The Language of Argument

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C H A P T E R 1 0 ■ C a u s a l R e a s o n i n g

and, just as obviously, writing well should not give you big feet. The cor-

rect explanation is that both foot size and handwriting ability are positively

correlated with age. Here, a noncausal correlation between two phenomena

(foot size and handwriting ability) is explained by a third common correla-

tion (maturation) that is causal.

At times, it is possible to get causal correlations backward. For example, a few

years ago, sports statisticians discovered a negative correlation between for-

ward passes thrown and winning in football. That is, the more forward passes

a team threw, the less chance it had of winning. This suggested that passing is

not a good strategy, since the more you do it, the more likely you are to lose.

Closer examination showed, however, that the causal relationship, in fact, went

in the other direction. Toward the end of a game, losing teams tend to throw a

great many passes in an effort to catch up. In other words, teams throw a lot of

passes because they are losing, rather than the other way around.

Finally, some correlations seem inexplicable. For example, a strong posi-

tive correlation reportedly holds between the birth rate in Holland and the

number of storks nesting in chimneys. There is, of course, a background

theory that would explain this—storks bring babies—but that theory is not

favored by modern science. For the lack of any better background theory, the

phenomenon just seems weird.

So, given a strong correlation between phenomena of types A and B, four

possibilities exist:

1. A is the cause of B.


2. B is the cause of A.


3. Some third thing is the cause of both.


4. The correlation is simply accidental.
   Before we accept any one of these possibilities, we must have good reasons
   for preferring it over the other three.
   One way to produce such a reason is to manipulate A or B. If we vary fac-
   tor A up and down, but B does not change at all, this finding provides some
   reason against possibility 1, since B would normally change along with A if
   A did cause B. Similarly, if we manipulate B up and down, but A does not
   vary at all, this result provides some reason against alternative 2 and for

Courtesy of Randall Munroe

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