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Demography 353
graphic research. It is true of course that observation can cause disturbance in
the facts observed, so much so that the description given no longer corresponds
to the phenomenon under investigation.
Further, if and when the results of demographic research become known to
the populations concerned, they can, through a boomerang effect, produce
changes of behaviour, a knowledge of the facts being one of the factors deter-
mining such behaviour. This is one side of the question that has been little
studied until now but which should engage the attention of demographers in
the future.
6. These relations between observers and facts observed determine to a certain
extent the choice of the means of observation. They must be such as to disturb as
little as possible the phenomena being studied.
Sample surveys fulfil this requirement. No doubt they disturb the fraction of
the population under survey, but as this fraction is small, the disturbance of the
bulk of the population is small also and it may be inferred that, on completion
of the survey, no significant variation has occurred in the characteristics of the
entire population.z Neither do observations derived from census returns of
population and civil status statistics have much effect on people’s behaviour. In
this case they extend to the whole population, but are carried out in such a way
as not to seem connected with demographic research; consequently they have
little effect on the objects of that research.3
But it would be difficult to imagine, for example, a survey on the rate of
fertility taking in the whole population. Let us choose as an example the situa-
tion in France. In that country, a law of 1920 regulates the sale of contracep-
tives. Sample surveys carried out withsmall groups ofthepopulationshowed that
58 % of the persons interviewed did not know that law existed. If, instead of
such surveys, the entire population were to be questioned, obviously, once the
total survey was over, there would no longer be many people left who were una-
aware of that law. The figure of 58 % that would be obtained4 would refer to a
population of the past which no longer existed.
No doubt the above example is exceptional and the disturbing influence
exercised by observer on observed has so far been fairly slight. If, as may be
thought, however, demography ought to direct its attention to the study of in-
dividual motivations, disturbance is then likely to become less and less negligi-
ble.
7. Another uncertain quantity arises from the fact that demography confines
itself to the study of group manifestations. The demographer’s indications con-
tain no precise information concerning a given individual’s destiny. All they
show is the possibility that that destiny wil lead to such and such a situation.
The result of this is that it is nearly always possible to find individual cases
which are decidedly at variance with the average laws enunciated. These cases
then appear to contradict such laws and this apparent contradiction sometimes
leads, quite wrongly, to doubts being cast on the soundness of the demographer’s
conclusions.