Genetic & environmental influences on intelligence

Heritability: statistic used in the fields of breeding and genetics that estimates the degree of variation in a phenotypic trait in a population that is due to genetic variation between individuals in that population. Studies of heritability generally ask questions such as how much genetic factors play a role in certain characteristics.

Heritability of intelligence: heritability of IQ for adults is between 58% and 77%. Heritability of IQ increases with age.

DNA (deoxyribonucleic acid): thread-like chain of nucleotides that carry the genetic instructions used in growth, development, functioning and reproduction of all known living organisms and many viruses. Most DNA molecules consist of 2 biopolymer strands coiled around each other to form a double helix.

Genes: a sequence of DNA or RNA which codes for a molecule that has a function. Genes can make up different DNA sequences called genotypes

What we know from studies on behavioural genetics (Plomin, DeFries, Knopic, & Neiderheiser (2016)

• All psychological traits show significant and substantial genetic influence
• No traits are 100% heritable
• Heritability is caused by many genes of small effect
• Phenotypic correlations between psychological traits show significant and substantial genetic mediation
• The heritability of intelligence increases throughout development
• Age-to-age stability is mainly due to genetics
• Most measures of the 'environment' show significant genetic influence
• Most associations between environmental measures and psychological traits are significantly mediated genetically
• Most environmental effects are not shared by children growing up in the same family
• Abnormal is normal

• 5-12% of the variance in mathematics at age 12 is explained by drawing at 4
• Correlation between drawing at 4 and mathematics at 12 is mostly explained by genetic factors (82%) and shared environmental factors (Malanchini et al, 2016)
• Genetics affects choice of academic subjects as well as achievement. Heritability estimates were 44% for choosing to do A-levels and 52-80% for choice of subject. Achievement after 2 years was also highly heritable (35-76%). This shows DNA differences substantially affect differences in subject choice as well as aptitude
• Meta-analysis identified approximately 74 independent genome-wide significant loci associated with educational attainment
• Since monozygotic twins raised apart do have slightly lower intelligence correlations than monozygotic twins reared together, genes are not the whole story

Environment seems to account for between .50 - .30 of the variance among intelligence scores. Environmental influences can be shared (e.g family environment, some conditions in the womb), and unshared (e.g brain damage after birth, measurement error).

Children who are adopted from deprived backgrounds show an increase in IQ, from around 10-12 points (Schiff & Lewontin, 1986). Children who remain in deprivation show no increase over time, and can actually show some decrease (Sepell, 2000). IQ scores correlated about .40 with the kind of home the child is raised in (Lubinski, 2004). The longer the child has been in this deprived environment, the lower their IQ. Also, the older the child, the lower the IQ.

Enriched environments have a good effect on IQ. In an isolated community in East Tennessee around 1930, the introduction of schools, roads, and radios led to the community becoming less and less isolated. Between 1930-1940, the average IQ of people in this community rose by 10 points from 82-92 (Wheeler, 1942). Similar effect found in the Venezuelan Intelligence Project (Hernnstein et al, 1986) which provided underprivileged children with extensive, theoretically-based training in various thinking skills. Assessments after such training showed substantial benefits on a wide range of tests

Schooling fosters the development of the cognitive processes that underpin performance on most IQ tests. Much of the associated between IQ and schooling related to the years spent in education (Ceci, 1991).

Winship & Korenman (1997) suggest that every year in school is worth about 2.7 IQ points. School attendance raises IQ, and truanting is associated with lower IQ scores (Ceci & Williams, 1997). Schooling is associated with increases in crystallised and fluid intelligence (Cliffordson & Gustafsson, 2008)

Genes not only exert an effect on traits like intelligence, but also on the environmental choices that a person makes. Biological inherited factors can play an active role in shaping experiences from early age to adulthood.