Participants. Participants were registered with the Netherlands Twin Register (NTR). The NTR collects longitudinal data on health, personality and lifestyle from twin families in the Netherlands. These families were recruited by NTR across the Netherlands through city councils, newsletters, and the NTR website. The present study is based on data collected between 2009 and 2012. Participants first received a written invitation including a link to a webpage, where they could log on to a web-based version of the survey with a unique, personal login name and password. If subjects did not access the web-based survey within the 6 weeks after the invitation, they received a paper version of the survey. Between 3–9 months after the paper versions of the survey were sent, subjects who had not responded received a reminder by post, or a reminder by email, if an email address was available. Several groups of non-responders (e.g. twins from incomplete twin pairs) were reminded in a phone call [34] We selected nuclear family members (parents and offspring), resulting in a sample of 19,135 participants. The survey was collected in two versions, a longer and a shorter version; a short version, filled out by 3,421 participants, did not include questions concerning EA. Our sample comprises twins registered with NTR and their parents and siblings. We refer to the twins and their siblings as the offspring generation. There are 12,234 offspring aged between 16 and 97 years and 6,901 parents (2,817 fathers and 4,084 mothers) aged between 21 and 94 years. Note that the age distributions overlap, as twins registered with the NTR include younger twins, often registered with their parents and older twins, usually registered without their parents.
The total number of families was 8,866 with an average of 1.4 offspring per family, where some families (18%) had respondents only from the parental generation. In addition, 45% of families included twins without their parents; 18% included twins and 1 parent; and 19% included twins and 2 parents (see supplementary table 1). There is information for both parent and offspring generation on BMI in 3,214 families (with N = 10,650), and on EA in 1,089 families (with N = 3,563) (see supplementary tables 2 and 3). EA from participants below the age of 25 years was not included, as they may not have yet reached their highest educational level, resulting in a sample size of 12,078 for EA. The data were checked for outliers and 5 individuals were excluded due to extreme values for height or weight.
Measures/Phenotyping. BMI was based on self-reported height and weight, and was analyzed as a continuous variable. EA was self-reported and categorized according to the highest achieved educational level. The EA scale has 8 levels, and was analyzed as a continuous variable (see Table 1). EA and BMI were adjusted for age of participant at the time of completing the survey. Age adjusted measures were used in the analyses. Outlying BMI data were checked against measurements obtained for the same subject from other surveys in the NTR database. Participants were grouped by sex in the analysis, and separate means were estimated for fathers, mothers, sons and daughters.
Table 1
Age, BMI, and EA in parents and offspring
| | Fathers n=2,817 | Mothers n=4,084 | Sons n=3,999 | Daughters n=8,235 |
Age | N available data | 2,817 | 4,084 | 3,999 | 8,235 |
mean | 55.88 | 53.01 | 31.64 | 31.91 |
SD | 7.90 | 8.04 | 14.74 | 14.35 |
BMI | N available data | 2,758 | 3,960 | 3,865 | 7,924 |
mean | 26.10 | 25.66 | 23.53 | 23.06 |
SD | 3.27 | 4.46 | 3.49 | 3.84 |
EA | N available data | 2,030 | 2,897 | 1,902 | 4,163 |
EA level | Dutch equivalent | | | | |
Elementary school | Basisschool | 1.7% | 2.3% | 1.1% | 1.3% |
Lower vocational education | Vmbo/vocational stream | 13.2% | 11.3% | 7.5% | %6.3 |
Lower general secondary school | Mulo, mavo, vmbo/theoretical stream | 9.1% | 19.5% | 6.1% | 9.2% |
Intermediate vocational education | Mbo | 20.0% | 23.1% | 20.8% | 27.3% |
Upper general secondary school | Havo, hbs, atheneum, gymnasium | 5.3% | 8.7% | 2.9% | 4.9% |
Higher vocational education | Hbo | 27.9% | 25.5% | 29.3% | 29.1% |
University degree | Post-hbo degree | 18.8% | 8.9% | 25.7% | 18.4% |
Post-graduate degree | PhD degree | 4.1% | 0.8% | 6.6% | 3.6% |
Abbreviations: BMI, body mass index; EA, educational attainment; SD, standard deviation |
Statistical Analyses. We used SEM to simultaneously analyze the influence of parental EA and BMI on EA and BMI in their offspring, including BMI-EA correlations within person and across persons within each generation. Paternal and maternal transmission to sons and daughters were specified separately, i.e., we estimated 4 sets (mother-daughter, mother-son, father-daughter, and father-son) of 4 coefficients (EA to EA, BMI to BMI and the cross-transmission coefficients). To accommodate multiple offspring, we included a maximum of 6 offspring (3 males and 3 females) We decided to limit the analysis to a maximum of 6 offspring on the basis of computational considerations. This did not result in an appreciable loss of data (we analyzed 99.37% of the original study sample). Thus, in the full model (figure 1), all coefficients were estimated separately for sons and daughters and for mothers and fathers. Subsequently we imposed equality constraints on transmission parameters across genders of parents and offspring to test the differences in influence across various parent-offspring gender combinations. For these tests, we applied a Bonferroni correction given a family-wise α of 0.01 and, given 8 tests, a test-wise alpha of .01/8 = 0.00125. We used IBM SPSS (version 26) to obtain frequency tables and descriptives. In the structural equation modeling, we used full information maximum likelihood estimation, as this exploits all available data and is more efficient than list- or pairwise deletion. We carried out the structural equation modeling in the Lavaan package (version 0.6-6) in R (version 3.6.1).
The following parameters were specified, with estimates for offspring of the same gender (3 sons and 3 daughters per family) constrained to be equal:
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16 parent-offspring transmission parameters: 2 traits (EA & BMI) x 2 types of transmission (within- and cross-trait) x 2 parental genders x 2 offspring genders
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4 spousal correlations: 2 measures (EA & BMI) x 2 types of correlation (within- and cross-trait)
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2 within person EA/BMI correlations in the parental generation: fathers and mothers
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2 residual EA/BMI correlations in the offspring generation: sons and daughters
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4 variances in the parental generation: 2 traits x 2 genders (fathers and mothers)
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4 residual variances in the offspring generation: 2 traits x 2 genders (sons and daughters)
In addition, we estimated all intercepts for EA and BMI (in 2 parents and 6 offspring) and the within and across-trait correlations for all residuals to account for the conditional association of BMI and EA in the offspring generation (i.e., conditional on parental effects). These included 15 within-trait sibling correlations for EA and 15 for BMI and 30 EA/BMI cross-trait sibling correlations.