Here's something worth questioning about how we measure population trends. The total fertility rate model makes a pretty bold assumption: it treats young women's childbearing patterns as if they'll match not just their peers today, but also women who are five, even ten years older.

That's a stretch. Demographic patterns don't work in a vacuum. Economic cycles, policy shifts, access to resources—these all shift how generations actually behave over time. A woman's fertility rate at 25 isn't necessarily predictive of her rate at 35 just because some older cohort followed that pattern. Life happens between those years.

When we're building models for long-term trends—whether it's for economics, market cycles, or resource planning—these kinds of statistical shortcuts can compound into serious blind spots. Worth keeping in mind the next time you see demographic forecasts being used to argue anything about the future.