The Great Debate: Nature v. Nurture
Psychologists today generally agree that all human attributes, including intelligence, are shaped by both genetic and environmental influences. There is much debate among researchers and scientists as to which type of influence reigns supreme in determining overall intelligence. Both nature and nurture have been scientifically established as having significant impact on intelligence. Recent discoveries have complicated this debate by proving that the relationship between internal predispositions and external circumstances not only varies among populations, but also changes over time. Genetics and the environment are constantly interacting, so the question of supremacy in the nature versus nurture debate in human intelligence will probably never be fully answered.
Genes are the unit of heredity by which traits are passed on through generations of human beings. Heritability is the degree of variability in a given population in a given trait . A score of 1.0 indicates no variance, which means that all of the variety in that trait is due to genetics. A score of 0.0 indicates no correlation, which means that none of the variety in that trait is due to a difference in genes. 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 even more complicated as it is a polygenic trait, influenced by a large percentage of human genes.
Studies in the Western world have found that the heritability of IQ in adult twins is between 0.7 and 0.8. The heritability of IQ in child 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.
There are many different environmental influences that have been found to shape intelligence. These influences generally fall under two major 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. Proper nutrition is especially critical in the early stages of life, as it establishes a base-line for further intellectual development. Malnutrition can disrupt neural connections and pathways, and leave a person unable to recover mentally. Stress also plays a part in human intelligence. Along with maternal age, stress can put undue pressure on the development of a human body such that it can cause irreparable damage. Exposure to toxins and other perinatal factors have also been proven to affect intelligence, and in some cases, cause issues such as developmental delays.
Sociocultural influences are much more fluid in nature. The most basic of these influences is the family unit, or lack thereof, in which a child develops. Having an encouraging home-life that is conducive to learning has a direct effect on intelligence test scores. A child's position in birth order has also been found to influence intelligence. Moving outside of the family unit, human beings are highly shaped by their respective peer groups. Stereotype threat is the idea that people belonging to a specific group will perform in line with generalizations with which they are associated, regardless of aptitude; this threat has been known to affect IQ scores. A person's access to education, and specific training and intervention resources, also determines one's life-long intelligence level.
Interaction of Genetics and Environment
The natural genetic make-up of the body interacts with the environment from the moment of conception. While extreme genetic or environmental conditions can predominate behavior in some rare cases, such as the inability of a mute person to speak regardless of their environment, these two factors generally work together to produce a person's intelligence level. They are so intertwined that it remains difficult to determine which influence holds the supreme position in shaping intelligence .