MAN IN THE MODERN WORLD
the differential birth-rate is the cause of the increase of feeble-minded-
ness : for by so doing we are being scientifically disreputable.
And, of course, the inevitable obverse of the principle of multiple
cause is the principle of multiple effect. I need not labour the point,
save to stress the need for the working out of suitable methods, of
partial correlation and the like, to deal with this multiple complexity.
Another peculiarity of the social sciences, closely linked with the
first, is that we cannot make rigorous and repeatable experiments,
because we cannot isolate our material or control all its variables.
Again a different technique from that of the natural sciences has to
be worked outóhere a different practical technique. Properly
planned regional experiments are an example.
But perhaps the most fundamental difference between natural and
social science is that the social scientist is himself part of his own
material, and that the criteria for judging the outcome of an experi-
ment are partially subjective. Thus the social scientist cannot escape
bias, and he cannot hope to check his work against objective criteria
that will be accepted by all normal men.
As regards bias, we may compare this with experimental error in
natural science. Just as it is possible to reduce experimental error,
but never entirely to eliminate it, so it is clearly possible to a large
extent to discount and reduce bias. Discovering the technique of
reducing bias will be as important in social science as has been in
natural science the long and often tiresome process of discovering the
technique of reducing experimental error.
The difficulty of finding an objective criterion of truth in social
science cuts deeper. But it is based upon an intellectualist philosophy
which hankers after abstract truth. It largely disappears if we take
the more robust view that science is control as well as knowledge, and
that these two aspects cannot be separated. There can be some
measure of general agreement on the practical results of social experi-
ments, especially if these are properly planned. Thus in social science,
experiment is not the remote preliminary to action that it is in natural
science, but is itself partly actionóboth pure and applied science
simultaneously. Solvitur operando should be the working principle of
the social sciences. It implies that progress in social science and its
applications will be slower and more sprinkled with practical mistakes
than progress in natural science; but it does not mean that we should
deny its possibility.
These general considerations have many particular applications to
our subject. Eugenics is not, as some of its devotees have perhaps
unconsciously assumed, a special branch of natural science: it is a