In the course of arguing here and here for merit pay for teachers Yglesias dismisses the usual seniority based pay scales for teachers as an ineffective version of merit pay (because merit and seniority are not strongly related). But I don't think this is the main argument for seniority pay.
Research has shown people's happiness depends more on whether their condition is improving or deteriorating than on its absolute level. Seniority based pay systems take advantage of this to increase worker satisfaction. Compare a pay scale that rises from $40000 to $80000 over the course of a 30 year career to one than is a flat $60000 independent of seniority. In the seniority based system workers will get annual raises of about 2.3% (in addition to any increases in general pay levels). So their pay will go up a little every year increasing satisfaction. Unions like this as it makes the contracts they negotiate appear better than they actually are.
Of course seniority pay has disadvantages too. It requires workers to trust their employers to stay in business and not to lay off their older workers. So it is especially attractive for jobs like teacher where these aren't major worries. I doubt it will be easy to get rid of.
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