Twin Research and Human Genetics Volume 16 Number 5 pp. 923–930 C The Authors 2013 doi:10.1017/thg.2013.54 The Wilson Effect: The Increase in Heritability of IQ With Age Thomas J. Bouchard Jr. Psychology Department, University of Minnesota, Minneapolis, USA Ronald Wilson presented the first clear and compelling evidence that the heritability of IQ increases with age. We propose to call the phenomenon ‘The Wilson Effect’ and we document the effect diagrammatically with key twin and adoption studies, including twins reared apart, that have been carried out at various ages and in a large number of different settings. The results show that the heritability of IQ reaches an asymptote at about 0.80 at 18–20 years of age and continuing at that level well into adulthood. In the aggregate, the studies also confirm that shared environmental influence decreases across age, approximating about 0.10 at 18–20 years of age and continuing at that level into adulthood. These conclusions apply to the Westernized industrial democracies in which most of the studies have been carried out. Keywords IQ, age, intelligence, heritability, twins, adoptees Monozygotic twins became increasingly concordant Figure 1 from Wilson (1983) is a plot of kin correlations with age and also paralleled each other for the spurts across age. The data were gathered by the justly famous and lags in development. Dizygotic twins became less Louisville Longitudinal Twin Study. The plot illustrates the concordant with age and eventually matched their sin- changes in kin correlations that mediate the effect; rela- gleton siblings as closely as one another. The overall re- tively stable monozygotic twin (MZ) correlations from 2 sults suggested that the course of mental development is guided by the intrinsic scheduling of the genetic pro- years of age and decreasing same-sex dizygotic twin (DZ) gram acting in concert with maturational status and correlations after 4 years of age. The converging of sibling– environmental influence. (Wilson, 1978, p. 439). twin correlations with the DZ correlations at 6 years of age and the increasing age-to-age correlations complement the twin correlations. The effect is well defined by 5–6 years of The fact that the heritability of IQ increases with age is age when the age-to-age correlations approximate the MZ now a widely accepted phenomenon (Boomsma et al., 2002; correlations. Deary, 2012; Johnson, 2010b). Such acceptance, even recog- Figure 2 puts together a number of key studies (large and nition, was not always the case. Early studies focused mostly representative twin samples and some smaller but statisti- on children and seldom explored the effect of age. The classic cally powerful adoption designs) that illustrate the general- review of family studies by Erlenmeyer-Kimling and Jarvick ity of the Wilson Effect. (1963) does not mention age effects on kin correlations nor The Dutch data are from Boomsma et al. (2002) and (embarrassingly for this author) does the 1981 review by consist of eight cross-sectional twin studies that provide Bouchard and McGue (1981) or the review by Plomin and a frame of reference for the other studies. For illustrative DeFries (1980). As late as 1997, Devlin et al. (1997) asserted purposes, a polynomial curve (solid black line) has been there were no age effects on the heritability of IQ. fit to the estimates of genetic influence (heritabilities) and The purpose of this paper is to provide a striking illus- it reaches an asymptote at about age 20. The estimates of tration of this phenomenon and provide it with a name — shared environmental influence (common environment) ‘The Wilson Effect’ — in honor of Ronald Wilson, the devel- opmental behavior geneticist who most clearly articulated its existence and emphasized the importance of putting the RECEIVED 6 July 2012; ACCEPTED 11 June 2013. First published findings in both a developmental and evolutionary context online 7 August 2013. (Wilson, 1978, 1983). For those not familiar with Wilson’s ADDRESS FOR CORRESPONDENCE: Thomas J. Bouchard Jr., 280 work, Plomin (1987) provides a thoughtful summary of his Storm Peak Court, Steamboat Springs, CO 80487, USA. E-mail: career. [email protected] 923 Downloaded from https://www.cambridge.org/core. IP address: 170.106.40.219, on 29 Sep 2021 at 04:53:23, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/thg.2013.54 Thomas J. Bouchard FIGURE 1 Mental development correlations for MZ twins, DZ twins, twin–sibling sets, parent–offspring sets; and for each child with itself, age to age. FIGURE 2 Estimates of genetic and shared environmental influence on g by age. The age scale is not linear (see text for details). 