T., a loyal MR reader, asked for a compendium. This is my off-the-cuff list, but in the interests of fairness I'm doing one on market-oriented economists as well. What are some of the common views found on the left which I consider not just disagreements but more along the lines of a mistake?
By no means is everyone is guilty of these mistakes, nor does it have to mean that the associated conclusions are wrong. Still I see these frequently:
1. Suggesting that money matters in politics far more than the peer-reviewed evidence indicates.
2. Evaluating government spending on a program-by-program basis, rather than viewing the budget as a series of integrated accounts. Cross check with the phrase "Social Security," or for use to take many discretionary spending cuts off the table.
3. A reluctance to incorporate sophisticated "public choice" theories into the analysis of favored programs.
4. Sins of omission: there are plenty of bad policies, such as occupational licensing, which fail to come under much attack from the left. Sometimes this is because the critique would run counter to the narrative of needing more government or needing more regulation.
5. Significantly overestimating the quality of the political economy of an America with more powerful labor unions and underestimating the history of labor unions as racist, corrupt, protectionist, and obstructions to positive change.
6. Overestimating the efficacy of fiscal policy, underestimating the power of monetary policy, and sometimes ignoring or neglecting how the two interact ("the monetary authority moves last").
7. Citing weak versions of structural unemployment theories and dismissing them with a single sentence or graph, while relying on stronger versions of structural theories in other, non-cyclical contexts.
8. Lack of interest in discussing ethnicity and IQ as relevant for social policy, except in preferred contexts.
9. Overly optimistic views of the fiscal positions of state governments. Since the states don't have the same tax-raising powers that the feds do, and since state government spending is favored, there is a tendency to see these fiscal crises as not so severe, or as caused by mere obstructionists who will not raise taxes to the required levels.
10. A willingness to think that one has "done one's best" in the realm of policy, and to blame subsequent policy failures on Republican implementation, rather than admitting that a policy which cannot be implemented by both political parties is perhaps not a good policy in the first place.
11. Use of a strong moral argument for universal health care coverage, combined with a fairly practical, hard-headed approach to the scope of the mandate, and not realizing the tension between the two. Failure to indicate where the "bleeding heart" argument actually should stop and at what margins we should (and will) let non-elderly people die, if only stochastically.
12. Implicitly constructing a two-stage moral theory, which first cordons off the sphere of the nation-state (public goods provision, etc.) and then pushing cosmopolitan questions off the agenda in the interests of expanding a social welfare state. (In fairness, many individuals on the right don't give cosmopolitan considerations even this much consideration, although right-oriented economists tend to be quite cosmopolitan.)
13. What about countries? Classical liberals are increasingly facing up to the enduring successes of the Nordic nations. There is not always a similar reckoning with the successes of Chile and Hong Kong and Singapore; often this is a sin of omission. (Addendum: comment from Matt here.)
14. Reluctance to admit how hard the climate change problem will be to solve, for fear of wrecking any emerging political consensus on taking action.
In most cases you can find evidence and links by searching back through the MR archives.