Psychologists have long debated how to best conceptualize and measure intelligence (Sternberg, 2003). These questions include how many types of intelligence there are, the role of nature versus nurture in intelligence, how intelligence is represented in the brain, and the meaning of group differences in intelligence. The concept of intelligence relates to abstract thinking and that includes our abilities to acquire knowledge, to reason abstractly, to adapt to novel situations, and to benefit from instruction and experience (Gottfredson, 1997; Sternberg, 2003). The brain processes underlying intelligence are not completely understood, but current research has focused on four potential factors:
- Brain size
- Sensory ability
- Speed and efficiency of neural transmission
- Working memory capacity
There is some truth to the idea that smarter people have bigger brains. Studies that have measured brain volume using neuroimaging techniques find that larger brain size is correlated with intelligence (McDaniel, 2005), and intelligence has also been found to be correlated with the number of neurons in the brain and with the thickness of the cortex (Haier, 2004; Shaw et al., 2006). It is important to remember that these correlational findings do not mean that having more brain volume causes higher intelligence. It is possible that growing up in a stimulating environment that rewards thinking and learning may lead to greater brain growth (Garlick, 2003), and it is also possible that a third variable, such as better nutrition, causes both brain volume and intelligence.
There is some evidence that brains of more intelligent people operate more efficiently than the brains of people with less intelligence. Haier, Siegel, Tang, and Abel (1992) analyzed data showing that people who were more intelligent showed less brain activity than those with lower intelligence when they worked on a task. Researchers suggested that more intelligent brains need to use less capacity. Brains of more intelligent people also seem to operate faster than the brains of those who are less intelligent. Research has found that the speed with which people can perform simple tasks, like determining which of two lines is longer or quickly pressing one of eight buttons that is lighted, was predictive of intelligence (Deary, Der, & Ford, 2001). Intelligence scores also correlate at about r = .5 with measures of working memory (Ackerman, Beier, & Boyle, 2005), and working memory is now used as a measure of intelligence on many tests.
Research using twin and adoption studies found that intelligence has both genetic and environmental causes (Neisser et al., 1996; Plomin, DeFries, Craig, & McGuffin, 2003). It appears that 40% – 80% of the variability (difference) in intelligence is due to genetics (Plomin & Spinath, 2004). The intelligence of identical twins correlates very highly at r = .86, which is much higher than the scores of fraternal twins who are less genetically similar (r = .60). Correlations between the intelligence of parents and their biological children (r = .42) is significantly higher than the correlation between parents and adopted children (r = .19). The intelligence of very young children (less than 3 years old) does not predict adult intelligence but by age 7 intelligence scores (as measured by a standard test) remain very stable in adulthood (Deary, Whiteman, Starr, Whalley, & Fox, 2004).
There is also strong evidence for the role of nurture, which indicates that individuals are not born with fixed, unchangeable levels of intelligence. Twins raised together in the same home have more similar intelligence scores than do twins who are raised in different homes, and fraternal twins have more similar intelligence scores than do non-twin siblings, which is likely due to the fact that they are treated more similarly than are siblings. Additionally, intelligence becomes more stable as we get older which provides evidence that early environmental experiences matter more than later ones.
Environmental factors also explain a greater proportion of the variance in intelligence and social and economic deprivation can adversely affect intelligence. Children from households in poverty have lower intelligence scores than children from households with more resources even when other factors such as education, race, and parenting are controlled (Brooks-Gunn & Duncan, 1997). Poverty may contribute to diets that under nourish the brain or lack appropriate vitamins. Poor children are more likely to be exposed to toxins such as lead in drinking water, dust, or paint chips (Bellinger & Needleman, 2003). Both of these factors can slow brain development and reduce intelligence.
Intelligence is improved by education and the number of years a person has spent in school correlates about r = .6 with intelligence (Ceci, 1991). There is a word of caution when interpreting this result. The correlation may be due to the fact that people with higher intelligence scores enjoy taking classes more than people with low intelligence scores, and may be more likely to stay in school. Children’s intelligence scores tend to drop significantly during summer vacations (Huttenlocher, Levine, & Vevea, 1998) a finding that suggests a causal effect of intelligence and education. A longer school year, as is used in Europe and East Asia, may be beneficial for maintaining intelligence scores for school-aged children.