4. GENETIC EFFECTS ON EDUCATIONAL SUCCESS IN CROSS-NATIONAL
4.3 Results
Figure 4.1 ACE Variance Decompositions Results for Twins’ School Grades and Years of Education –Adjusted for Assortative Mating
Sources: Add Health, Swedish Registers, and TwinLife; own calculations.
Independent of country, we find that genetic influences contribute substantially to differences in grades and, as expected, to a lesser extent to differences in years of education.
For grades, we find that genes account for more than half of the total variation in Germany, and in Sweden and the United States for about 80%. In comparison, genetic influences on school grades are least pronounced in Germany. In addition, shared environmental influences matter only in Germany and account for about one-fifth of the total variation in grades.
For years of education, genes and shared environmental influences contribute to equal shares (i.e., about 35%) in Germany. In Sweden, genes account for about 70%, while shared environmental influences are again absent. In the United States, genetic influences exceed the relative importance of shared environmental influences (50% vs. 30%). In comparison, genetic influences on years of education are more important in Sweden than in Germany and the United States.
With the exception of Sweden, findings of the substantial impact of shared environmental influences on education are in line with the findings based on an international meta-analysis (Branigan, McCallum, and Freese 2013). Different findings
for Sweden could be explained by the fact that previous research did not adjust for assortative mating, and also by differences in the samples under study (this is outlined in greater detail in the Discussion and Conclusion section).
Next, we test for stratified genetic effects for school grades in the three countries. As an indicator for social background, we used parents’ occupation based on the Erikson-Goldthorpe-Portocarero (EGP) class scheme (Ganzeboom and Treiman 1996).3 Higher EGP classes are indicated by lower numbers (see Appendix 4.B). Table 4.2 displays the means and variances of school grades by parents’ EGP class for each country. The results show that, in all countries, children from higher EGP classes have, on average, better grades and that the total variances for school grades are smaller in higher EGP classes.
Table 4.2 Means and Total Variances of Twins’ School Grades by Parents’ EGP Class
Germany Sweden United States
b/var c.s.e b/var c.s.e b/var c.s.e
EGP V–VII or non-employed
EGP III–VII or non-employed
Mean –0.36 0.07 –0.24 0.02 –0.20 0.07
Total var. 1.13 0.09 1.06 0.04 1.10 0.09
N(Pairs) 231 2542 181
EGP III–IV
Mean –0.16 0.07 0.03 0.02
Total var. 1.08 0.09 0.90 0.05
N(Pairs) 238 1256
EGP II EGP I–II
Mean 0.16 0.06 0.26 0.02 0.27 0.06
Total var. 0.89 0.08 0.87 0.04 0.78 0.06
N(Pairs) 213 1703 183
EGP I
Mean 0.34 0.06 0.50 0.03
Total var. 0.62 0.07 0.84 0.06
N(Pairs) 167 1009
Notes: Clustered standard errors are calculated at the twin pair level. Sources: Add Health, Swedish Registers, and TwinLife; own calculations.
3We used EGP since this information is available for all countries, and we intend to measure another dimension of social background related to parents’ occupation and the economic situation of the family.
Figure 4.2 visualizes the ACE variance decomposition results for school grades differentiated by parents’ EGP class. Detailed results are displayed in Appendix Table 4.D.7.
For Germany the results show that genetic influences on grades increase in the three lower EGP classes, while shared environmental influences decline. In EGP classes V–
VII, including the non-employed, genetic influences account for about 35% of the variation, while genes account for about 80% in EGP class II. However, in the highest EGP class genetic effects explain only about 30% of the total variation in grades.
In Sweden genetic influences decline with parents’ EGP class. In families belonging to the lower two EGP classes, genetic influences account for up to about 75% of the total variation, while shared environmental influences explain less than 10%. For families belonging to EGP classes I and II, genetic influences account for about half of the variation, and shared environmental influences account for about one-fourth.
For the United States, we can only differentiate between the upper two EGP classes and EGP classes III to VII, including the non-employed, due smaller sample sizes. The results support an interaction in line with the Scarr–Rowe hypothesis: For children from families belonging to the lower EGP classes, genetic influences account for roughly 60%, while for those belong to the higher EGP classes genes account for about 80%. Moreover, shared environmental influences are absent in the upper EGP class.
