IQ

What is the historical development of genetics?

Pythagoras suggested that the father provided all the essential characteristics of the offspring, and that the mother contributed the environment for the offspring to grow 

Aristotle suggested that both the mother and the father contributed to the offspring’s traits, by arguing that both purified blood (semen) and menstrual blood contributed 

Darwin proposed his theory of evolution through natural selection 

Mendel proposed his laws of Mendelian genetics (law of dominance, law of segregation, law of independent assortment) 

Galton conducted the first twin studies  

Fisher unified Mendelian genetics and biometric approaches  

What is heritability?

Heritability = the proportion of variation associated with genetic factors in a specific environment 

Twin studies compare similarities between monozygotic and dizygotic twins  
Adoption studies compare adopted children to both their biological and their adopted parents 
Limitations = these studies may overestimate the role of genetics due to gene-environment correlation (genes can influence the environments that people are exposed to) e.g. identical twins may seek out similar environments or adopted children may seek out similar environments to their biological parents 

Results of twin and adoption studies 
(Bouchard, 1990) conducted a long-term study in which monozygotic twins were reared apart, and they found that about 70% of intelligence is due to genetics 
Limitations = small sample size (~100 pairs of twins) 

(Plomin, 2015) showed that heritability increases from ~20% in childhood to ~80% in adulthood – as we grow older, our genetic predispositions have more opportunity to influence our behaviour 

(Rimfeld, 2018) showed that heritability can change over time as they found that, after Estonia’s shift to capitalism, genetic influences on education and job statuses increased, suggesting that in more merit-based societies, genetic traits played a greater role in determining success 
(Jelenkovic, 2016) showed that heritability can remain constant over time when they found that, over time, differences in height has increased, but the proportion of those differences explained by genetics (heritability) has remained constant 

What are the misconceptions of heritability?

Heritability doesn’t imply causality (genes do not directly cause something) 
If something is highly heritable, it doesn’t mean that it cannot be influenced by the environment 
Heritability tells us nothing about the average value of a trait

What is gene-environment correlation?

Passive correlation = parents provide both the genes and the environment 
Reactive correlation = a child’s natural cognitive abilities will provoke responses from the environment automatically 
Active correlation = individuals actively seek out environments that align with their genetic predispositions 
(Abdellaoui, 2019) suggests evidence of an active correlation – they showed that individuals with genetic variants associated with higher educational achievements were more likely to move out of areas where there were fewer educational and economic opportunities

How can we measure intelligence?

Galton attempted to measure intelligence by suggesting that there is a genetic basis to intelligence (his argument being that eminence runs in families) 
Galton’s theory of intelligence = the quicker a person can distinguish between subtle differences in things (which can be measured through RTs), the more intelligent they are 

Binet developed tests of mental ability for children based on what an average children of a specific age could solve 
Limitations = this method relies on comparing individuals to age-based norms, but because there’s no increase in mental age during adulthood, this approach can’t be applied to adults (there is no average age to compare them to) 

Weschler measured intelligence by looking at deviation IQs – how much a particular score deviates from the average score of a test 
Weschler Adult’s Intelligence Scale (WAIS-R) consists of 11 subtests, 6 verbal and 5 performance (intelligence can manifest itself a variety of ways, so the more subtests, the better) 
Raven’s Matrices is comprised of abstract reasoning and has been shown to correlate well with the WAIS-R 

Criteria for IQ tests 
Construct validity = scores on an IQ test should correlate with another independent measure of someone’s ability e.g. exam results 
Reliability = IQ scores should remain consistent 

What can be said about performance on different IQ subtests?

Spearman’s g = a single factor of general intelligence that measures the underlying performance on a range of subtests 
^The consistent positive correlation between IQ tests and subtests suggests that all cognitive abilities draw on a common underlying factor (g) 
(Jensen, 1998) found strong correlations between different IQ subtests and argued that these correlations reflect a general intelligence factor 

IQ tests don’t correlate perfectly, which suggests that intelligence is likely made up of several different abilities, rather than just one single ability (spearman’s g is an oversimplification)  

(Cattell, 1963) proposed a two-factor theory of intelligence in which he distinguished between fluid intelligence (solving novel problems) and crystallised intelligence (using prior knowledge to solve problems), and he argued that spearman’s g could be broken down into these factors 
Limitations = it is difficult to separate Gf and Gc in practice (many tasks involve both) 

(Carroll, 1993) proposed the hierarchical three-stratum theory – stratum III (g), stratum II (broad abilities like Gf, Gc), stratum I (narrow, specific abilities under each broad category) 
Limitations = this doesn’t account for how intelligence develops over time, or how contextual and cultural factors can shape cognitive abilities 

Raven’s Matrices measures Gf (a key component of g) 

How does intelligence differ across populations?

(Moore, 1986) found that Black children adopted by middle-class White families scored higher on the WISC than those adopted by middle-class Black families 
This is due to more positive and supportive behaviours from White adoptive mothers compared to Black adoptive mothers 

(Tucker-Drob, 2015) found that in the US, intelligence heritability is lower in poorer children, whereas in Europe, heritability of intelligence is more consistent among those with different socio-economic backgrounds (because there is more equal access to education opportunities) 
Environmental factors can reduce the influence of genetics on intelligence 

Flynn effect = the rise in average IQ scores across generations, likely due to improved nutrition, education, and the modern demands of modern society 

(Hsin, 2014) found that Asian Americans’ higher academic achievements compared to White students is mainly due to greater academic effort rather than differences in cognitive ability 
Intelligence can be impacted by environmental factors rather than innate cognitive ability alone 

What biases exist in IQ tests?

IQ tests are biased if they use upper-class English vocabulary, if they were designed by White (males) having other White people in mind, if the examiners are typically white, or if they reflect White values and culture 

(Wicherts & Dolan, 2010) showed that a Dutch IQ test was biased against ethnic minority students 

• Use culture-free IQ tests e.g. Raven’s Matrices 

• Control for socioeconomic and educational backgrounds 

• Rely more on visual and non-verbal problem-solving to minimise language load 

• Measure multiple domains of intelligence (Gf and Gc) 

(Gonthier, 2022) found that cultural differences in how people perceive visuo-spatial information can impact performance on tests like Raven’s Matrices 

How can we overcome these biases?

To reduce language and cultural biases, we should emphasise fluid intelligence e.g. visual working memory tasks such as Corsi block-tapping task 
We can further reduce cultural biases by using abstract and symbolic information (which isn’t reliant on language) and ensuring cross-cultural validity testing – piloting the test across different populations 

It is difficult to control for socioeconomic status at the level of the individual, but we can collect detailed background information (that might impact socioeconomic status), use statistical regression to determine whether socioeconomic status significantly mediates or moderates test performance 

We could also implement a modular structure, which involves designing the IQ test in independent sections so that researchers can administer the relevant sections, rather than needing to use the full test