Demographic Transition I

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Lecture 23

• Demographic Transition I

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Overview of the Demographic Transition

• 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.
• 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. 
• Technological innovations occurred both in
  production (the industrial revolution) and in
  medical care and public health

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Overview of the Demographic Transition (contd.)

• 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.
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• At the same time, fertility rates began to
  decrease as well.
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• The decrease in mortality and fertility rates has
  been called the demographic transition. It has
  been described as a process in which
• Mortality rates decline 1st and populations grow
  
• 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.

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Possible Explanations

• What is a theory of fertility?
• A good theory should explain
• Cross-cultural variation in fertility
• Within population variation in fertility
• Secular trends in fertility
• Relationships between relevant ecological
  variables and fertility outcomes (e.g. wealth,
  food intake, status)
• Why the demographic transition? Doesnt this
  contradict expectations based on evolutionary
  theory?

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Other Theories

• 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
• Group selection why should individuals behave
  for the good of the group?
• 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.
• Fails to explain population growth with baby boom
  and below replacement fertility.

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Evolutionary Approach
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Two Big Problems

• Why is fertility so low?
• Why dont 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.

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Natural Selection and Fertility The Problem

• The fertility of an organism has the most direct
  impact on the number of descendants it produces
  and therefore its biological fitness
• Natural selection on the mechanisms governing
  fertility is likely to be strong and will tend to
  optimize fertility so as to maximize number of
  descendants produced
• Over time, fertility will be adjusted to the
  environment so that observed fertility will tend
  to be fitness maximizing

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Natural Selection and Fertility The Problem
(contd.)

• However, the low fertility observed today is not
  fitness-maximizing, because 1) higher fertility
  would result in greater numbers of genetic
  descendants and 2) on average, wealthier
  individuals with more access to resources do not
  have more children than poorer individuals
• Humans today live under very different conditions
  than the long history of selection on the
  mechanisms controlling fertility. All but last
  10,000 years of human evolution was spent in a
  hunting and gathering economy

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A four-step process was employed to develop a
theory of modern fertility

• Model the effects of natural selection on
  fertility behavior
• Apply it to the ecology of hunter-gatherers
• Specify the proximate physiological,
  psychological and behavioral mechanisms that
  evolved to respond to ecological variation among
  hunter-gatherers
• Develop and test a model of how modern conditions
  might interact with those mechanisms to produce
  observed behavior.

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An Evolutionary Model of the Demographic
Transition

• As a result of human evolutionary history, human
  parents invest heavily in the embodied or human
  capital of offspring. The large human brain and
  the long period of juvenile dependence are the
  result of niche based skill-intensive techniques
  of resource accrual. Parents adjust investment
  in offspring depending on the returns to skill.

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An Evolutionary Model of the Demographic
Transition (contd)

• Models of investment in human capital show that
• Increased survival during adulthood increases the
  payoffs to investments in human capital because
  it increases the length of time over which the
  investment brings a return
• Increased payoffs to investments in human capital
  favor increased investment in staying alive
• Exogenous changes in offspring survival increase
  the expected cost of raising an offspring,
  because it will have a higher likelihood of
  reaching each age to receive investment. With a
  fixed parental income constraint, this
  necessarily reduces offspring quantity.
• These results also imply that if payoffs to
  investments in both offspring income and survival
  increase, large changes in fertility are expected

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An Evolutionary Model of the Demographic
Transition (contd)

• When all wealth/income is stored in the body and
  fertility is controlled by the physiological
  mechanisms affecting fecundity, investments that
  maximize lifetime income and the sum of the
  lifetime incomes of offspring also maximize
  fitness
• When wealth is also stored outside the body,
  there are no evolved mechanisms to ensure that
  increases in wealth are translated into fitness.
  Most wealth in modern society is now stored
  extra-somatically
• Increased mercantilization of the economy in
  conjunction with technological advances led to
  the emergence of skill- and education- based
  labor markets, and large wage premium for skill
  and education
• Increases in survival rates due to science and
  increased public and private investments in
  health further increased payoffs to investments
  in own and offspring embodied capital

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An Evolutionary Model of the Demographic
Transition (contd)

• Points 4-6, in combination with the evolved
  psychology of PI, lead parents to desire much
  lower fertility than the system of physiological
  controls would produce, generating a demand for
  birth control and fertility levels that do not
  maximize fitness
• The impacts of parental time investments on child
  educational outcomes depend positively on the
  education of parents. Children from more
  educated backgrounds can take better advantage of
  formal schooling. More educated parents are
  expected to invest both more time and more
  resources in their children. This increase in
  investment compensates for the increased income
  of more educated parents. Therefore, fertility
  does not vary with parental income.

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There are Three Main Evolutionary Hypotheses

• Quality-quantity trade-offs, economic costs of
  children, competitive environment? Parents focus
  on income and status rather reproduction
• Evolved psychology linked to sex, not
  reproduction per se, so current environment leads
  to maladaptive outcomes.
• Cultural transmission imitate successful
  individuals of low fertility, when individual
  learning costs are high.