Introduction

• All 3 key demographic processes are dependent on age

• All 3 key demographic processes are related to sex and gender

  • Survival advantage of women over men

• Composition: the distribution of the population in accordance with the intersecting characteristics of age and sex

Principles of Age and Sex Composition

Population Age Distribution

• Age distribution: those under 15 (youth), those aged 15-64 (working age), and those aged 65+ (post-retirement)

  • These are approximations

  • To get a sense of the economic dependency burden in a given society

• Total dependency ratio: the ratio of youth + old-age dependents ÷ working-age population

  • Obtain a measure of a society’s overall dependency on the workers, who must provide for those not in the labor force

• Dependency ratio:

  • If it is greater than 100 → more dependents than workers in the population

  • If it is below 100 → more workers than dependents in the population

• Populations with relatively large old-age components tend to have relatively small youth components and have a relatively high median age

Age Pyramids

• Age-sex pyramid: a pictorial representation of the age and sex composition of the population

• Median age: the age that divides the distribution in half (half the population is above the median and the other half is below)

Typology of Age Pyramids

• Demographic transition

  • Early stages: age pyramid = pagoda-like (wide base and narrowing structure)

  • Most explosive growth stage: triangular shape

  • Post-transitional phase: urn-shaped pyramid (narrow bottom, large bulge in the middle, sharp narrowing at the top)

• Aging transition: societies pass from an initially young to an eventual aging structure

• Canada: urn-shaped pyramid

  • Bulge in middle, largest segment of population → baby boom generation

• Stable population: structure with very low constant rates of natural growth

• Stationary age structure: structure in which annual rates of natural increase remain exactly zero indefinitely due to equal numbers of births and deaths annually

Determinants of Age Composition

• Shifts in age composition are caused by changes in fertility (mostly) and mortality

• Intrinsic rate of natural increase: the difference between the intrinsic birth rate and the intrinsic death rate

  • Intrinsic birth rate: rate at which mothers bear their daughters in a stable population, determined by a set of constant age-specific birth rates

  • Intrinsic death rate: death rate in a stable population, determined by a set of constant age-specific death rates

The Relative Importance of Fertility and Mortality

• Life expectancy at birth: measure of survival determined by a populations’ age-specific mortality rates

  • Higher life expectancy = better survival probability

• Gross reproduction rate (GRR): measures the average number of daughters born to a women, given a prevailing schedule of age-specific fertility rates

  • Greater GRR = higher fertility

• Declining mortality → makes the age distribution younger

  • Because more young people + more births

• When mortality declines are in infancy/early childhood = younger population

• When mortality declines are in the ages 45+ = aging population → advanced societies today

Effects of Migration on Age-Sex Structure

• Negligible! In large national populations, any amount of migration will only have a minor impact

• Age pyramid of foreign-born population: diamond-shaped (few immigrants below 15 and 65)

Age Distribution as Demographic Memory

• 2 populations with different age structures will converge to identical age compositions if they have identical age-specific birth and death rates over 70 years or more

  • Example: Sri Lanka + Sweden

• Ergodic property of populations: the tendency for populations to eventually “forget” their initial age distributions

• Time it takes for a population’s age structure to change completely: 100 years

Population Momentum

• Population momentum: M = the size of the stationary population ÷ size of the initial population

  • M is greater than 1: population increase

  • M = 1: no momentum

  • M is below 1: population decline

• Useful for planning purposes

• Net reproduction rate (NRR): measure of a population reproductivity. the extent to which mothers replace themselves by bearing daughters, taking into account mortality to women in the reproductive ages

Sex Ratios

Basic Principles

• Sex ratio: numerical balance between males and females in a population

  • Should be close to 100 males per 100 females

  • Primary sex ratio: ratio of males to females at conception

  • Secondary sex ratio: ratio of males to females at birth

  • Tertiary sex ratio: ratio of males to females at birth beyond infancy

Sex Ratio at Conception

• Human sex ratio at conception = 115/130 males for every 100 females

  • Cause: males are at greater risk of death than females throughout the entire lifecycle

Sex Ratios at Birth and in Early Childhood

• Secondary sex ratio depends on 2 factors:

  • The sex ratio at conception

  • Sex-selective loss during pregnancy

• Normal range: 103-107 boys born for 100 girls

  • More male than female fetuses are lost to spontaneous abortion or stillbirth

• Sex-ratio at birth is controlled by parental hormone levels at the time of conception

  • Severe stress

  • Highly traumatic events

• Evolutionary biology theory: in the animal kingdom, pregnant females when faced by severe stressors preferentially abort frail male fetuses

  • Not proven in humans

Sex Ratios in Adulthood and Old Age

• Determined by gender-based difference in mortality

• Females live longer than males

Societal Ramifications of Change in Age-Sex Composition

Sex Ratio Effects

• High sex ratio for black and white populations (more men than women): low levels of family disruption, high rates of violent crime

• Low sex ratio (more women than men): low rates of violent crime, high levels of family disruption

  → Direct positive effects on the sex ratio on rates of criminal violence and negative indirect effects via family disruption