Salt is fine for you! (poor logic in public health and educational pseudo-science)

I'm busy, but this caught my eye, so I'll do it quickly.  It turns out that low-salt diets are actually bad for you.  

This is another in a long line of reversals for overly simplistic public health guidelines (butter is bad, mammograms are good, breastfeeding is bad, eat lots of carbs, etc.), and it should make us very wary of the educational pseudo-science, which often uses the exact same simplistic logic.

Salt was considered bad because salt consumption was associated with slightly higher blood pressure, and slightly higher blood pressure was associated with slightly more heart attacks.  So A was associated with B, and B was associated with C--but no one had actually checked that A actually led to C--that increased salt consumption meant more heart attacks or earlier death.  It turns out that it doesn't--quite the opposite.

This isn't so surprising--the human body is a very complicated system, and there are a lot of other billiard balls on the table besides just salt and blood pressure--but it is worth noting as a cautionary tale, because so much “evidence-based” discourse in the education world is highly dubious and likely to be disproven in the future.

Some examples 

I have seen this kind of logic--A is correlated with B, and B is correlated with C, so A must cause C—in arguments about all sorts of questions.  Here's an example that I wrote about a long time ago:

A.      Explicit vocabulary instruction can lead to some increase in vocabulary

B.      Good readers tend to have larger vocabularies

C.      Therefore, one of the most evidence-based ways to increase reading comprehension is explicit vocabulary instruction

Sometimes the arguments are even weaker:

A.      The texts assigned in schools are less complex than they were 40 years ago.

B.      Students are marginally less good at reading complex texts than they were 40 years ago (I think the evidence for this is very weak, but I’ll accept it for the sake of argument)

C.      Therefore, we should assign more difficult texts in school.

And sometimes they're painfully comical:

A.      Schools are spending more money

B.      Test scores are flat

C.      Therefore, we should get rid of teachers’ unions

Why not more logical arguments?

These arguments would seem to be self-evidently silly, and it might not be worth taking the time to respond to them, except that they are so widely accepted and so central to the major education debates of our time.  So we might ask, why not the following arguments?

A.      Many of our students go whole years without reading a single book.

B.      No one has ever become a good reader without reading a lot

C.      Therefore, we should spend a lot of time and money and thought on getting kids to read more.

A. In an appropriately leveled book, 1% of the words will be new, and readers will learn on average about 15% of those new words.

B. If you read 100 pages a week, you will learn about 45 new words, which is far higher than the ten to fifteen that kids are given in typical vocabulary instruction.

C. Therefore, teachers should stop spending time on explicit vocabulary instruction and should instead devote more time to independent reading.

A.  If you read books that are too hard, you don't improve as much as if you read books that are at the appropriate level.

B. If you read books that are too hard, you will read less.

C. Therefore, students should read books at their reading level.

A. The educational achievement of poor kids is much, much worse than that of rich kids.

B. No school has ever succeeded in educating poor kids up to the level of rich kids.

C. Therefore, if we want all kids to have the same opportunity, we should eliminate poverty.

But that last argument is its own refutation, because of course it is unthinkable anymore to seriously consider attacking poverty directly.  So instead we get ridiculous logic.