Demographic Transition
1. The same forces that led people to go on
crusades, and colonize the world, led to the growth of cities. Non-inheriting
children went to cities to work as laborers and servants.
2. Death rates exceeded birth rates in cities until
the 19th century. However, the cities continued to grow because of
migration. This and increased trade created a stimulus for a mercantile economy
to grow and for technological innovation.
3. Technological inovations occurred both in production (the industrial revolution) and in medical care and public health
4. At the end of the 19th century the
germ theory of disease started to impact mortality rates, and so did public
health measures. In the last 5 years of the 19th century, many of
the cities in Europe and America cleaned up the water supply, and this
drastically reduced mortality rates, especially in the summer months.
5. At the same time, fertility rates began to
decrease as well.
6. The decrease in mortality and fertility rates
has been called the demographic transition. It has been described as a process
in which 1) mortality rates decline 1st and populations grow; 2)
people realize the decrease in mortality and respond by having smaller
families. So fertility and mortality are back in balance and populations are
stable. There have been many attempts to explain the transition, which happened
in the developed world from about 1890-1920, and in the developing world after
1960 or so.
Demographic transition in Sweden and Mexico (mortality
drops below fertility and populations grow, then they are equal, and
afterwards, below replacement fertility occurs when fertility rates are lower
than mortality rates; process is faster Mexico, because of modern medicine and
modern economic conditions, but population grew faster as well.)
Possible Explanations
What
is a theory of fertility?
A good theory should explain:
1) cross-cultural
variation in fertility
2) within
population variation in fertility
3) secular
trends in fertility
4) relationships
between relevant ecological variables and fertility outcomes (e.g. wealth, food
intake, status)
Why the demographic transition? Doesn’t this
contradict expectations based on evolutionary theory?
There are three main evolutionary hypotheses:
1) quality-quantity trade-offs, economic costs of children, competitive environment – time frame of payoff?
2) evolved
psychology linked to sex or status, not reproduction per se, so current
environment leads to maladaptive outcomes.
3) cultural
transmission – imitate successful individuals of low fertility, when individual
learning costs are high.
1) Cultural equilibrium – Cultures
respond to mortality so that values regarding reproduction serve to maintain a
balance between births and deaths. There is a lag between the timing of
mortality decline and the realization that fertility must also decline to avoid
population growth.
Problems
are:
1) Group selection – why should individuals
behave for the good of the group.
2) The theory was used in the developing world
with very poor results. The idea was that by improving medical care, people
would have smaller families, but instead populations grew very fast.
3) Fails to explain population growth with baby
boom and below replacement fertility.
Note baby boom and explain difference between crude birth rate (births/1000 in population), completed fertility (total births in a woman’s lifetime) and total fertility rate (synthetic expected completed fertility using the cross section).
(note
that fertility decline had already occurred in Europe and went to below
replacement).
2) Caldwell’s wealth flows theory
Predemographic
transition- Net flow of wealth is upwrd from offspring to parents. Children
worked for the good of the kin group. Investment in kids when they are young,
then kids give more back as they age. They also provide old-age insurance. When
kids are a net asset, parents want as many as possible.
Post-demographic
transition – kids are a net cost and parents only want a few of them – more
like a consumption good. Kids are into their own selfish interests and do not
give back. Emphasis on individualism as opposed to collectivism.
Contrast with
evolutionary theory where net flows are always downward.
Problems:
1) Logical – such a system is not possible because it would lead to runaway population growth.
2) Emprical – Kaplan showed that even in traditional, predemographic transitions societies, like the Ache, children are a net cost; yet fertility is high and people really want lots of kids.
Evolutionary approach
Two-stage embodied capital model including
offspring income
Male income as a function of sibling size from ABQ
men
(there is a tradeoff between quantity and
quality in terms of income, if not survival)
Fathers children on respondants children from ABQ
men
(even if there is an effect of offspring income,
there is no effect on fertility)
(no effect of income on fertility, but big
effect of baby boom. Fertility is low before and after)
(note the negative effect of income on fertiity
in earlier cohorts. Perhaps there is a negative effect at early stage of demo
transition when only people involved in the education-oriented part of the
society, with higher incomes, have small families)
(because income and fertility are unrelated,
those that have the highest number of children have the most grandchildren,
even if they have less income- no tradeoff)
(bars show frequency distribution in percents of
fertility, most people have 2 or 3 children, line shows linear relationship
between fertility and fitness as measured by grandchildren- this is a big
problem for evolutionary theory: Why is the most fit strategy not the most
prevalent? Why did people respond to increased weath with less rather than more
kids)
Two big problems (to be on exam)
1) why is
fertility so low
2) why
don’t wealthier people have more kids
People are living in a novel environment. Can we
understand the modern response by understanding the proximate mechanisms that
have evolved in the past to respond to environmental variation? We need a
theory with two characteristics: 1) Predicts optimal responses under traditional
conditions most similar to our evolutionary past; 2) predicts nonfitness
maximizing response under modern circumstances.
End part 1
