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\textit{Nature} \textbf{182} (1958 August 30), 596.
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\Large{CANCER AND SMOKING}
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\textsc{The curious associations} with lung cancer found in relation to
smoking habits do not, in the minds of some of us, lend themselves
easily to the simple conclusion that the products of combustion reaching
the surface of the bronchus induce, though after a long interval, the
development of a cancer.  If, for example, it were possible to infer
that inhaling cigarette smoke was a practice of considerable
prophylactic value in preventing the disease, for the practice of
inhaling is rarer among patients with cancer of the lung than with
others.

Such results suggest that an error has been made of an old kind, in
arguing from correlation to causation, and that the possibility should
be explored that the different smoking classes, cigarette smokers, cigar
smokers, pipe smokers, etc., have adopted their habits partly by reason
of their personal temperaments and dispositions, and are not lightly to
be assumed to be equivalent in their genotypic composition.  Such
differences in genetic make-up between those classes would naturally be
associated with differences of disease incidence without the disease
being causally connected with smoking.  It would then seem not so
paradoxical that the stronger fumes of pipes or cigars should be so much
less associated with cancer than those of cigarettes, or that the
practice of drawing cigarette smoke in bulk into the lung would have
apparently a protective effect.

A letter of mine in \textit{Nature}\footnote{Fisher, R.\ A.,
\textit{Nature}, 108 (1958).} included a brief first report of some data
on the smoking habits of twins in Germany kindly supplied by Prof.\ v.\
Verschuen.  What was evident in these data, which concerned only males,
was that the smoking habits of monozygotic, or one-egg, twins were
clearly more alike than those of twins derived from two eggs.  The
monozygotic twins are identical in genotype and the clear difference in
these data gave \textit{prima facie} evidence that among the many causes
which may influence the smoking habit, the genotype is not unimportant.

Unfortunately, considerable propaganda is now being developed to
convince the public that cigarette smoking is dangerous, and it is
perhaps natural that efforts should be made to discredit evidence which
suggests a different view.  Assumptions are put forward which,
\textit{if true}, would show my inference from von Verschuen's data not
indeed to be false but at least to be inconclusive.  I may refer to an
anonymous writer Geminus'' in the \textit{New
Scientist}\footnote{Geminus'', \textit{New Scientist}, \textbf{4}, 440
(1958).}, who supports in this way what is rapidly becoming an
accepted truth---that smoking can cause lung cancer''.

If it could be assumed as known facts (\textit{a}) that twins greatly
influence each other's smoking habits, and (\textit{b}) that this
influence is much stronger between monozygotic than between dizygotic
twins, then an alternative explanation would be afforded for the result
I have emphasized.  The assumptions can be supported by
eloquence$^*$, but they should, for scientific purposes, be
supported by verifiable observations.

Since my letter was written, however, I have received from Dr.\ Eliot
Slater, of the Maudsley Hospital (London, S.E.5), some further data, the
greater part of which concern girl twins, and in this way supply a
valuable supplement to Verschuer's data, and in which, moreover, a
considerable number of pairs were separated at or shortly after birth.

For the resemblance in smoking habits, these female pairs give:
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& \textit{Alike} & \textit{Unlike} & \textit{Total} \\
Monozygotic &             44 &               9 &             33 \\
Dizygotic   &   \phantom{4}9 &               9 &             18
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So far, there is only a clear confirmation of the conclusion from the
German data that the monozygotic are much more alike than the dizygotics
in their smoking habits.  The peculiar value of these data, however, lie
in the subdivision of the monozygotic pairs into those separated at
birth and those brought up together.  Those are:
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& \textit{Alike} & \textit{Unlike} & \textit{Total} \\
Separated     &             23 &               4 &             27 \\
Not separated &             21 &               5 &             26
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Of the 9 cases of unlike smoking habits, only 4 occur among the 27
separated at birth.  It would appear that the small proportion unlike
among these 53 monozygotic pairs is not to be ascribed to mutual
influence.

There is nothing to stop those who greatly desire it from believing that
lung cancer is caused by smoking cigarettes.  They should also believe
that inhaling cigarette smoke is a protection.  To believe this is,
however, to run the risk of failing to recognize, and therefore failing
to prevent, other and more genuine causes.

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$^*$ \footnotesize{The quotation from Geminus'' was too short to do
justice to the techniques of modern publicity''.  The two paragraphs
which follow deserve careful reading.  They show how a simple
assumption, which \textit{might} have been true (though the first
factual evidence at once showed it not to be) is progressively built up
into confident assertions that both my method and my results were
erroneous; and as it is built up, so it is progressively ornamented.

The public should not think that publicity, even if supported by the
Ministry of Health, is always aimed at \textit{improving} public
knowledge.

But things are not really as simple as this.  Comparisons of identical
and non-identical twins are unimpeachable when they are used to assess
the inevitability of purely physiological characteristics, but the habit
of smoking is not necessarily physiological at all.  And in the
formation of psychological attitudes towards smoking, one would expect
that identical twins would be more likely to go along with each other
than would non-identical twins.  For one thing they must constantly be
reminded of their identity by all those around them, and they are bound
eventually to be blessed with a conviction that they ought always to do
similar things.  This, after all, is what society expects of them.

Such a correlation of all kinds of habits might easily account for Sir
Ronald Fisher's results.  So it is too much to say that these imply the
inheritance of smoking and of a susceptibility to lung cancer may be
jointly inherited.  There is therefore no support for the corollary
that those who are going to die of lung cancer will do whether they
smoke or not.  I hope that heavy smokers will not seek some kind of
solace in this latest smoke-screen between them and what is rapidly
becoming an accepted truth---that smoking can cause lung cancer.''

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\textit{Nature} \textbf{182} (1958 August 30), 596.
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