On the Relationships between Period and Cohort Fertility

Lei Lu, Independent Consultant

Since the classic paper of Norman B. Ryder on demographic translation (1964), demographers have studied the relationships between period and cohort fertility from different perspectives and achieved important results. This paper looks at the demographic translation from a cohort-to-period perspective, based on a unified analytical framework. The paper addresses issues of fertility level (quantum) and timing (tempo) and how they are intertwined. The discussions show that measures of period and cohort fertility are linked in complex ways. The results gained in the paper may help to further improve the understanding and interpretation of period measures of fertility, especially the conventional period total fertility rate (TFR).