The measure of how much human intelligence is affected by genotype has been a topic of great fascination to psychologists, educators, and parents. Hypothesis have been posed in order to discover whether a person’s intelligence is a result of genes, or mainly a factor of the shared and non-shared environment that surrounds a person as he or she grows, or a measure of both aspects. And if both, what percentage can be attributed to either sides of the spectrum.
To begin, there have been many interpretations as to the definition of “intelligence”—though perhaps a common association is that of an IQ test and score. David Wechsler, creator of the Wechsler Adult Intelligence Scale, stated that intelligence was—“The aggregate or global capacity of the individual to act purposefully, to think rationally, and to deal effectively with his environment.” Alfred Binet, creator of the first IQ tests said that it was, “Judgment, otherwise called ‘good sense,’ ‘practical sense,’ ‘initiative,’ the faculty of adapting one’s self to circumstances . . . auto-critique.”
Studies on the measure of heritability in intelligence have been conducted on monozygotic and dizygotic twins—those raised together and apart. These studies have shown that monozygotic twins, those sharing identical genes, had a high IQ correlation coefficient, non-identical twins also achieved the same high correlation coefficient when taking IQ tests, though not as high as identical twins. And siblings raised together had a higher IQ correlation coefficient as compared to siblings raised apart, and overall, these various family groupings had a higher IQ correlation coefficient when compared to the IQ of an unrelated individual of the same age.
Brain size, which has a heritability 0.90 (Baare, et al., 2001), has been observed to have a relationship with intelligence. A larger brain has been shown to have more neurons, more neurons means a greater capacity for the speed with which information is processed. This has also been shown to be true with animals: species with a greater head (brain) size to body ratio have been shown to have greater intelligence when compared to animals with a lesser head to body ratio (Lynn 2006).
Studies such as the above support the theory that intelligence is a highly genetic trait. However, the measure of genotype expression does also depend on environmental factors. A child born with a high intelligence potential, but not given proper nutrition and the right stimulus, or who is neglected during his formative years, could fail to fully realize the potential in his genetic make up.
Nutrition is an environmental factor which plays a role in the development of intelligence. The cells that make up the brain are composed of 60% fat, a deficiency in certain fats (Alpha-linoeic Acid found in flax seed and flax seed oil, various nuts, and dark-green, leafy vegetables, Linoeic Acid found in corn, various seeds and nuts, Docosahexaenoic acid found in breast milk) in a child’s diet can greatly hinder the brain’s proper development and in some cases lead to ailments such as Attention-deficit disorder, Alzheimer’s disease, Parkinson’s disease, and depression.
A stimulating environment has also been shown by various studies to improve the overall intelligence of children. Maternal deprivation has been linked to retardation of language and intellectual development, but it’s also likely that it is the lack of stimulation in the form of being read to or talked to or played with that causes children suffering from maternal deprivation to fail to realize their full intellectual potential. Attachment studies conducted over the last few decades have shown that infants and young toddlers were less likely to learn and explore the world if a secure attachment with a parent figure had not been formed.
While the scientific community remains divided as to what degree intelligence is derived from genes versus environment, and while recent research favors intelligence being a highly genetically determined trait, we know enough to be responsible to do what we can to provide the kind of care and nurture to our children that will allow their full intelligence potential to be realized.