Heritability Defined: Heritability is the proportion of variation among individuals in a group that can be attributed to genes.
Estimates of intelligence heritability range from 50% to 80% (Madison et al., 2016; Plomin et al., 2016; Plomin & Von Stumm, 2018).
Heritability applies to populations, not individuals.
Understanding Heritability:
Heritability is population-specific and varies from study to study.
High heritability in a uniform environment: If environments are equal, differences in intelligence are mainly due to heredity.
Low heritability in diverse environments: If heredity is equal (e.g., clones), differences in intelligence are mainly due to environment.
Twin and Adoption Studies
Identical twins (same genes) share similar mental abilities, even when raised separately.
Intelligence test scores of identical twins raised apart are highly correlated.
Twins also show similarity in specific talents like music, math, and sports (Haworth et al., 2009; Licken, 2006; Plomin et al., 2016).
The Polygenetic Nature of Intelligence:
There is no single "genius gene."
Intelligence is polygenetic, involving many genes (Kaiser, 2020; Johnson, 2010).
Gene variations account for a small percentage of differences in educational achievement (Savage et al., 2018; JayLee et al., 2018).
Similar to height, intelligence is determined by a combination of many genes (Sniakers et al., 2017).
Environmental Influences:
Fraternal twins, sharing similar environments, have more alike intelligence scores than non-twin siblings.
Adoption studies help assess environmental influence by comparing adopted children's scores with biological and adoptive parents.
Shared environment has a modest influence on intelligence.
Adoption into middle-class homes enhances intelligence scores (Nisbik et al., 2012).
A Swedish study showed adopted children's IQ scores were 4.4 points higher than non-adopted siblings (Kendaller et al., 2015).
Adoption of mistreated/neglected children enhances intelligence (Almas et al., 2017).
Virtual twins (unrelated, same-age, raised together) show a positive correlation in intelligence scores (Segal et al., 2012).
The Diminishing Role of Shared Environment:
As adopted children age, their intelligence scores resemble biological parents more than adoptive parents (Lowland, 2016).
Genetic influences become more apparent with life experience.
Identical twin similarities increase into their eighties.
Variability of general intelligence increases with age (from 41% in middle childhood to 66% in young adulthood) (Power Fettol, 2010; and his colleagues, 2009a, 2012).
General intelligence variability increases from 30% in early childhood to over 50% in adulthood.
Dynamic Interaction:
Genes and experience together shape intelligence.
Epigenetic studies explore the dynamic biology of nature-nurture interaction.
Genes shape experiences that further shape us.
Natural aptitudes lead to more practice and experience, increasing skills (Cheesman et al., 2020; Sauce and Matzil, 2018).
Small genetic advantages can trigger social experiences that multiply original skills.
Adverse Environmental Conditions:
Severe deprivation negatively impacts cognitive development.
Neglect in orphanages can lead to developmental delays (Khan, 1982).
Extreme deprivation crushes native intelligence (Nelson et al., 2009, 2013; Van Eijssendorn et al., 2008).
Benefits of Early Intervention:
Training programs for caregivers can improve infant development.
Early intervention can lead to significant improvements in cognitive abilities.
Poor environmental conditions associated with poverty can depress cognitive development (Head Roland Carter, 2015; Turk, 2005).
Environmental differences are more predictive of intelligence scores among children of less educated parents (Tucker drop and Bates, 2016).
Worries and distractions consume cognitive bandwidth and diminish thinking capacity (Mani et al., 2013).
Enriched Environments:
Normal exposure to sights, sounds, and speech is essential.
Special educational lessons for babies may not be effective (Scares, 1984).
Intensive preschool programs, especially for children in poverty, can be beneficial (Dodge et al., 2017; Sasser et al., 2017; Tucker drop, 2012; Gormley et al., 2013; Heckman and Karen Pakula, 2019; Magnuson et al., 2007).
Intelligence scores rise with nutritional supplements, quality preschool, and interactive reading programs (Protsko et al., 2013).
Schooling and intelligence interact, enhancing later income (Cece and Williams, 1997, 2009).
Motivation and Mindset:
Study motivation and skills rival aptitude in predicting academic achievement (Credit and Kunsel, 2008).
Intelligence test performance can be affected by motivation (Duckworth et al., 2011).
Believing intelligence is changeable fosters a growth mindset (Dweck, 2018; Dweck and Yeager, 2019).
Praising effort and challenges helps children link hard work and success (Gunderson et al., 2013).
Growth mindset can make children more resilient (Pennantellus, 2020; Walton and Wilson, 2018).
Growth mindset interventions can improve grades (Yeager et al., 2019).
Companies with growth mindset endorsed by mission statements have more trusting and committed employees (Canning et al., 2020).
Ability + opportunity + motivation = success (Ericsson et al., 2007).
Self-discipline, belief in effort, and a curious mind are crucial (Murayama et al., 2013; Richardson et al., 2012; Vonsdom et al., 2011).
Observed Differences:
Racial and ethnic groups differ in average intelligence test scores.
High-scoring groups are more likely to attain higher education and income.
Examples: New Zealanders of European descent outscore native Maury, Israeli Jews outscore Israeli Arabs, White Americans have outscored Black Americans (Dickens and Flynn, 2006; Nisbet et al., 2012).
Interpreting Group Differences:
Group differences should not be used to judge individuals.
Heredity contributes to individual differences, but group differences can be environmental.
Environmental Factors:
Language exposure affects test scores.
Genetic research shows humans are remarkably alike under the skin (Cavalli Sforza et al., 1994; Rosenberg et al., 2002).
Race is a social construction without well-defined physical boundaries (Helms et al., 2005; Smedley and Smedley, 2005).
Racial categorization is increasingly complex due to mixed ancestries (Punker et al., 2009).
Test scores of today's populations exceed those of past generations (Flynn, 2012; Pete Snig and Boruchak, 2015; Trahan et al., 2014).
Socioeconomic Factors:
Wealth inequality correlates with intelligence test score gaps (Nismuth, 2009).
Rising income inequality means less equal access to education (Jackson and Holtzman, 2020).
The Role of Education:
Each additional year of school predicts an increase in IQ points (Ritchie and Tucker drop, 2018).
Cultural Factors:
Math achievement and grades reflect conscientiousness more than competence (2014).
Students in Asia spend more time studying (2018).
Defining Bias:
Scientific definition: A test is biased if it predicts future behavior differently for different groups.
Major US aptitude tests are generally not biased (Barry Anjou, 2015; Nieser et al, 1996; Wictor and Garner, 1982).
Predictive validity is similar across gender, race, ethnicity, and socioeconomic level.
Everyday language: A test may be considered biased if it is unfair.
Culture-Fair Tests:
Culture-neutral questions assess ability to learn novel information (Fagan and Holland, 2007, 2009)
It is important to not blame tests for exposing unequal experiences and opportunities.
Expectations and attitudes influence test performance.
Stereotype threat: Self-fulfilling prophecy where negative stereotypes undermine performance (Spencer et al., 1997, 2016; Steele et al., 2002).
If people worry that their group doesn't do well, self-doubts impair performance (Hutchison et al., 2013; Inslichten Kang, 2010; Rydell et al., 2010).
Critics argue that stereotype threat effects are weaker than originally thought (Flora and Witchards, 2015; Flora et al., 2019).