Individuals vary in their physical and psychological characteristics, referred to as individual differences. People differ in their ability to understand complex ideas, adapt to the environment, learn from experience, engage in reasoning, and overcome obstacles.

The mean IQ score in the population is 100. People deviate considerably from the normal population in two directions: the intellectually gifted and the intellectually disabled.

Intellectual disability is defined as "significantly sub-average general intellectual functioning existing concurrently with deficits in adaptive behaviour and manifested during the developmental period". Persons having IQs below 70 are judged to have sub-average intelligence.

There is wide variation among the intellectually disabled, categorized into different levels: mild (IQs 55 to approximately 70), moderate (IQs 35–40 to approximately 50–55), severe (IQs 20–25 to approximately 35–40), and profound (IQs below 20–25).

The functioning of these individuals varies significantly. People with mild disability have slower development but can function independently, hold jobs, and have families. However, as the level of disability increases, difficulties become strongly marked. People with moderate disability lag behind in language and motor skills, while those with severe or profound disabilities may require institutional care throughout their lives.