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Human intelligence is shaped by both internal genetic factors and external environmental circumstances.
Relate human intelligence to both genetic and environmental factors
Genetics and the environment are so intertwined in their influence on human intelligence that it remains difficult to determine which, if either, is most responsible for determining a person's intelligence.
Up to 80% of the variation found in adult human intelligence is thought to be attributable to genetics, despite the fact that it is a complicated, polygenictrait.
Both sociocultural and biological influences in the environment affect the development of human intelligence.
The natural genetic make-up of the body interacts with environment from the moment of conception. While extreme genetic or environmental conditions can predominate behavior in some rare cases, these two factors usually work together to produce individual intelligence. There is much debate among researchers and scientists over which influence, genetics or environment, has the largest role in determining overall intelligence, because both have been scientifically established as having a significant impact on intelligence. Recent discoveries have further complicated this debate by proving that the relationship between internal predispositions ("nature") and external circumstances ("nurture") not only varies among populations, but also changes over time. Genetics and environment interact constantly, so the question of supremacy in the nature versus nurture debate in human intelligence will probably never be fully answered.
A gene is the unit of heredity by which a biological trait is passed down through generations of human beings. Heritability describes what percentage of the variation of a trait in a population is due to genetic differences in that population (as opposed to environmental factors). Some traits, like eye color, are highly heritable and can be easily traced. However, even highly heritable traits are subject to environmental influences during development. Intelligence is generally considered to be even more complicated to trace to one source because it is a polygenic trait, influenced by many interacting genes.
Twin studies in the western world have found the heritability of IQ to be between 0.7 and 0.8, meaning that the variance in intelligence among the population is 70%-80% due to genetics. Conventional twin studies reinforce this pattern: monozygotic (identical) twins raised separately are more similar in IQ than dizygotic (fraternal) twins raised together, and much more than adoptive siblings.
However, the heritability of IQ in juvenile twins is much lower at 0.45. Heritability measures of IQ have a general upward trend with age (from as low as 0.2 in infancy to 0.8 in late adulthood), leading psychologists to believe that either we rely on or reinforce our genes as we age. This is thought to occur through human interaction with external circumstances, whereby people with different genes seek out different environments. Thus, despite the high heritability of IQ, we can determine that there is an environmental influence as well.
Genetics and Intellectual Disabilities
As mentioned, under normal circumstances intelligence involves multiple genes. However, certain single-gene genetic disorders can severely affect intelligence. Genetic causes for many learning disabilities, such as dyslexia, and neural disorders, such as Down syndrome, autism, and Alzheimer's disease have been investigated by the field of cognitive genomics, the study of genes as they relate to human cognition. Down syndrome, for example, is a genetic syndrome marked by intellectual disability, and has implications for the ways in which children with Down syndrome learn. While experts believe the genetic cause for Down syndrome is a lack of genes in the 21st chromosome, the gene(s) responsible for the cognitive symptoms have yet to be discovered. And like most traits, the occurrence of neurobehavioral disorders is influenced by both genetic and non-genetic factors, and the genes directly associated with these disorders are often unknown.
Many different environmental influences have been found to shape intelligence. These influences generally fall into two main categories: biological and sociocultural. Biological influences act on the physical body, while sociocultural influences shape the mind and behavior of an individual.
Biological influences include everything from nutrition to stress, and begin to shape intelligence from prenatal stages onward. Nutrition has been shown to affect intelligence throughout the human lifespan; malnutrition during critical early periods of growth (particularly the prenatal period and during the second year of life) can harm cognitive development. Inadequate nutrition can disrupt neural connections and pathways, and leave a person unable to recover mentally.
Stress also plays a part in the development of human intelligence: exposure to violence in childhood has been associated with lower school grades and lower IQ in children of all races. A group of largely African American, urban first-grade children and their caregivers were evaluated using self-report, interview, and standardized tests, including IQ tests. The study reported that exposure to violence and trauma-related distress in young children was associated with substantial decreases in IQ and reading achievement. Exposure to toxins and other perinatal factors have also been proven to affect intelligence, and in some cases, cause issues such as developmental delays.
The family unit is one of the most basic influences on child development, but it is difficult to untangle the genetic from the environmental factors in a family. For example, the quantity of books in a child's home has been shown to positively correlate with intelligence... but is that due to the environmental impact of having parents who will read to their children, or is it an indicator of parental IQ, a highly heritable trait?
A child's position in birth order has also been found to influence intelligence: firstborn children have been found in some studies to score higher, though criticism has been offered to these studies for not controlling for age or family size. Moving outside of the family unit, human beings are substantially shaped by their respective peer groups. Stereotype threat is the idea that people belonging to a specific group will perform in line with generalizations assigned to that group, regardless of their own aptitude; this threat has been known to affect IQ scores both positively and negatively. That is, if a person belongs to a group that is told they are intelligent, they will appear more intelligent on IQ tests; if they are told they belong to a group that is unintelligent, they will perform worse, even if these distinctions are random and fabricated (as in lab studies). People's access to education, and specific training and intervention resources, also determines their life-long intelligence level.