Nature vs nurture in childhood intelligence

Introduction

Intelligence is reasoning, planning, solving problems, thinking abstractly, comprehending complex ideas, and learning from experience. This broad and deep capacity for understanding our surroundings can be measured using standardised tests, such as Intelligence Quotient (IQ) tests, picture vocabulary tests for verbal ability and matrix reason tasks for non-verbal ability. This article explores how nature facilitates the development of intelligence in childhood through twin studies, which highlight genome-wide association studies. On the other hand, how nurture can further aid in developing intelligence in childhood through social environmental influences and investigate the association between parent socio-economic status (SES) and intelligence. However, there is no definite answer to whether only nature or nurture leads to the development of intelligence in childhood. Therefore, genotype-environment correlations are also explored.

Nature Argument

Nativists such as Jensen (1969) believe that intelligence is determined by nature-genetic makeup and estimate that 80% of the heritability in IQ is accounted for by genes. One way to determine the heritability of intelligence is by using twin studies. McGue and Bouchard (1998) reviewed five studies of monozygotic (MZ) twins who were reared apart and found that when accounting for heritability of intelligence, there was a correlation of 0.86 in MZ twins compared to the 0.60 correlation in dizygotic (DZ) twins. Hence, nature does play a role in the development of intelligence in childhood, because the monozygotic/MZ twins didn’t share the same environment but did share the same genes and had a higher correlation than dizygotic/DZ twins, suggesting that intelligence is heritable. Genome-wide association studies (GWAS) further investigate the relationship between genetic sequences and intelligence by examining individual chromosomal markers, such as single nucleotide polymorphisms (SNPs). Butcher et al (2008) conducted a genome scan using 7000 subjects and found six SNPs associated with intelligence, indicating that it is polygenic. The correlation between SNP-set scores and g scores is 0.105. Squaring this correlation indicates an effect size of 1.1% comparable to the sum of the effect sizes of the six SNPs. Figure 2 depicts a genotype-by-phenotype plot illustrating the relationship between SNP-set scores and standardised g. Identifying target alleles and SNP associations in genome polygenic scores has helped account for the heritability of intelligence in childhood. However, due to intelligence being polygenic, the contribution of any individual locus is small. Therefore, genomic variance only explains 10%, which makes it very difficult to detect relevant SNPS without huge samples. Foreseeable advances in genetic technology can mitigate this problem. 

Nurture Argument

Alternatively, empiricists emphasise the family environment, socioeconomic status, and schooling, where schooling is a social influence. Sternberg et al., 2001 claim that pursuing higher education and schooling is associated with higher IQs. Dearey et al (2007), in their 5-year longitudinal study, recruited approximately 70,000 children and found a large overall contribution of intelligence to educational attainment, with an average chance of 58% of attaining grades between A and C. Therefore, their study establishes educational attainment for intelligence as an environmental outcome. However, the decision to pursue education may not be motivated by intelligence but may result from social causation, suggesting that social-economic conditions influence intelligence.  The relationship between a parent's SES and a child's intelligence also exemplifies the role of nurture in the development of intelligence. This is further supported by Turkheimer et al (2003), where the authors concluded that in families with low levels of SES, 60% of the variance in IQ is explained by the shared environment, while in affluent families, all variation was accounted for by genes. However, parents with higher levels of intelligence may qualify for better-paying jobs. Hence, they have higher levels of SES, referring to social selection — when individuals influence the quality of their socio-economic environment — and genetics. Meanwhile, impoverished families do not get to develop their full genetic potential, and thus, the heritability of IQ is very low. Conversely, adoption can be seen as a social intervention that moves children from lower to higher SES homes and explores the gene-environment interplay in the development of intelligence. Kendler et al. (2015) studied 436 full male siblings, separated at birth, and tested at 18–20 years. A comparison was made between pairs of separated siblings (one raised in their biological family, the other in an adoptive family). Adopted-away siblings tested 7.6 points higher than their biological siblings when their adoptive parents had higher education levels than their biological parents (such as high school versus some postsecondary education).

Gene-environment interplay

According to Lerner et al. (2015), nature and nurture are inextricably linked and never exist independently of each other. In this way, the nature-nurture dichotomy presented in the title may be false. Gene-environment (GE) interplay offers two concepts: GE interaction and GE correlation. GE interaction is where the effects of genes on intelligence depend on the environment. GE correlation can be explained through adoption studies that compare genetically unrelated and related individuals. Supporting evidence from Van Ijzendoorn et al (2005) indicates that children who were adopted away from institutions had a better IQ than those children who remained in institutional care. Using 75 studies involving 3,800 children from 19 countries, a meta-analysis compared the intellectual development of children living in orphanages to those living with their adoptive families. On average, children growing up in orphanages had an IQ of 16.5 points lower than their adopted peers.  This illustrates how adoptive families who typically have higher SES levels can assist children in achieving higher levels of IQ. However, the generalisability of Ijzendoorn's findings can be questioned as they used participants who were highly deprived in institutional settings, suggesting that their cognitive development is at risk. Furthermore, Neiss and Rowe (2000) contradicted Ijzendoorn’s findings by comparing adopted children to birth children to estimate the genetic-environmental effect of the mother's and father's years of education on the child's verbal intelligence. In biological families, mother-child (0.41) and father-child (0.36) correlations were significantly higher than in adoptive families (0.16). This implies that the adoptive parent's home environment has modest effects on the children's cognitive abilities, whereas the heredity and environment of the birth parents exert a profound influence. 

Conclusion

In conclusion, both nature and nurture represent their significant role in childhood intelligence development, as they both offer testable evidence through twin studies and socio-economic correlations. Nevertheless, scientists have claimed that both genetics and environmental factors will predominantly influence the development of intelligence in childhood. This essay and future research in this field demonstrate that intelligence can be malleable, especially in children, through major social interventions and that the environment will continuously affect gene action. 

Written by Pranavi Rastogi