Rethinking age-specific fecundability: Selection bias can generate apparent rapid decline with age | CU Experts

Age-specific fecundability among conception seekers is frequently treated as evidence of how much individuals should expect their own fecundability to decline. Demographers caution against this practice because of negative selection on fecundability among people seeking to conceive at older ages. We revisit this classic result, using simulation to investigate the plausible degree of selection introduced by two processes: on average, first, those with higher fecundability have more births before seeking to conceive; and second, these individuals achieve their desired births sooner, and thus seek to conceive higher-parity births at younger ages. We simulate full reproductive lives for cohorts in which fecundability is constant with age for individuals but varies across individuals. Thus, any change in the age-specific fecundability of conception seekers comes from selection. Our results demonstrate that selection alone can generate substantial depression in observed fecundability at later ages. We call for greater caution in interpreting age patterns of observed fecundability as straightforward evidence of individual age-related decline.

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