Fig 1.
Evolution of the main indicators of Spain’s educational system from 1940 to 1974.
(A) Educational attainment in Spain by year of birth. (B) Mean years of schooling by sex and year of birth for cohorts at 19 and 24 years old. Data source: Historical Statistics of Spain. 19th and 20th Centuries. Fundación BBVA [23]
Fig 2.
Path diagram for the basic univariate twin model used estimation of sources of variance in EA.
The additive genetic factors (A) have a correlation of 1 between MZ twins and 0.5 between DZ twins, respectively. Shared family environment (C) is correlated 1 for both MZ and DZ twins. Unique environment (E) is the source of variance that will result in differences among members of one family and is, thus, uncorrelated between members of MZ and DZ pairs. The regression coefficients for A, C and E effects, are ‘a’, ‘c’ and ‘e’, respectively.
Table 1.
Distribution of completed educational levels by sex and cohort.
Table 2.
Polychoric twin correlations with 95% Confidence Interval (CI) by zygosity, sex, and cohort.
Fig 3.
Proportions of variance explained (95% CI), by sex and cohort.
Table 3.
Unstandardized estimates of variance explained by additive genetic influences (A), common environment (C) and unique environment (E) with 95% Confidence Interval, by sex and cohort.