IQ to the Test

Eleven facts about intelligence that will astound the experts. Who has it and how do we measure it?

By Stephen Ceci, published on July 01, 2001 - last reviewed on May 09, 2006

Every culture has a word for "smart," and for "stupid." And everyone feels entitled to have an opinion about intelligence. Unlike, say, brain surgery, intelligence is not an area of expertise that is considered off-limits. Because it's something that our society particularly values, just about everyone has taken a test that measures intelligence, whether it's billed as an IQ test or not. Will you be assigned to the radar corps or the mess tent?

For over 20 years, a national debate has raged about intelligence: what is it, who has it and how do we measure it? The argument is fueled by findings from two camps of research. There are the psychometricians, who look at the statistics and biology of IQ and try to determine how much of intelligence is innate. And then there are the cultural ecologists, who focus on environment and point out the mutability of intelligence and the unfairness of IQ tests. Unfortunately, the two lines of study seldom meet because their methods are so different. Rarely does one camp communicate with the other.

That leaves most ordinary citizens on the outside of the debate, free to cling to their personal beliefs about intelligence. The only trouble is our theories of intelligence are too narrowly constructed. They tend to ignore real data, even though a voluminous literature exists on the topic.

At the very least, intelligence can be defined as the ability for complex thinking and reasoning. One thing the research shows for sure: much of the ability for complex reasoning depends on the situation. A person can be a genius at the racetrack but a dolt in the stock market, even though both pursuits require comparable mental activities. But the knowledge is organized in the mind differently in different domains, so what a person knows about the track can lie fallow on Wall Street.

I would like to present eleven research-supported facts about intelligence that most people, including some IQ experts, might find surprising:

FACT 1: IQ is affected by school attendance

Although intelligence does influence the decision to stay in school, staying in school itself can elevate IQ. Or, more accurately, prevent it from slipping. Each additional month a student remains in school may increase his IQ above what would have been expected had he dropped out. The idea that schooling increases IQ may surprise anyone who views it as a measure of innate intelligence.

The earliest evidence comes from the turn of the last century, when the London Board of Education studied children who had very low IQ scores. The report revealed that the IQ of children in the same family decreased from the youngest to the oldest. The youngest group—ages 4 to 6—had an average IQ of 90, and the oldest children—12 to 22—had an average of only 60. This suggests that factors other than heredity are at work. The older children progressively missed more school, and their IQs plummeted as a result.

A few other facts about school attendance:

IQ is affected by delayed schooling. Researchers in South Africa studied the intellectual functioning of children of Indian ancestry. For each year of delayed schooling, the children experienced a decrement of five IQ points. Similar data has been reported in the U.S.
IQ is affected by remaining in school longer. Toward the end of the Vietnam War, a draft priority was established by lottery. Men born on July 9, 1951 were picked first so they tended to stay in school longer to avoid the draft; while men born July 7 had no incentive to stay in school longer because they were picked last in the lottery. As a result, men born on July 9 not only had higher IQs, they also earned more money—approximately 7% more.
Dropping out of school can also diminish IQ. In a large-scale study, 10% of all males in the Swedish school population born in 1948 were randomly selected and given an IQ test at age 13. Upon reaching age 18 (in 1966), 4,616 of them were tested again. For each year of high school not completed, there was a loss of 1.8 IQ points.
IQ is affected by summer vacations. Two independent studies have documented that there is a systematic decline in IQ scores over the summer months. With each passing month away from school, children lose ground from their end-of-year scores. The decline is pronounced for children whose summers are least academically oriented.

FACT 2: IQ is not influenced by birth order

The idea that birth order influences personality and intelligence is long-standing. First-borns are allegedly smarter and more likely to become leaders than are later-born siblings. Recently, however, this belief has come up against scrutiny. The idea that large families make low-IQ children may be unfounded because researchers have discovered that low-IQ parents actually make large families.

The truth is that smart people tend to have small families, but it is not small families per se that make people smart. Hence, birth order doesn't predict IQ, and there is no causal role for family size in determining a child's IQ.

Also, no structural aspects of family size influence a child's IQ. Otherwise two siblings closer in age would have more similar IQs than two siblings spaced far apart. But this is not the case.

FACT 3: IQ is related to breast-feeding

My colleagues and I were skeptical when we first heard claims that breast-fed infants grew into children with higher IQs than their siblings who were not breast-fed. There are factors that differ between breast-fed and non-breast-fed children, such as the amount of time mother and child spend together through nursing and the sense of closeness they gain from nursing.