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  » Types of Intelligence

  » Intelligence, Heredity, and Environment
        Evidence for Nature
        Evidence for Nurture
        Comments on Research

  » Neuropsychological Testing
        Normal Intelligence
        Abnormal Examination             and Brain Trauma

  » Spectroscopy Data

  » Disorders related to Intelligence

  »  Gender Differences
       Self-Estimated              Intelligence
       Anatomical Differences
       Gray vs. White Matter

  » Artificial Intelligence
        A Timeline of AI
        Ancient History of AI
        Modern History of AI
        The Future of AI

  » Age and Intelligence
        Areas of Function
        Effects of Lesions

  » References

Evidence for ‘Nurture’

We saw earlier that, while not discounting that genetic factors may exist, supporters of the nurture theory believe that our behavioral aspects originate mostly from the environmental factors of our upbringing. Studies on infant and child temperament have revealed the most crucial evidence for the nurture theory. The nurture camp also took advantage of the nutrition studies, twin studies, and adoption studies for collecting evidence for their hypothesis.

In the 1980s, a New Zealand-based political scientist, James Flynn, noticed that IQ was increasing in all countries all the time, at an average rate of about 3 IQ points per decade i.e. the average IQ across the world has risen over 1 standard deviation (i.e. 15 points) since WWII - predominantly due to environmental effects.




Could this be due to diet? Possibly but IQ scores are still rising just as rapidly in well-nourished western countries. So, it is not exactly the nutrition that causes this increase in IQ. Could it be schooling? It has been found that interruptions to schooling only have temporary effects on IQ. One researcher, Ulric Neisser suggests that the Flynn effect is due to the way we are being saturated with sophisticated visual images: ads, posters, videogame and TV graphics etc - rather than written messages. He suggests that children experience a much richer visual environment than in the past and that this helps them with visual puzzles of the kind that dominate IQ tests. There have been posed many such environmentalist hypotheses to explain the Flynn effect; yet, even today, it is still not known what exactly causes the steady increase in IQ as found by James Flynn. One lesson to be learned from the Flynn example is that when we say environment plays an important in intelligence, in fact we are talking about many different environmental factors, such as nutrition, schooling, parental behavior… Those factors, circumstances and attributes have been found to vary to a greater or lesser (but always significant) extent in relation with IQ - note that not all of these relationships increase intelligence, yet they all support the environmental view. This means that although some of those factors negatively affect intelligence, this is still a sign that environment can affect the level of intelligence and mental abilities. It has been found that intelligence can vary with:

  • Infant malnutrition (negative)
  • Number of years in school
  • Social group of parental home
  • Father's profession
  • Father's economic status
  • Degree of parental rigidity (negative)
  • Parental ambition
  • Mother's education
  • Average TV viewing (negative)
  • Average book-reading
  • Self-confidence according to attitude scale measurement
  • Age (negative relationship, applies only in adulthood)
  • Degree of authority in parental home (negative)
  • Criminality (negative)
  • Alcoholism (negative)
  • Mental disease (negative)
  • Emotional adaptation

"No single environmental factor seems to have a large influence on IQ. Variables widely believed to be important are usually weak....Even though many studies fail to find strong environmental effects....most of the factors studied do influence IQ in the direction predicted by the investigator....environmental effects are multifactorial and largely unrelated to each other." (Bouchard & Segal (1985), p.452) So, it would appear that there are many environmental factors and attitudes each contributing a small fraction to the variance in IQ scores. Now, let’s explore some of these relationships more explicitly.

American psychologist John Watson, best known for his controversial experiments with a young orphan named Albert, demonstrated that the acquisition of a phobia could be explained by classical conditioning. Watson used an 11 month-old Albert to prove that a person could be conditioned to be afraid of something by which he was not previously affected. Albert was put into a room with no other human and no other distracters present. Watson placed a white rat in the room. Albert seemed to like the rat; he even showed affection towards it. Some time later, Watson would produce a very loud noise every time Albert would reach out to touch the rat. As a result, the baby became terrified of every white and furry object in which he came in contact. This distinguished investigation became known as the Albert experiment. A strong proponent of environmental learning, Watson said: ‘Give me a dozen healthy infants, well-formed, and my own specified world to bring them up in and I'll guarantee to take any one at random and train him to become any type of specialist I might select...regardless of his talents, penchants, tendencies, abilities, vocations and race of his ancestors.’ Experiments such as these ones prove that a person’s environment can have a crucial effect on him and on his manner of thinking.

Harvard psychologist B. F. Skinner's early experiments produced pigeons that could dance, do figure eights, and play tennis. Today known as the father of behavioral science, he eventually went on to prove that human behavior could be conditioned in much the same way as animals. In addition, the amount of nourishment an individual receives has been proven to play a very large part in a person’s mental ability. This is especially true concerning infants and young children. Moreover, the human brain critically needs nutritious food and antitoxins to grow and function properly, particularly in early years of development. A study done in Great Britain in the late 1980s shows that nutrition plays a very large role in a person’s development. Adolescents aged twelve to thirteen were given vitamin and mineral supplements for eight months. These subjects were then administered intelligence tests. Test scores were recorded before the test and after the test. These scores were also compared to other adolescents who were not given the supplements. The scores showed that the students who had taken the supplements scored higher on the tests after taking the supplements (Herrnstein and Murray, 292). This study, thus, proves that nutrition (which is a part of the environment) plays a role in intelligence and mental aptitude.

Adoption studies have also somewhat shown that a person environment plays an important role in his mental ability. For example, a study done with adoptive children raised in the same house had very similar IQs, given that these children were in no way related genetically. The environment that they we raised in provided them with similar abilities for learning and for retaining information (Kagan and Havermann, 39). In addition, fraternal twins (who share approximately half of their genes) present an informative contrast. Because they are raised in the same environment but are not genetically identical, they help us to see the influence of environmental factors (Segal, 69). These factors are all valuable to the environmentalist argument. In addition, if environment didn't play a part in determining an individual's traits and behaviors, then identical twins should, theoretically, be exactly the same in all respects, even if reared apart. But, as we saw earlier, a number of studies show that they are never exactly alike, even though they are remarkably similar in most respects.


Several recent US studies have also demonstrated improvements in children's IQ's by improving the lives of infants in disadvantaged circumstances. These studies employed random assignment of children and families to treatment and control conditions. These studies selected experimental families with low parental IQ, low parental education, and minimal financial resources. The experimental group received enriched, educational day care outside the home every weekday from 3 months to start of schooling. The control group received nutritional supplements and pediatric medical care or crisis intervention but no educational day care. Even though the children returned to their home environment every day and spent holidays and weekends with their families (mostly unemployed, single mothers) in poverty-stricken neighborhoods, there were large gains in IQ. This shows that education as a part of an individual’s environment has a huge effect on that individual’s intelligence and mental aptitude.