No matter how we choose to define and assess intelligence, it is true that there will be a wide range of individual differences. For example, the psychometric approaches compare people's scores to averages of others of the same chronological age, so most people by definition show average intelligence scores. But what about those whose IQ scores are significantly below or above average? What outcomes are common for these individuals?

Mental Retardation

Children with mental retardation learn more slowly than other children, have more difficulty solving problems, and show language and communication deficits. As a result, they perform less well in school and have more difficulty making friends and engaging in social activities. With special services and support, children with mild-to-moderate levels of retardation can adjust to many of the normal challenges in life. They can attend regular classrooms, learn to care for themselves, and develop friendships with peers. With more severe levels of retardation, a child may need extensive support merely to negotiate everyday activities such as brushing teeth and getting dressed. Approximately 2 to 3% of the U.S. population has mental retardation. There are three components to the formal definition of mental retardation (MR):

  1. below-normal intellectual functioning (usually indicated by an IQ of less than 70 or 75);
  2. deficits in adaptive behavior, the daily activities required for personal and social independence (e.g., communicating needs to others, eating, dressing, grooming, toileting, following rules, and working and playing with others); and
  3. an onset early in life (before age 18) (Hodapp & Dykens, 2003).

Literally thousands of biological and environmental factors can cause mental retardation. The most severe forms of MR tend to result from genetic disorders. Down syndrome and fragile X syndrome are the two most common types of genetic disorders that cause mental retardation. Together these two disorders alone affect 1 in every 500 children born, and more than 700 other genetic diseases also can contribute to MR (Hodapp & Dykens, 2003). Mental retardation can also result from prenatal damage to the brain and nervous system by toxins such as alcohol and drugs. Prenatal alcohol exposure is the leading known cause of mental retardation in the United States (Abel & Sokol, 1987; Institute of Medicine, 1996). MR can also occur when infants suffer oxygen deprivation or other traumas during birth, and when they are born prematurely.

After children are born, numerous factors in the environment can retard mental development. The best-known environmental factors related to MR include exposure to lead and other toxins, poor nutrition, lack of stimulation, and parents who are illiterate or mentally retarded themselves. Rates of mental retardation are higher among children living in poverty, minority children, and males (Hodapp & Dykens, 2003). Mental health researchers often refer to retardation caused by lack of educational opportunity and stimulation as cultural-familial retardation. Mental retardation also can have multiple causes. For example, children may inherit low intelligence from their parents; on top of this, they may suffer poor nutrition, and their parents may fail to provide a stimulating learning environment. When both parents have mental retardation, the odds are more than 40% that their children will also have MR. The odds drop to 20% when only one parent has MR and to less than 10% when neither parent has MR (Mash & Wolfe, 2005).

Approximately 85% of people with mental retardation are in the mild category (Mash & Wolfe, 2005). Toddlers and preschoolers with mild MR usually show only small delays. When they reach early elementary school, however, they fall behind in academic subjects. With some special education and support, these children can learn up to the sixth- or seventh-grade level. They may have only minor problems with peers and other social relationships, and after finishing school they can live and work independently or with a modest amount of supervision. At the other end of the scale, 1 to 2% of all people with mental retardation are in the profound category. As infants, they show serious delays in sensory and motor functions, and by the age of 4 they are still responding like typical 1-year-olds (Mash & Wolfe, 2005). These children need considerable training to learn to perform self-care activities such as eating, dressing, and toileting. They will need lifelong care. At present, most people with profound MR in the United States eventually go to live in group homes or residential facilities. Almost all cases of profound MR have a genetic or biological cause.

There are several things family members and other people can do to help children with mental retardation improve the quality of their lives (Mash & Wolfe, 2005; Ramey & Ramey, 1992). They can encourage children with MR to explore the environment so they can learn and gather information, and work with them on basic learning skills such as labeling, sorting, and comparing objects. Children with MR need consistent care from a responsible adult—someone they can trust and depend on. Caregivers can also help by celebrating the achievements and developmental milestones of children with MR, and protect them from harmful teasing, punishment, and criticism. None of these steps can erase the retardation, but they can go a long way in helping the child live a more happy and satisfying life.

Gifted and Talented Children

Gifted (or talented) children show achievement that is well above average in one or more areas—usually in language, math, music, art, or athletics. Some children are globally gifted: They show exceptional talent in all areas. Other children are unevenly gifted: They are exceptional in one or two areas but are at (or below) average levels in others. While a high IQ score may be one indicator of giftedness, it is not the only one; some talent areas are not included on intelligence tests, and such tests do not consider a child's cultural context when used as indicators of talent (Sternberg, 2007). Winner (1996) describes three characteristics that are typical of gifted (or talented) children:

  • Gifted children are precocious. They begin learning early and progress faster than others.
  • Gifted children march to their own drummer. They don't need much assistance to master information in their favorite subjects. They often teach themselves, have their own ways of learning, organizing, and sorting information; and they don't always conform to the conventional learning methods of schools.
  • Gifted children have a rage to master—an intense craving for information and an obsessive need to make sense out of their favorite topics. They devour information, spend endless hours on their chosen subjects, and rarely engage in any other pursuits. Parents don't push them to achieve; instead, gifted children push their parents for more materials and stimulation.

One of the most ambitious longitudinal studies in history was begun by Lewis Terman in 1921 to study the development of highly gifted individuals. Contrary to common stereotypes, Terman found that gifted and talented individuals were not neurotic, frail, eccentric, or emotionally sensitive individuals. Instead, they were larger, healthier, and generally more well-adjusted than most other children. Overall, they tended to live longer, enjoy better health, have a lower divorce rate, and be happier than most people (Shurkin, 1992; Terman, 1925). More recent research has found that gifted and talented adolescents are more focused in school, spend much of their free time working in their talent areas, and spend more time alone than their "average" peers. Their parents tend to have more education, and their families have higher incomes, as well as more supportive and positive family environments. For example, most talented teenagers rate their family interactions as more affectionate, cohesive, flexible, and happy than other students (Csikszentmihalyi, Rathunde, & Whalen, 1997; Shurkin, 1992; Terman, 1925). If others had these benefits, how many more would show exceptional talent?