Genetic and Environmental Influences on Intelligence

Heritability and Intelligence

• 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).

Environment and Intelligence

• 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.

Gene-Environment Interaction

• 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).

Group Differences in Intelligence Test Scores

• 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).

The Question of Bias in Testing

• 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.

Test Takers’ Expectations

• 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).