Why do a population’s sex ratio and age structure affect fertility rates?

A population’s sex ratio and age structure are fundamental determinants of fertility rates, as they dictate the availability of potential parents and the reproductive capacity of the group.

Sex Ratio

The sex ratio—the relative number of males and females—directly influences marriage patterns, which are a primary social constraint on fertility. A scarcity of males in a specific age group can depress marriage rates for women, leading to delayed marriage or reduced childbearing. Conversely, a "marriage squeeze"—where the number of eligible partners is insufficient—can force shifts in social conventions, such as changing the typical age gap between spouses, which ultimately impacts the timing and number of births.1

Age Structure

Age structure, often visualized through population pyramids, reflects the proportion of individuals in different age categories. Because fertility is concentrated among women of reproductive age, a population with a large base of young people (a youthful age structure) typically exhibits higher fertility rates. In contrast, older populations with more uniform age distributions generally experience lower fertility. Changes in fertility rates themselves further influence this structure by altering the number of newborns, creating a feedback loop that shapes the population's future reproductive potential.2