Main trends of research in the social and human ... - unesdoc - Unesco

Demography 357
return to this question later on. The same can be said of differential migration,
and for the same reason.6
The result of all this is that in populations the average frequency of each gene
(or group of genes) giving expression to the information it bears within it
evolves from generation to generation. The essential purpose of population
genetics is the study of this development, or in other words, it endeavours to
describe and to estimate the effects of different kinds of behaviour and different
conditions of existence on the collective genetic heritage. It is a comparatively
young science calling for new mathematical instruments and which ought to
develop in the years ahead. We find here a mixture of biology (transmission of
genes) and sociology (differential marriage customs, fertility, mortality and
migrations).
Here are a few examples of research in progress: to start with, theoretical
research. We take a hypothetical case of panmixia, that is to say, a situation
wherein marriages take place haphazardly within an inter-marriage area of
given size. The average size of the family when the children reach marriageable
age and the variation in that average determine then the proportions of con-
sanguineous marriages.’ Similar calculations are conducted by taking hypoth-
eses other than panmixia. The application of these theoretical researches is
to be found by reversing thecalculation procedure: byobservingtheconsanguini-
ty incidence, the size of the inter-marriage area can be calculated. The manner in
which inter-marriage areas - isolates, as they are called - develop and disinte-
grate has important effects on the health of the population, for while there are
relatively few hereditary diseases, the organism always inherits a certain type of
internal environment which can be more or less favourable to the incubation
of diseases. Population genetics has hardly begun the study of all these phe-
nomena, but it is a branch of demography which is expanding rapidly.
Intra-uterine mortality
15. But let us return for a moment to the fertilized ovule which has just started a
new life - first of all an intra-uterine life during which it wil be exposed to
dangers such that, for very many of them, this life wil be a short one indeed
and will end long before the birth of a living child. We know quite a lot about
that intra-uterine mortality during the last months of pregnancy, which is known
as still-birth; but we are largely ignorant of what affects the rest of intra-uterine
life. Strange as it may seem, we know next-to-nothing about intra-uterine
mortality during the first month of pregnancy, and as for the succeeding months,
our knowledge is confined to two or three tables referring to observations car-
ried out in the case of a few thousand conceptions. That is not much in a world
where each year there are IOO million births.
The question of male births has been discussed for a long time, but we do not
know what makes for maleness at the moment of conception. It is thought that
there are differences in intra-uterine mortality between populations and the
various social groups belonging to a given population, but at present we are not
in a position to estimate such differences-still less to discover the factors re-
sponsible for them. Someof them are biological. Actually, the formation proce-