924 TWIN RESEARCH AND HUMAN GENETICS Downloaded from https://www.cambridge.org/core. IP address: 170.106.40.219, on 29 Sep 2021 at 04:53:23, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/thg.2013.54 The Wilson Effect show a linear decrease (black squares) from about 0.55 at age from Scottish Mental Survey of 1932 and 1947’ (Benyamin 2 2 5 to 0.00 at age 12 and remain at 0.00 through 50 years of age. et al., 2005). The estimates are hb = 0.70 and c = 0.21. These findings are from an advanced industrialized society The large and intentionally population-representative Min- with a sophisticated social welfare system and provide a nesota Twin Family Study carried out a cognitive ability baseline against which other data can be compared. All assessment at age 11 as well. The results were almost iden- genetic estimators in this study and those to be added below tical, h2 = 0.67 and c2 = 0.26 (Johnson et al., 2005). 2 are of the broad heritability (hb ), which includes additive These combined data are shown as filled boxes on the line genetic variance, dominance genetic variance, and genetic drawn at age 11 years in Figure 2. There is also a Norwe- variance due to assortative mating (Vinkhuyzen et al., 2012; gian data set based on military recruits (again a compre- Visscher et al., 2008). Common or shared environmental hensive sample that includes all males tested in their late estimates (c2) differ slightly depending on the particular teens to early twenties) gathered from 1931 to 1960 (Sundet design, but the values are all quite low so it would take very et al., 1988). When we add those values, 0.68 and 0.16 and large samples to partition any differences. Most twin studies shown in braces {}, they fall right on the lines connecting the set c2 to zero if the estimate is not statistically significant. Haworth and MZA data. This is an unfortunate practice in our opinion. The very modest shared environmental influence by mid- How do the Dutch results compare to data sets generated childhood revealed by twin studies always puzzles psychol- in other settings? Haworth et al. (2010) combined the raw ogists, as they intuitively expect siblings to become more IQ data from six large twin studies (three from the United similar as they spend more time together in the same fam- States, one from the United Kingdom, one from Australia, ily and its social and educational environment. Therefore, and one from the Netherlands). These data were divided it is important to subject this counter-intuitive finding to into three age groups; mean ages of 9, 12, and 17 years. The independent tests. The most powerful design for estimating findings are shown as numbers with no associated coding this effect is to study unrelated siblings raised in the same 2 2 in the figure (hb = 0.41, 0.55, and 0.66; c = 0.33, 0.28, family (URT). When we examine this literature, we find an and 0.16). The heritabilities go up with age, and the line estimate of 0.26 in childhood (13 studies) and an estimate of runs parallel with the Dutch data but lies about 10% below 0.04 in adulthood (Bouchard, 2009, Figure 5). We note here it. There may be some overlap in the Dutch sample in this that Devlin et al. (1997) found these values to be extremely collection of samples with the Boomsma et al. (2002) Dutch heterogeneous and dropped them from their analysis. There data. are only five studies in adulthood; estimates from four clus- What happens if we add data from monozygotic twins ter are very close to zero and one (a United States study reared apart (MZA)? These twin correlations estimate the with participants at about age 20 is 0.19). Three of the five broad heritability directly (Bouchard, 2009). The mean of studies are longitudinal and all show the decline character- five studies of adults is 0.74. The line from the Haworth data istic of the twin studies in the figure. The childhood and at age 17 is projected (- - ->) to the MZA data and it largely adulthood data points are shown in Figure 2 as two aster- parallels the Dutch data. We can also estimate shared envi- isks connected with an arrow pointed to the later age. There ronmental influence on adult IQ by subtracting the adult are no twins in these studies. Thus, different methods (con- MZA correlation from the monozygotic twins reared to- structive replication) lead to the same conclusion and this gether (MZT) correlation. An estimate of the MZT corre- conclusion does not appear to be driven by the assumptions lation in advanced industrialized countries (Norway, 1181 underlying twin studies. A second way to estimate shared pairs, age 18 to 21) is 0.81 (Sundet et al., 2005), although environmental influence is by using a comprehensive adop- an earlier sample suggests 0.84 (Sundet et al., 1988).