The age distribution of motherhood across societies fits a bell curve with approximately a 98% R-squared match.
"if you look at the age of motherhood, it falls into a bell curve shape... And that curve is almost perfectly smooth. If you want to throw R-squareds at it, and then we're not getting too scientific here, but it's like 98% R-squared match to a perfect bell curve." (said at 1:13:30)
The claim that the age distribution of motherhood fits a smooth bell curve is contextually accurate in direction, as demographic models of age-specific fertility rates (ASFR) are unimodal and hump-shaped. However, standard demographic literature shows that actual fertility distributions across societies are characteristically asymmetric and positively skewed (right-skewed), rather than fitting a symmetrical Gaussian (normal/bell) distribution. Demographic functions specifically designed to accommodate skewness—such as the Hadwiger function, Coale-Trussell function, gamma distributions, or generalized skew-normal/skew-logistic models—are required to achieve high goodness of fit ($R^2 > 0.95$ or $0.98$). Fitting a strict symmetrical normal bell curve directly to raw age-specific fertility data yields a poorer fit due to early-onset childbearing and biological/social bounds on maternal age.
- context: Experiments in Modelling Recent Danish Fertility Curves (Demography 1981)
"Functions fitted were a cubic spline, the Hadwiger and Coale-Trussell functions, the gamma and beta densities, two versions of a polynomial, and two of Brass's relational procedures, as well as the Gompertz curve. The spline function fitted all curves far better than any of the others. The Coale-Trussell procedure and gamma density were about equal, followed by the Hadwiger function. All of these functions fit the data well." (abstract, passage verified)
openalexfull study (doi) - context: Fitting Age-Specific Fertility Rates by a Flexible Generalized Skew Normal Probability Den… (Journal of the Royal Statistical Society Series A (Statistics in Society) 2014)
"This model can fit symmetric and skew patterns as well as reflect humps that are observed in some fertility patterns. It is more parsimonious than mixture distributions and more flexible, showing a good fit with several shapes (bimodal or unimodal) of fertility patterns." (abstract, passage verified)
openalexfull study (doi) - context: Modeling fertility in modern populations (Demographic Research 2007)
"The age-specific fertility pattern has a typical shape common in all human populations through years. In order to describe this shape a number of parametric models have been proposed." (abstract, passage verified)
openalexfull study (doi)