The fertility rate multiplied by the number of women of child-bearing age determines the number of babies born in any time period- that is, the cohort size, Cohort size and fertility normally change together, but the echo effects of previous shifts in fertility can produce divergent trends such as those that emerged in the late 1970s. At that time the large cohorts born in the 1950s reached child-bearing age; thus, the number of births rose even though the fertility rate did not.
Does it matter to children whether they are born as part of a small or a large cohort? On average, the answer is certainly yes, Economic theory suggests that the demand for goods and services (such as schooling) that are cohort-specific will, other things equal, increase as cohort size increases. If the supply of those goods and services is not completely elastic–will not increase without limit unless the price rises – this increase in demand will result in a higher price, or poorer quality. Similarly, when a large cohort enters the labor market, the increase in supply will depress the relative wages of entry-level jobs, unless the demand for such labor is completely elastic.