded individuals and publicly expressing their beliefs. Material incentives, on the other hand, offer tangible benefits like health insurance and discounts.