and to lead to inconclusive results. The bottom line is that there is no way to extract
cause-effect conclusions from correlational data.
You overlooked that the causal direction might be reversed. In the case of The French
Paradox finding, I can readily see a plausible alternative interpretation as to how the
observed data could have arisen. The data do seem to show that as drinking declines
from a high to a moderate level, longevity increases. This accords with the notion that
alcohol is toxic, and that its effects are deleterious. What constitutes The French
Paradox, however, is that when one goes even farther along the drinking continuum from
moderate drinking all the way down to no drinking at all, instead of longevity
increasing still higher, the opposite happens - longevity shrinks.
What distinguishes the scientifically-trained mind from that of the layman in this case
is that the layman thinks of a single interpretation, and seizing on that as the only
one possible, stops thinking. That is, the layman thinks "Drinking not at all is
unhealthy, therefore I can improve my health by drinking." The scientifically-trained
mind, in contrast, recognizes that in correlational data a large number of
interpretations is possible, acknowledges the first interpretation that springs to mind
as one among the many that are possible, and keeps looking, and keeps finding, a number
of alternative interpretations, and ultimately acknowledges the impossibility of
choosing among them.
As illustrated in my own case. Specifically, I happen to find myself in a
naturally-constituted zero-alcohol group. That is, I drink not at all, or very close to
not at all. There is a reason for this, and that is that the effects of alcohol upon me
are toxic. Mainly, I get splitting headaches, even from the ingestion of small amounts
of alcohol, particularly if the alcohol comes in the form of wine. I take this to mean
that my constitution is weak, that I am unable to process alcohol efficiently, that I am
unable to detoxify my body of alcohol the way that others can, that my body chemistry is
not up to par. In other words, I am unwell, and as a result I do not drink.
Please mark well what I have just done - I have reversed the cause-effect conclusion
that you had come to. You concluded that not drinking causes deteriorated health, but
what I am proposing to you at the moment is that deteriorated health can cause not
drinking. The insight that I offer you is that when we observe a correlation, we don't
know what caused what, and one of the possibilities to be considered is that the causal
direction may be the opposite of our first impression, that a situation in which we
first conjectured that A causes B may prove upon more thoughtful examination to be a
situation in which B really causes A. In short, it may be the case that people who are
destined not to live as long as others tend to find themselves unable to drink alcohol.
That's all that the French Paradox may have discovered, and that's not a very good
reason for anybody to follow your recommendation to go out and start drinking.
Common sense alone invalidates The French Paradox conclusion. In other contexts, a
correlation being misinterpreted to mean that drinking promotes either health or
longevity will be obviously laughable. For example, a researcher who observes that
hospitalized patients don't drink will not conclude that teetotalling causes
hospitalization. Or, a researcher who visits death row and discovers that the inmates
don't drink and do have short life expectancies will not conclude that teetotalling
shortens life. In such examples, anyone with a modicum of common sense instantly
recognizes that a correlation between zero wine intake and either poor health or short
life does not mean that zero wine intake causes either poor health or short life. All
that is required to recognize the invalidity of your conclusion in The French Paradox is
to apply this same common sense to an only slightly more subtle case.