Between Nurture and Nature: The Shifting Determinants of Female Fertility in Danish Twin Cohorts Hans-Peter Kohler 1 Joseph Lee Rodgers 2 Kaare Christensen 3 February 6, 2003 1 Associate Professor of Sociology, Population Studies Center, University of Pennsylvania, 3718 Locust Walk, Philadelphia, PA 19104-6299, USA, email: hpkohler@pop.upenn.edu, www: http://www.ssc.upenn.edu/∼hpkohler. 2 Professor of Psychology, Department of Psychology, University of Oklahoma, Norman, Ok 73019. Email: jrodgers@ou.edu 3 Research Professor, Institute of Public Health and Danish Center for Demographic Research, University of Southern Denmark-Odense, Sdr. Boulevard 23A, 5000 Odense C, Denmark, Tel: +45- 6550 3049, email: kchristensen@health.sdu.dk