23rd November 2023

Nature and Nurture

‘Give me a dozen healthy infants … and I’ll guarantee to take anyone at random and train him to become any type of specialist I might select – doctor, lawyer, artist, merchant-chief and, yes, even beggar-man and thief, regardless of his talents, penchants, tendencies, abilities, vocation, and race of his ancestors.’– John Watson

Temperament

Academic psychologists have come a long way from the radically behaviouristic and naively pragmatic era in which John Watson could boast those words above. A generation of developmental scholarship has demonstrated to us that human beings are anything but blank slates at birth. From shyness to stimulation seeking, we know that people’s attributes are genetically influenced and cannot be seen as merely the result of their upbringing. But how does one affect the other?

The nature of a person is their temperament and other fixed attributes. As articulated by Arnold Buss and Robert Plomin (1987), temperament refers to ‘traits observable by 2 years of age that are genetic in origin’. The three core dimensions of temperament include emotionality, activity, and sociability, which are believed to be precursors for adult personality.

Alexander Thomas and Stella Chess (1968) employed factor analysis in their New York longitudinal study, a 30-year longitudinal study (initially 6 years) of 138 children, observing childhood temperaments during their infancy and followed up at 5, 18, and 22 years. The study identified nine temperamental features, including activity level, biological rhythms, sensitivity, intensity of reaction, adaptability, approach/withdrawal, persistence, distractibility, and mood, which they subsequently grouped into four distinct clusters.

• ‘Easy child’ – 40%
• ‘Difficult child’ – 10%
• ‘Slow to warm up a child’ – 15% 
• Does not fall into a category – 35%

Children’s temperament is only one example of the unchangeable aspects of people’s individual psychologies. But what about the things that can be changed – their nurture?

Examination of gene-environment interactions 

In the RCPsych syllabus, the gene-environment debate is focused on intelligence.

We must approach the concept of human intelligence with a nuanced understanding of its complex nature. While studies below estimate a heritability of 40-50% for IQ, the relationship between genes and IQ can vary depending on the environmental context.

For example, the impact of genetic factors on IQ is more pronounced in individuals from high socioeconomic status (SES) families than in individuals from low SES families. According to Turkheimer et al.’s (2003) meta-analysis, in low SES families, environmental factors such as poor nutrition, lack of access to education, and exposure to environmental toxins may hinder the expression of genetic potential for intelligence.

Tucker-Drob et al. (2013) examined the relationship between family SES and the heritability of IQ in a sample of over 10,000 twins and also found that the heritability of IQ was higher in high-SES families compared to low-SES families and that the environmental context accounted for a significant portion of the variation in the heritability estimates.

The Minnesota transracial adoption study (MTAS) (Scarr & Weinberg, 1977; Weinberg, Scarr, & Waldman, 1992) also provides evidence for gene-environment interaction in the development of IQ. This study found that Black and interracial children whom White families adopted had higher IQ scores than those who remained in foster care or were adopted by non-White families. The study also found that the IQ scores of the adopted children were more strongly correlated with the IQ scores of their adoptive parents than their biological parents, suggesting that the environmental context had a greater impact on their IQ than their genetic background.

Plomin and Deary (2015) argue that intelligence is a highly polygenic trait, with many genes involved in its development, each contributing only a small amount to the variation in IQ scores. They suggest that genetic influences on intelligence are likely to be highly complex and interactive, with multiple genetic variants working together in different ways at different stages of development.

Savage, Jansen and Stringer et al. (2018) conducted a genome-wide association study (GWAS) to identify genetic variants associated with IQ and found that the effects of individual genetic variants on IQ were small and highly variable, with no single variant accounting for more than a small fraction of the variation in IQ scores. While genetic influences on intelligence are highly polygenic and complex, there is evidence for some genetic overlap between different cognitive abilities.

These studies suggest that the development of IQ is influenced by a complex interplay between genetic and environmental factors, with the impact of genetic factors on IQ being modulated by the environmental context in which the individual develops.

References 

• Goldsmith, H. Hill, et al. “Roundtable: What is temperament? Four approaches.” Child development (1987): 505-529.
• Thomas, A., Chess, S., & Birch, H. G. (1968). Temperament and behaviour disorders in children. New York: New York University Press
• Turkheimer, E; Haley, A; Waldron, M; D’Onofrio, B; Gottesman, II (November 2003). “Socioeconomic status modifies heritability of IQ in young children”. Psychological Science. 14 (6): 623–8. doi:10.1046/j.0956-7976.2003.psci_1475.x. PMID 14629696.
• Briley, Daniel A. and Elliot M. Tucker-Drob. “Explaining the Increasing Heritability of Cognitive Ability Across Development.” Psychological Science 24 (2013): 1704 – 1713.Weinberg, Richard A., Sandra Scarr, and Irwin D. Waldman. “The Minnesota Transracial Adoption Study: A follow-up of IQ test performance at adolescence.” Intelligence 16.1 (1992): 117-135.
• Plomin, Robert, and Ian J. Deary. “Genetics and intelligence differences: five special findings.” Molecular Psychiatry 20.1 (2015): 98-108.
• Savage, J.E., Jansen, P.R., Stringer, S. et al. Genome-wide association meta-analysis in 269,867 individuals identifies new genetic and functional links to intelligence. Nat Genet 50, 912–919 (2018)