Because if you don't add money as incentive the only other strong incentive is for people who join the social sciences because they want to push some kind of po

Because if you don't add money as incentive the only other strong incentive is for people who join the social sciences because they want to push some kind of policy that they feel strongly about (In this case, their personal beliefs and values serve as their incentive). There is even a joke among psychologists that people who join psychology departments are mostly those who themselves need psychotherapy (and they choose to study psychology instead).