Family environment and the malleability of cognitive ability: a Swedish national home-reared and adopted-away cosibling control study

Abstract

Cognitive ability strongly aggregates in families, and prior twin and adoption studies have suggested that this is the result of both genetic and environmental factors. In this study, we used a powerful design--home-reared and adopted-away cosibling controls--to investigate the role of the rearing environment in cognitive ability. We identified, from a complete national Swedish sample of male-male siblings, 436 full-sibships in which at least one member was reared by one or more biological parents and the other by adoptive parents. IQ was measured at age 18-20 as part of the Swedish military service conscription examination. Parental educational level was rated on a 5-point scale. Controlling for clustering of offspring within biological families, the adopted siblings had an IQ 4.41 (SE = 0.75) points higher than their nonadopted siblings. Each additional unit of rearing parental education was associated with 1.71 (SE = 0.44) units of IQ. We replicated these results in 2,341 male-male half-sibships, in which, controlling for clustering within families, adoption was associated with a gain of IQ of 3.18 (SE = 0.34) points. Each additional unit of rearing parental education was associated with 1.94 (SE = 0.18) IQ units. Using full- and half-sibling sets matched for genetic background, we found replicated evidence that (i) rearing environment affects IQ measured in late adolescence, and (ii) a portion of the IQ of adopted siblings could be explained by the educational level of their adoptive parents.

Keywords: adoption; cognitive ability; cosibling control; environment; rearing.

Fig. 1.

Magnitude of IQ difference (black bars and left y axis) between adopted and nonadopted full-siblings as a function of the difference in educational level between biological and adoptive parents of the adopted siblings (x axis). The four bars represent (from left to right) −4 to −2 steps; −1.5 to 0 steps; 0.5–2 steps; and 2.5–4 steps difference on the education scale. The gray line (right y axis) illustrates the number of pairs in each group.

Fig. 2.

Magnitude of IQ difference (black bars and left y axis) between adopted and nonadopted half-siblings as a function of the difference in educational level between biological and adoptive parents of the adopted siblings (x axis). The four bars represent (from left to right) −4 to −2 steps; −1.5 to 0 steps; 0.5–2 steps; and 2.5–4 steps difference on the education scale. The gray line (right y axis) illustrates the number of pairs in each group.

Fig. 3.

A flowchart showing the creation of the full- and half-sibling databases.

Fig. 4.

The distribution of the standardized IQ score in the full- and half-sibling samples.

Fig. 5.

The distribution of the mean educational level of biological and adoptive parents in the full-sibling sample. The five levels indicated on the x axis are: (1) <9 y, (2) 9 y, (3) 10–11 y, (4) 12 y, and (5) >12 y.

Fig. 6.

The distribution of the educational level of biological and adoptive parents in the half-sibling sample. The five levels indicated on the x axis are: (1) <9 y, (2) 9 y, (3) 10–11 y, (4) 12 y, and (5) >12 y.