This fact can be derived from very simple assumptions. Let's assume people value pay, and let's assume they value helping others. Different people value pay more/less.
People will then choose jobs on the "efficient frontier". Which point on the frontier depends on how much you value pay.
Now it follows from the efficient frontier that if job A is both better paid and helps other people more than job B, then no one will do job B.
So for all pairs of jobs (A,B) that people do, one will be better paid and the other help others more.
People will then choose jobs on the "efficient frontier". Which point on the frontier depends on how much you value pay.
Now it follows from the efficient frontier that if job A is both better paid and helps other people more than job B, then no one will do job B. So for all pairs of jobs (A,B) that people do, one will be better paid and the other help others more.