Figure 4.2 ACE Variance Decompositions Results for Twins’ School Grades by Parents’ EGP Class
Sources: Add Health, Swedish Registers, and TwinLife; own calculations.
Analyses using parents’ education instead of EGP class are presented in Figure 4.E.2 in the Appendix. The results for Sweden and the United States provide no support for a social stratification of genetic effects in line with the Scarr–Rowe hypothesis. For Germany, we find support for the Scarr–Rowe hypothesis: genetic influences are about 40% in less and about 60% in more educated families. However, differences are not statistically significant.
In sum, we find indications for socially stratified genetic effects in line with the Scarr–
Rowe hypothesis for school grades by parents’ EGP class in the United States and by parents’ education in Germany.
Next, we turn to the results that put the Scarr–Rowe hypothesis for educational attainment to the test. To address the accumulation of years of education on the upper tail of the distribution, we estimate Tobit ACE models (see Appendix 4.C). Table 4.3 reports the means and variances of years of education by parents’ EGP class for each country.
Table 4.3 Means and Total Variance for Twins’ Years of Education by Parents’ EGP Class
Germany Sweden United States
b/var c.s.e b/var c.s.e b/var c.s.e
EGP V–VII or non-employed
EGP III–VII or non-employed
Mean 0.06 0.13 –0.14 0.04 –0.12 0.07
Total var. 2.88 0.27 1.72 0.07 1.71 0.13
N(Pairs) 279 1290 277
EGP III–IV
Mean 0.56 0.14 0.29 0.05
Total var. 3.23 0.28 2.13 0.11
N(Pairs) 272 865
EGP II EGP I–II
Mean 1.29 0.18 0.85 0.06 0.80 0.10
Total var. 3.98 0.38 3.07 0.14 2.02 0.22
N(Pairs) 228 1034 262
EGP I
Mean 1.84 0.23 1.77 0.10
Total var. 4.55 0.49 4.09 0.24
N(Pairs) 177 684
Notes: Clustered standard errors are calculated at the twin pair level. Sources: Add Health, Swedish Registers, and TwinLife; own calculations.
In line with previous research (e.g., Breen and Jonsson 2005; Sirin 2005) and similar to the results for school grades, we find that twins’ years of education increase with
parents’ EGP class in all countries. Moreover, the total variance for years of education is larger for the higher EGP classes in Germany and Sweden, while the total variance is only slightly higher in the United States. Figure 4.3 displays the results from Tobit ACE models for years of education differentiated by parents’ EGP class (estimates are provided in Appendix Table 4.D.8).
Figure 4.3 Tobit ACE Variance Decompositions Results for Twins’ Years of Education by Parents’ EGP Class
Sources: Add Health, Swedish Registers, and TwinLife; own calculations.
Results for Germany do not support the Scarr–Rowe hypothesis. In EGP classes V–
VII, including the non-employed, as well as in EGP class II, genetic influences account for about 40%, and in EGP classes III–IV, for about 55% of the total variation in years of education. In EGP class I, genes account for only about 5%. However, the uncertainty of this estimate is very high.
In Sweden, genetic effects on years of education are rather stable across EGP classes (about 55 to 68%), while genetic effects decline from EGP III–IV class to EGP II class.
However, the dip in genetic effects in the EGP class II should not be overstated, since genetic influences in the EGP class I are about 65%.
In the United States, we find –similar to Sweden– that the relative importance of genes decreases. Genetic influences on years of education account for about 50% in EGP classes V–VII, including the non-employed, and for around 35% in the upper EGP classes.
However, using parents’ education the results do not support a systematic decline, neither in Sweden nor in the United States (see Appendix Figure 4.E.3). In Germany, results tend in the direction of the Scarr–Rowe hypothesis, but again, differences in genetic effects between the single educational groups are not statistically significant.
Thus, we only find indication for a stratification for Germany for years of education and only if parents’ education is used as an indicator for social background.