Environmental and Genetic Factors as Contributors to Intelligence

Environmental and Genetic Factors as Contributors to Intelligence

Introduction

In addition to disagreements about the basic nature of intelligence, psychologists have spent a great amount of time and energy debating the various influences on individual intelligence. The debate focuses on one of the major questions in psychology: Which is more important - nature or nurture? Today, nearly all psychologists recognize that both genetics and the environment play a role in determining intelligence. It now becomes matter of determining exactly how much of an influence each factor has (Akamatsu, 1999).

First, it is important to note that genetics and the environment interact to determine exactly how inherited genes are expressed. For example, if a person has tall parents, it is likely that the individual will also grow to be tall. However, the exact height the person reaches can be influenced by environmental factors such as nutrition and disease.

Evidence of Genetic Influences

While showing impressive growth prenatally, the human brain is not completed at birth. There is considerable brain growth during childhood with dynamic changes taking place in the human brain throughout life, probably for adaptation to our environments (Bouchard, 1981).

Evidence is accumulating that brain structure is under considerable genetic influence. Puberty, the transitional phase from childhood into adulthood, involves changes in brain morphology that may be essential to optimal adult functioning. Around the onset of puberty gray matter volume starts to decrease, while white matter volume is still increasing.

Recent findings have shown, that variation in total gray and white matter volume of the adult human brain is primarily (70-90%) genetically determined [Baare et al, 2001] and in a recent magnetic resonance imaging (MRI) brain study with 45 monozygotic and 61 dizygotic 9-year-old twin-pairs, and their 87 full siblings also high heritabilities have been found. Thus, while environmental influences may play a role in later stages during puberty, around the onset of puberty brain volumes are already highly heritable (Hughes, 1999).

Genetic Influences And Functional Relevance

Twin studies have also shown that genetic effects vary regionally within the brain, with high heritabilities of frontal lobe volumes (90-95%), moderate estimates in the hippocampus (40-69%), and environmental factors influencing several medial brain areas. However, the mechanisms by which interaction between genes and environment occur throughout life as well as dynamics of brain structure and its association with brain functioning still remain unknown. Twin and family studies and newly evolving genetic approaches start to give us a glimpse as to which genes and (interacting) environmental influences are shaping our brains (McCall, 1973).

Brain structure - measured macroscopically using MRI - and the dynamic changes therein, have a functional relevance. Studies revealed that total brain volume is positively correlated with general intelligence. In healthy subjects, the level of intellectual functioning has been positively associated with whole brain, gray, and white matter volumes. More focally, several brain areas were found to be correlated with intelligence. Interestingly, it was also shown that the trajectory changes in cortical thickness throughout adolescence are associated with the level ...