Environmental Factors> Family Size

There is a small negative correlation between family size (how many siblings a child has) and intelligence, especially in families with lower incomes. This means that, on average, when families have more children, each child might score slightly lower on intelligence tests.

• Research Evidence: Studies (Gille, 1954; Heuyer, 1950) show that intelligence test scores tend to decrease as the number of siblings increases, especially in low-income families.
• Negative Correlation: This means as one factor (family size) increases, the other (intelligence scores) decreases.
• Resource Dilution Theory: More children mean resources like time, money, and parental attention are spread thinner for each child.
• Socio-economic Factors: This effect is stronger in lower-income families, where resources are already limited.

Imagine a parent’s attention like a pizza. If there’s only one child, the child gets the whole pizza. In a family with five children, each gets just one slice. Smaller slices mean less attention, help, or opportunities for each child.

• A negative correlation exists between family size and intelligence, due to shared resources.
• This trend is mainly observed in families with limited economic means.
• Intelligence outcomes are influenced by many factors—family size is just one part of the picture.
• Not all large families will experience lower intelligence scores.