we estimated the fertility of the general Danish population at ages 34, 35, and 40 and combined different fertility indices (number of children, parity progression measures, age at first birth) with corresponding measures for the twins population (combined and separately by zygosity). The comparison was restricted to 14,600 twins in complete same-sex pairs born in 1945-1965, including the twins pairs from the present study. The comparison in Kohler et al. (2002c) found a very close correspondence between the fertility level and its change across cohorts in both the twins and the general population. There exist only a few statistically significant differences; the primary difference pertained to the fact that female twins seem to have a slightly later onset of childbearing, which may be due to sibling influences because twins always have at least one sibling (e.g., see Murphy and Knudsen, 2002). There are virtually no relevant differences between the fertility patterns of MZ and DZ twins.


Our first set of analyses considers jointly the genetic and shared environmental variance components in completed education and fertility. We restrict these analyses to cohorts born prior to 1963—that is, the subset of cohorts at least 35 years old in 1998 when our fertility data were censored. For these cohorts we can therefore investigate completed or almost-completed fertility. We then analyze the number of children born to twins, together with a measure of completed education that was obtained by converting the education categories in Table 3-2 into years of tertiary education.4

The data include 539 female MZ, 844 female DZ, 524 male MZ, and 822 male DZ twin pairs with nonmissing information on education and fertility. The correlations for within-twin pairs obtained for our measure of completed education (years of tertiary schooling) are 0.37 for DZ males, 0.49 for MZ males, 0.34 for DZ females, and 0.50 for MZ females and for fertility 0.17 for DZ males, 0.30 for MZ males, 0.16 for DZ females, and 0.38 for MZ females. These within-variable patterns clearly suggest the presence of genetic components because the MZ twin correlations are nota-


In particular, the categories were scaled as follows: (a) no education beyond elementary school, semiskilled workers, and standard basic training at apprentice school: zero years of tertiary education; (b) less than 1 year of education: 0.5 years; (c) 1 to 3 years of education, possibly practical: 2 years; (d) 3 years of education (e.g., technician, pedagogue): 3 years; (e) academic education of more than 3 years: 5 years; (d) still in education: 2 years or number of years corresponding to already completed education (if more than 2). For cohorts born prior to 1963, only 7 percent of females and 5 percent of males were still in training.

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