Figure 2.

Six‐variate model to which the data were fitted. For simplicity, only the diagram for Twin 1 is shown. The additive genetic components A1 to A6 of Twin 1 are connected to those of Twin 2 with a correlation of 1 in MZ twins and 0.5 in DZ twins. The nonadditive genetic components D1 to D3 of Twin 1 are connected to those of Twin 2 with a correlation of 1 in MZ twins and 0.25 in DZ twins. The common environmental components C4 to C6 of Twin 1 are connected to those of Twin 2 with a correlation of 1 by definition for both MZ and DZ twins. For i in {1,6}, A_i represents the additive genetic factor acting on trait i. D_i represents the dominant genetic factor acting on trait i. Similarly, C_i represents the common environmental factor acting on trait i and E_i represents the unique environmental factor acting on trait i. The additive and dominant genetic effects were combined to compute broad heritability of the individual traits as ( $a_{11}^2$ +  $d_{11}^2$)/( $a_{11}^2$ +  $d_{11}^2$ +  $e_{11}^2$) for network measure at Wave 1 and ( $a_{21}^2$ +  $d_{21}^2$ +  $a_{22}^2$ +  $d_{22}^2$)/( $a_{21}^2$ +  $d_{21}^2$ +  $e_{21}^2$ +  $a_{22}^2$ +  $d_{22}^2$ + $e_{22}^2$) for network measure at Wave 2. A phenotypic correlation between IQ and network measures at, for example, Wave 1 was defined as (a ₁₁ × a ₄₁ + e ₁₁ × e ₄₁₎/√( $a_{11}^2$ +  $d_{11}^2$ +  $e_{11}^2$) × √( $a_{14}^2$ +  $a_{24}^2$ +  $a_{34}^2$ +  $a_{44}^2$ +  $c_{44}^2$ +  $e_{14}^2$ +  $e_{24}^2$ +  $e_{34}^2$ +  $e_{44}^2$). Furthermore, a (broad sense) genetic correlation between the broad sense genetic factors was computed as (a ₁₁ × a ₂₁ + d ₁₁ × d₂₁)/(√( $a_{11}^2$ +  $d_{11}^2$) × √( $a_{21}^2$ +  $d_{21}^2$ +  $a_{22}^2$ +  $d_{22}^2$)