A while back, I favorably quoted this line from Larry White: “One can’t explain an unusual cluster of errors by citing greed, which
is always around, just as one can’t explain a cluster of airplane
crashes by citing gravity.”  Soon afterward, Brad DeLong retorted:

Larry White writes that those who blame the crisis on greed are wrong
because “greed… is always around” and you cannot explain a variable
result by a constant cause “just as one can’t explain a cluster of
airplane crashes by citing gravity.” I say that the same is true of the
CRA. It has been around in more-or-less its current form for a

While there’s a sense in which both White and DeLong are right, I’ve realized that there’s a more important sense in which they’re both wrong.  Yes, you can’t explain variation in terms of constants; this much is true.  But you can explain variation in terms of a variable bundled with a constant. 

Non-economic example: If a soldier gets shot and killed, can you say “The reason is that he wasn’t wearing a bulletproof vest”?  In one sense, you can’t; after all, what about all the days where he didn’t wear a vest and didn’t get killed?  But the common sense interpretation of the facts still allows us to blame the lack of a vest; the soldier died as a result of a variable (the bullets) bundled with a constant (the lack of a vest).

My favorite economic example: European labor market regulation and unemployment.  They had these regs back in the sixties when their unemployment was low.  So how can you blame their unemployment on their regs?  Simple: The cause of high European unemployment is a variable (various shocks) bundled with a constant (the regs).

None of this shows, of course, that e.g. the Community Reinvestment Act was an important cause of the mortgage crisis.  My point, rather, is that you can’t rule out such hypotheses merely by pointing out how long some legislation has been in place.  Indeed, if you really believed that, you couldn’t blame someone’s declining health on a lifetime of bad diet and exercise, could you?