Much of formal demography (techniques, methods) can be viewed also as
theory, that is, as a collection of substantive models about how populations and cohorts
behave;
b) Many ideas from behavioural demography that have been rejected as
empirically false or too simplistic can be viewed as perfectly good theory, especially if
they were to be stated more rigorously.
Indeed, at the theoretical level, the classic distinction between formal and
behavioural demography loses much of its force. In both sub-areas of demography,
theoretical models have essentially the same epistemological standing, even if they may
differ on other dimensions such as scope and complexity, and even if different kinds of
day-to-day work may be involved in their development and use.
I recognise that the word
theory
is ambiguous in the non-pejorative sense of
‘having two or more meanings.’ It means different things to different people, both in
everyday speech and in scientific discourse. In such circumstances, it is futile to try to
establish the ‘correct’ definition or the ‘true meaning’ of theory. But it is possible and
useful to suggest a new – though certainly not entirely new -- approach to theory that
might prove more fruitful than the older ideas to which many of us are accustomed. In
the next section, I summarise the main elements of the model-based view, noting some
ways in which it differs from, but also agrees with, logical empiricism. A key part of
this exposition is a partial re-definition of such terms as
model
and
theory.
But
terminology is not crucial, and some may want to define these words differently, and to
preserve a sharp distinction between
theory
and
model.
The central ideas I wish to
convey are an emphasis on formal demography as substantive knowledge, and a plea
that empirical exceptions to otherwise useful behavioural theories should not lead to
their discard.
In the logical empiricist view of science, theory comes from data through a process
of induction and generalisation. Theoretical knowledge and empirical knowledge are
seen as occupying different but parallel planes, layered upward into ever more general
and abstract propositions. In the model-based view, theory and empirical studies are
seen as occupying non-parallel planes. The planes must intersect, of course, since we
are discussing empirical science. But the origin and character of the two kinds of
knowledge are qualitatively different
