It is surely part of loving altruism to bring up someone else’s child. It is not entirely selfless, for adoptive parents seek to have children by the best means possible, and to reap the rewards of the love they give in return for the child’s love of them. But, and it is a big proviso, adoptive parents have to be sanguine about how much they can influence their adopted infant. All the loving and hoping in the world will not change the child’s abilities a single jot. The child remains someone else’s child, product of other seeds and eggs.
Kevin M. Beaver, Joseph A. Schwartz, Mohammed Said Al-Ghamdi, Ahmed Nezar Kobeisy, Curtis S. Dunkel, and Dimitri van der Linden. A closer look at the role of parenting-related influences on verbal intelligence over the life course: Results from an adoption-based research design. doi:10.1016/j.intell.2014.06.002
Beaver et al. have taken the Add Health sample (a representative sample of 90,000 schoolchildren collected in 1995 which included a sample of adoptees) and used it to have a very close look at the putative effects of adoption on intelligence. If nurture has any effect on intellect, adoptive parents should have an impact on the abilities of their adopted children. We already know that about 60% of the variation in intelligence is due to genetic factors, (and that 56 to 94% of the covariance between SES and childhood intelligence is due to shared genetics), so it is a matter of great interest to find out what causes the remaining 40% . Being “socialized” or cultivated by specific family practices seems, prima facie, a possible cause of variation in mental ability. Very probably, if families have an influence on intelligence it is equally likely they would have an influence on behaviour, which should show up on personality assessments. Hence the interest in trying to disentangle purely genetic from truly familial environmental factors.
Sample attrition over the years resulted in a final sample of 286 adoptees, a good number as these things go, given the comparative rarity of adoption today. Intelligence was measured using the Peabody Picture Vocabulary Test, which the authors correctly describe as assessing verbal intelligence, though it is also one of the highest predictors of general intelligence. Eight parenting measures were used. By the way, you might want to think about what those measures should be. We assume that parenting does something, but what aspects of parenting? The sample were studied for both father and mother’s disengagement, attachment, involvement and education. The measure included how much children talked with each parent, and how close they felt to them. Of course, education is also a part surrogate for intelligence, so it is hardly a pure environmental measure, though it is often incorrectly treated as such. All 8 measures were related to children’s intelligence in the whole sample, but very weakly, at betas of .14 or less, often much less. For the adopted sample the highest beta was .16 and that was for paternal education. So, overall, parenting does not have much effect on children’s intelligence.
The authors conclude: The results thus far have revealed that parenting measures tend to have very little influence on variation in IQ scores in adolescence and young adulthood.
This is a very instructive negative result.
The authors then carry out a one-egg two-egg comparison: we use the MZ difference-scores method, where difference scores for IQ and all of the covariates are estimated and the twin pair is the unit of analysis. By using this approach, genetic and shared environmental influences are held constant (i.e., the only reason that there would be differences between MZ twins is because of nonshared environmental influences) and, in line with the adoption design, the effects of the parenting measures are not confounded by genetic influences. Importantly, though, the MZ difference score directly models the parenting measures as nonshared environments. This is particularly salient because findings from behavioral genetic studies have shown that nonshared, not shared, environments are the most influential for adolescent and adulthood IQ (Plomin & Spinath, 2004).
“Nonshared” is not an easy concept. I was really warming to Beaver et al. and now they have put their foot in it and require nomological re-education.
I think they mean that “personally created niches” account for variance, not the “standard family environment” provided by parents. You will see in my post, linked above, that one person described to me how as a child she build her own shed in the garden so that she could study on her own and avoid her disruptive parents. Apparently, this counts as a “non-shared” environment. I have difficulty believing that counting this as an environmental factor is being done with a straight face. The kid, being bright and sensible, decided she would do better in life if she avoided noisy family rows. It was her choice to move to the shed, and tidy it up and make it into a study. She created the environment: an act of creation, not a passive response to the woody smell of a garden shed.
Anyway, when MZ twins are studied, none of the differences they experienced in the way they were parented were significantly related to differences in intelligence.
In summary, this is a carefully presented analysis, showing an importantly negative result. The authors go through a number of possible explanations, and here is my immodest account of why they got their results. They had a much better sample than usual. They measured over a longer period than usual, from adolescence to young adulthood, which is when the finished product of family life hits the streets. They did a thorough job, and have shown that an expected effect of parenting was not present. Contrary to all expectation from strong environmentalism, the supposed formative effect of social class and family life does not accumulate: it diminishes as children leave home.
In the 60s we really thought that socio-economic status was like an artillery shell that fired a shell into the distance. The more wealthy, privileged and powerful the gun, the further the shell was fired (whatever the genetics of the child). It was all those books on the shelves, and the proper use of multisyllabic words at the dinner table that drummed ability into the crania of privileged brats. Take a child, any child, and put them at the dinner table (after a good wash and a medical examination) and by 7 they are on the road to mental adroitness, and by 17 surely ready to rise to the top and triumph over all. However, it turns out that by 17 kids are more like their real parents (whom they may have never met) than their adoptive parents. The “family pushes you forward by environmental means” hypothesis is not supported by the best available data.
The authors make a very good pair of final points:
While the results of the current study revealed convergence across all of the modeling strategies, we would advocate that the most methodologically defensible approach to use is a genetically sensitive research design. Two reasons inform this recommendation. First, there is ample empirical evidence showing that genetic influences account for a significant proportion of variance in IQ scores and in parenting measures (Jensen, 1998 and Kendler and Baker, 2007) and that at least some of these genetic influences overlap between IQ and family/parenting (e.g., Trzaskowski et al., 2014). What this necessarily creates is a prime example of a confounding variable, one that must be taken into account in order to rule out spuriousness. Second, even though the results from our non-genetically informative analysis revealed very little parenting influences on IQ, a large body of existing studies shows a very different pattern of results. It is quite possible that these significant parenting effects are simply due to genetic confounding (Harris, 1998) and the only way to know for certain is to employ methodologies capable of ruling out this explanation. Moving forward, therefore, studies need to more fully rule out genetic confounding before claiming that family and parenting influences represent causal contributors to IQ. Failure to do so will leave them open to attacks based on model misspecification and erroneous conclusions regarding the true effect of parental and family socialization effects on IQ.
In the plain language of this blog: when studying children’s progress into adult life, particularly their abilities and achievements, you must include measures of early IQ. If you don’t, you will misinterpret your results. If you misinterpret your results, this blog will hunt you down, and politely point out your error. Then you will be very embarrassed, and have difficulty explaining yourself, and your colleagues will shake their heads very sadly behind your back, whilst seeming to be supportive in public. In sum, it would be better to let intelligence measures play a part in your investigations.