Measured Futures
A short story about fertility, timing, and chance
by Stephen J. Shaw
Written to explain the Microdemographic Framework (MDF) and the nuance of fertility measurement —
including why a single headline statistic can hide the structure of demographic change.
This short story accompanies the paper On a Microdemographic Framework for Analyzing Contemporary Fertility Dynamics,
published in Scientific Reports (Nature Portfolio), Aug 21, 2025.
In a quiet school
where every year, without fail, exactly one hundred girls were enrolled in each grade,
and where every former student had lived past the age of fifty, a teacher had discovered an intuitive
way to teach demography and fertility rates.
Every spring she gave her class of fifteen-year-olds an unusual assignment:
calculate the Total Fertility Rate (TFR) for the school.
This year, as always, the girls were baffled. A ripple of whispers moved through the room.
They understood that TFR tells demographers something about how many children a woman has in her life.
But how could they possibly know how many children women who had once attended the school have?
The teacher offered just one cryptic hint:
“The school secretary sends flowers to every former student who has a baby—
large bouquets for first births, smaller ones for later births.”
The girls rushed to the secretary’s office.
The girls asked:
“Do you really send flowers to every mother?”
The secretary replied:
“Yes,” she replied. “I scan every birth announcement for former students’ names. I’ve never missed one.”
The girls beamed.
The girls said:
“Please show us all the flower-delivery records ever sent!”
But the secretary shook her head.
The secretary explained:
“I’m sorry, I only keep one complete year of courier records. This year isn’t complete yet,
so I can only show you births from last year.”
The girls returned to the teacher, discouraged.
The teacher said:
“I’ll give you another hint,” the teacher said. “One year of data is all you need to calculate TFR.”
Still confused, one girl said:
“Then the one year she must mean is the year when former students turn fifty!
If we ask all the fifty-year-olds how many children they’ve had, we can work it out.”
Back they hurried to the secretary.
The secretary smiled and said:
“Fifty-year-olds? Funny you should ask,” she smiled.
“I’ve just organized their reunion—and all one hundred replied with life updates, including how many children they have.”
The girls counted eagerly. Exactly 220 children.
The girls said:
“Simple! 100 women, 220 children—so the TFR must be 2.20!”
Back in the classroom however, the teacher shook her head gently.
The teacher explained:
“You’ve just measured the completed fertility of one historical group—what demographers call a ‘cohort’.
But that tells us nothing about what is happening to younger women today.
If we used that method, we’d have to wait until you are fifty to know your fertility.”
The girls fell silent until one suddenly remembered the flowers.
One girl said:
“What if we just use last year's pattern instead?
If we imagine we all experience the same chance of having a baby at each age that last year’s women did?”
Another jumped in:
“None of the fifteen-, sixteen-, or seventeen-year-olds got flowers last year.
But one eighteen-year-old became a mother…”
The teacher said:
“Now you’re thinking like demographers,” the teacher said.
They rushed back for last year's records. Conveniently every age from 15 to 49 had exactly 100 former students,
so no adjustments were needed.
Last year the secretary had sent:
- 154 flower deliveries in total (154 babies),
- of which 70 were large bouquets sent only to first-time mothers.
The girls calculated quickly:
The girls said:
“154 babies for 100 women… so the TFR is 1.54.”
They wondered why it was so much lower than the 2.20 they had found for the older cohort.
The teacher then unrolled a long chart of TFR estimates from previous years.
The teacher said:
“Last year’s class estimated 1.56. Now we add 1.54.”
The chart revealed a long decline stretching back to the 1970s.
The teacher then posed a challenge:
The teacher asked:
“But TFR alone can’t tell us why fertility is low.
Are fewer women becoming mothers? Or are mothers having fewer children? Or both?”
One girl had already been crunching the numbers:
She said:
“Seventy large bouquets last year means seventy of us would become mothers if we followed the same pattern.
So the Total Maternal Rate (TMR) is 70 out of 100—70%.
And with 154 babies, those seventy mothers would have 154 / 70 = 2.2 children each.
That’s the Children per Mother (CPM).
So:
- 70% will become mothers (TMR = 0.70)
- 30% will remain childless (TCR = 0.30)
- Mothers will have 2.2 children each (CPM = 2.2)
And TFR = TMR × CPM = 1.54.”
The teacher replied:
“Exactly,” the teacher said. “The single number ‘1.54’ hides the structure. The flowers reveal it.”
The teacher then showed them two more charts: one tracing how the share of former students who became mothers (TMR)
had steadily fallen for decades, and another showing that the number of children per mother (CPM) had stayed almost
unchanged at just above two.
The teacher said:
“This is the key,” the teacher said. “Most of the decline comes from fewer women becoming mothers,
not from mothers having fewer children.”
A girl asked softly:
She asked:
“So thirty of us will never have children?”
The teacher replied:
“Statistically, if the birth pattern stays the same, yes—but no one can know who.
Some will choose it. Later some may change their minds. Some will face medical challenges — that's called involuntary childlessness.
Others may never meet the right partner at the right time or face other circumstantial challenges — that's called unplanned childlessness.”
Another girl asked:
She asked:
“But what if former students last year were simply delaying motherhood?”
The teacher replied:
“Maybe. That’s the ‘tempo effect’,” the teacher replied. “Delays raise the average age of motherhood for a while
and reduce births in the short term. Births can rebound—but once the share of women who ever become mothers falls,
it rarely returns to earlier highs.”
Another girl asked:
She asked:
“How does this apply to the whole country?”
The teacher explained:
“You’ve done exactly what national statistical offices do — only they do it without the flowers,”
the teacher said with a smile.
“They count every birth in a single calendar year — a ‘period’ — and calculate the societal TFR.
And when first births are recorded, they also calculate societal TMR, TCR, and CPM — what we call
the Microdemographic Framework, or MDF. These are all period measures describing the society
right now — not the life stories of one older cohort.”
As the girls left, one muttered:
She muttered:
“Well, I don’t like children anyway.”
Another laughed:
She laughed:
“Wait until you fall in love!”
The teacher called after them:
The teacher said:
“Remember—this is only a snapshot of one year. Your future is still entirely ahead of you.”
THE END
