Q.What is IQ? How do psychologists classify people on the basis of their IQ scores?

IQ (Intelligence Quotient), devised by William Stern (1912), is a person's mental age divided by chronological age, multiplied by 100 (IQ = (mental age / chronological age) × 100). Mental age (MA) is intellectual development relative to one's age group; chronological age (CA) is age from birth. Multiplying by 100 removes decimals; when MA equals CA, IQ is 100. IQ scores form a bell-shaped normal curve around a mean of 100, and psychologists classify people using descriptive labels — e.g., above 130 very superior, 90–109 average, below 70 intellectually disabled. …

What IQ is

Binet and Simon (1905) made the first successful attempt to measure intelligence, and in 1908 introduced Mental Age (MA) — a measure of a person's intellectual development relative to their age group (a mental age of 5 means performing like an average 5-year-old). Chronological Age (CA) is the biological age from birth. A bright child's MA is above her CA; a dull child's MA is below it.

In 1912 the German psychologist William Stern devised the Intelligence Quotient (IQ), defined as:

IQ = (mental age / chronological age) × 100

The multiplier 100 avoids decimals. When MA equals CA, IQ is exactly 100; if MA is greater than CA, IQ exceeds 100; if MA is less than CA, IQ falls below 100. For example, a 10-year-old with a mental age of 12 has an IQ of 120, while one with a mental age of 7 has an IQ of 70. The average IQ in the population is 100, irrespective of age.

Distribution — the normal curve

IQ scores are distributed so that most people fall in the middle range and only a few score very high or very low. The frequency distribution approximates a bell-shaped normal curve, symmetrical around the mean of 100.

Classification on the basis of IQ

Psychologists classify people using descriptive labels (Table 1.1):

• Above 130 — Very superior (≈2.2%)
• 120–130 — Superior (≈6.7%)
• 110–119 — High average (≈16.1%)
• 90–109 — Average (≈50%) …