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Why your mother affects your intelligence...
Differential psychology deemphasizes the impact of the family environment in general and of mothers’ behavior in particular. It is wrong to do so...
Written by Curtis Dunkel.
Tell me about your mother
What else would you expect a psychologist to inquire about? The belief that a mother’s behavior has a profound and lasting impact on her children is a foundational principle of many eminent theories in psychology. This makes intuitive sense; memories of one’s mother are often the most cherished or, alternatively, upsetting. We continue to have deep feelings concerning our mother’s loving or cruel behavior.
Differential psychology deemphasizes the impact of the family environment in general and of mothers’ behavior in particular. For example, it is now well established that by adulthood individual differences in general intelligence are primarily the result of differences in genetics and experiences unique to an individual. Given this evidence, some prominent psychologists have concluded that, absent abuse or neglect, parenting is of little importance.
The so-called Wilson effect, defined as the increasing heritability of intelligence from childhood into adolescence and adulthood, is often invoked to highlight the diminishing effect of the environment and the ultimate preeminence of genes. The term ‘Wilson effect’, coined by Thomas Bouchard, was named after Ronald Wilson who is credited as being the first to uncover this developmental trend. The effect can be seen in the figure below which is reproduced from Bouchard’s article.
The heritability of intelligence by adulthood is estimated ≈ .80, while shared environment (i.e., aspects of the environment that make siblings similar) may amount to zero. In other words, any impact of the early, shared family environment appears to get washed away with time. Conversely, the shared environment accounts for the majority of the variance in intelligence in the first years of life, and continues to account for a significant amount at least through early adolescence. At five years of age the shared environment is estimated to account for over 50% of the variance in intelligence (see red arrow in the Figure). Thus, from a developmental perspective, in which growth is a dynamic process, one could just as well reference the Wilson effect to demonstrate the importance of the shared environment in the first decades of life.
While the shared environment captures several factors outside the family (e.g., neighborhood, school, peers) there is also direct evidence for the effect of family. For example, Petrill et al. found that the shared-environmental effects associated with a chaotic home and low socioeconomic status negatively impact cognitive ability in early childhood. Likewise, in his book, Does your Family Make You Smarter? Nature, Nurture, and Human Autonomy, James Flynn reasoned that the intellectual level of the family environment may not perfectly match a given child’s genotypic potential, and this mismatch would be more extreme for children who were outliers. As such, the family environment would act as a slight pull downward for those children with higher genetic potential and a substantial push upward for those with lower genetic potential. Using data from IQ test manuals, he showed that the family environment can exert up to one-half a standard deviation effect on verbal intelligence and that this effect lasts through adolescence. However, these findings do not mean that a mother’s behavior is in any way the catalyst, only the family environment in the broadest sense.
Does a mother’s behavior specifically influence her child’s general intelligence and does this influence extend beyond the first decade of life? Simple associations between maternal supportiveness and her children’s intelligence are suggestive of such a link, though they are amenable to alternative explanations. For example, the arrow of causation could be reversed. The association between maternal supportiveness and a child’s intelligence might be because intelligent children elicit more maternal supportiveness. Or the association could be due to genetic confounding.
Several authors have attempted to address the potential confounds of simple correlational studies. Using a sample of twins, Roisman and Fraley examined the role of supportive parental behavior on the academic skills of young children. They found a significant association (r = .32). Consistent with the Wilson effect, they found that the shared environment not only accounted for the majority of the variance in each parental support and academic skills individually, but fully 95% of the variance in the correlation between the two variables. The remaining 5% was due to the nonshared environment, with the additive genetic component accounting for 0%. As expected, they also found that the specific variable of parental support, measured prior to kindergarten, was predictive of a child’s subsequently measured academic skills. As suggestive as the results from this study are, they do not directly address the questions of whether the effect of supportiveness is on general intelligence and whether its effect extends beyond childhood.
Several years ago, Michael Woodley of Menie and I conducted a study which suggested that maternal supportiveness had both a significant and a lasting impact on general intelligence. Specifically, we found maternal supportiveness at age five, as judged by several trained independent raters observing mother-child interactions, predicted IQ scores at ages 11 and 18 (see the scatterplot below). This effect remained, at least for verbal IQ, after several possible confounding variables were statistically controlled. However, the sample size of the study was quite small, increasing the need for replication.
Recently, Dimitri van der Linden, Tetsuya Kawamoto, and I were able to conduct a replication using the much larger Early Head Start Research and Evaluation Study 1996-2010 study. Our results were similar to the earlier work. Maternal supportiveness predicted intelligence and continued to do so throughout childhood. When several possible confounding variables were statistically controlled, the associations were diminished in strength, though they remained intact. Additional checks showed that while a mother’s tutelage may focus on specific skills like reading or vocabulary, it also appears to be on general intelligence.
In summary: given the weight of the evidence, it seems reasonable to conclude that maternal supportiveness in early childhood has a positive influence on a child’s general intelligence, and that while this impact wanes it remains present at least until early (if not late) adolescence.
But, if the effect washes away, does having a supportive mother really matter?
Yes. To begin with, I think it is wrong to conceptualize adulthood as the destination of development. A day in your life when you are 14 seems just as important when experienced, if not more so, than a day in your life at age 44. For example, there is increased recall for experiences that occur during adolescence (the reminiscence bump), which highlights the importance of this period in development. If maternal supportiveness enhances ability in adolescence, then it also increases the extent of our successes during this important period, while reducing the extent of our failures. Since adolescence is a period of intense identity exploration, these successes and failures become part of the personal narrative of our life.
Second, while the focus of this article was on cognitive ability, the effects of maternal supportiveness are much more extensive than cognitive ability. For example, in the study by Roisman and Fraley that was previously referenced, they found that maternal supportiveness also had a positive impact on both social-skills and mental health. This suggests that maternal supportiveness has a wholistic positive influence. The ability to form prosocial friendships and maintain a sense of well-being are clearly beneficial in and of themselves at any age. However, even if we focus solely on outcomes that are often seen as being primarily dependent on cognitive ability (e.g., academic success) it should be clear that social ability and mental health may play at least a supporting role.
Third, while the effect of maternal supportiveness on general intelligence eventually fades, the indirect effects may remain. A slight edge in cognitive performance at critical junctures during a child’s development may have a sizeable impact on important outcomes. Being accepted to an elite college or university is highly valued and is considered by many to be the ticket to a better life. There are various examples of parents going to extreme lengths for a seemingly minor increase in the chance of an acceptance. Thus, if maternal supportiveness has just a slight effect on intelligence in the 2nd decade of life, this should lead to better scores on standardized tests like the SAT. Such a boost would be seen as valuable to many parents.
Curt Dunkel is a former Professor of Psychology. He is now an independent scholar with interests in evolutionary psychology and individual differences.
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