A large body evidence points to the increasing heritability of intelligence with age (McGue et al., 1993; Plomin et al., 1997; Posthuma et al., 2002; Haworth et al., 2010). In adulthood, shared environmental influences may fall close to zero. These findings are consistent with the persistent failure of educational intervention in raising the cognitive abilities of poor children. Although several studies (Rowe et al., 1999, pp. 1157-1158; Turkheimer et al., 2003, pp. 625-627; Kremen et al., 2005, pp. 419, 427-429; Harden et al., 2007) found that the genetic influences on IQ vary with socio-economic status, also known as GxE interaction, research seems to be highly controversial. Numerous studies (Thompson et al., 1993; Asbury et al., 2005, pp. 653, 656; Nagoshi and Johnson, 2005, pp.776-780; van der Sluis et al., 2007, p. 358; Hoekstra et al., 2007, p. 112; Grant et al., 2010, pp. 443-444; Hanscombe et al., 2012, pp. 8-14) have also shown that the genetic influences do not decrease among low-SES families. But this debate neglects the very heart of the entire matter. As Herrnstein and Murray have noted in The Bell Curve, page 314 :
For practical purposes, environments are heritable too. The child who grows up in a punishing environment and thereby is intellectually stunted takes that deficit to the parenting of his children. The learning environment he encountered and the learning environment he provides for his children tend to be similar. The correlation between parents and children is just that: a statistical tendency for these things to be passed down, despite society’s attempts to change them, without any necessary genetic component. In trying to break these intergenerational links, even adoption at birth has its limits. Poor prenatal nutrition can stunt cognitive potential in ways that cannot be remedied after birth. Prenatal drug and alcohol abuse can stunt cognitive potential. These traits also run in families and communities and persist for generations, for reasons that have proved difficult to affect.
To some extent, environments are heritable and, therefore, hard to manipulate. Further evidence by Robert Plomin in “Behavioral Genetics in the Postgenomic Era” (2003, pp. 189-190). And don’t mention that SES level is mainly determined by IQ. Furthermore, a mother’s IQ is more important than her SES in predicting the IQ of her children. Home environment is also influenced more by mother’s IQ than by her SES. See The Bell Curve (1994, pp. 230-231, 222-223). Consistent with the increasing heritability of IQ with age, the test-retest correlation increases with age, which means that IQ tests in two different points of time show less volatility as children grow. But even at age 6, IQ test scores seem to stabilize (Randy W. Kamphaus, 2005, pp. 66-68). Clinical Assessment of Child and Adolescent Intelligence by Randy W. Kamphaus
Intelligence test scores seem to stabilize at about age 6. Schuerger and Witt (1989) reviewed 34 studies of test-retest reliability for the WAIS, WAIS-R, WISC, WISC-R, and several editions of the Stanford-Binet, exclusive of the Binet 4. Using multiple regression procedures they found that age and interval between tests were the two variables most predictive of changes in intelligence test scores. [...] Schuerger and Witt (1989) found stability coefficients to be high, even for 6-year-olds. The coefficients for 6-year-olds ranged from .85 for a 1-week interval to .67 for a 20-year interval. Stability was still better for 39-year-olds where coefficients ranged from .99 for 1 week to .82 for 20 years. This relationship between age/interval and stability of test scores is depicted graphically in Figures 3.1 and 3.2. Stability is maximized as age increases and interval between test administration decreases.
Wechsler Verbal, Performance, and Full Scale scores have all been found to be very stable for children participating in special education classes (Canivez & Watkins, 1998). Cassidy (1997) compared the Wechsler score for 592 children who were enrolled in special education classes for a 3-year period. She found that for the group as a whole the scores did not significantly differ over this time period. There was also a tendency for deviant scores to regress toward the mean upon retest. Children with Full Scale scores below 90 score higher upon retest by 1 or 2 points, whereas children with scores above 109 scored lower by 3 or 5 points when retested. Interestingly, IQ test scores display a slight increase due to familiarity. This finding is consistent with the decline in g-loadedness of intelligence tests through training. See Section 9, 10 and 11. These same Wechsler scores are remarkably consistent in late adulthood. A study of 70-year-olds (mean age = 72) who were administered the WAIS-R one year apart produced a remarkably high test-retest coefficient of .90 (Raguet, Campbell, Berry, Schmitt, & Smith, 1996). Of particular interest is the finding that there was a slight rise in scores from time one to time two of about 3 standard score points for the Full Scale (mean = 111.5 at time one and mean = 114.7 at time two). The authors attributed this mild increase to practice effects, that is, the tendency for scores to improve due simply to familiarity with the item types.
What might account for the stability in IQ test scores ? In the words of Kamphaus (2005, p. 69) :
It is possible that the lack of change in scores for most children is due to more than one “fixed” entity (i.e., genetics). Environments may be “fixed” for most children as well (Reiss, et al., 2000).
That IQ test scores increase with training, due to familiarity, is quite expected since IQ tests are not ordinarily designed to withstand practice. That does not mean, however, that these IQ gains represent an increase in real intelligence, as suggested by Susanne Jaeggi. In fact, IQ tests lose their g-loadedness over time as the result of training and retesting (te Nijenhuis et al., 2007).