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

It turns out, however, that even when researchers control for such factors, there still appears to be a gain of 3 to 8 IQ points for breast-fed children by age three. Exactly why is unclear. Perhaps the immune factors in mother's milk prevent children from getting diseases that deplete energy and impair early learning. Breast milk may also affect nervous system functioning. Mother's milk is an especially rich source of omega-3 fatty acids that are building blocks of nerve cell membranes and crucial to the efficient transmission of nerve impulses.

FACT 4: IQ varies by birth date

Most states have restrictions on the age of students entering schools, as well as policies mandating attendance until age 16 or 17. School attendance drops off for students born during the final three months of the year, as they are more likely to enter school a year later. When these individuals come of age, they have been in school one year less than their classmates.

Researchers have shown that for each year of schooling completed, there is an IQ gain of approximately 3.5 points. Students born late in the year, as a group, show a lower IQ score. Given the random processes involved in being born early versus late within a given year, we can assume that the genetic potential for intelligence is the same in both groups.

FACT 5: IQ evens out with age

Imagine interviewing two biological siblings, adopted by two different middle class families, at age five and again at 18. Will their IQs be more alike when they are younger and living in the homes of their adoptive parents, or when they are older and living on their own? Many people reason that IQs will be more alike when they're younger because they are under the influence of their respective middle class parents. Once they are on their own, they may diverge as they become exposed to different experiences that may influence their intelligence differently.

But according to data, this isn't true. As these siblings go out on their own, their IQ scores become more similar. The apparent reason is that once they are away from the dictates of their adoptive parents, they are free to let their genotypes express themselves. Because they share approximately 50% of their segregating genes, they will become more alike because they are propelled to seek similar sorts of environments. Genes may be more potent in making siblings alike than similarities in home environments.

FACT 6: Intelligence is plural, not singular

Regardless of their views about the existence and the strength of so-called general intelligence, researchers agree that statistically independent mental abilities exist—such as spatial, verbal, analytical and practical intelligence.

In 1995, Yale psychologist Robert Sternberg and colleagues developed new evidence that practical and analytical intelligence are two different things. They demonstrated that the skills of practical intelligence, such as common sense, were important in predicting life outcomes, but were not associated with IQ-type analytic intelligence. There may even be at least seven or eight different kinds of intelligence, says researcher Howard Gardner of Harvard, including interpersonal, intrapersonal, linguistic, motoric and musical intelligence.

FACT 7: IQ is correlated with head size

The relationship between head size and IQ has long been a subject of controversy. Popular writers such as Stephen J. Gould have rightly objected to the crude and biased means 19th-century scholars used to establish this correlation, which were based on head size and contour. But modern neuroimaging techniques demonstrate that cranial volume is correlated with IQ. Evidence also comes from studies of the helmet sizes of members of the Armed Services, whose IQs were measured during basic training. The correlations, however, are quite small.

FACT 8: Intelligence scores are predictive of real-world outcomes

People who have completed more school tend to earn more—over a lifetime, college graduates earn $812,000 more than high school dropouts, and those with professional degrees earn nearly $1,600,000 more than the college grads. But more schooling can't be the only factor in earning differences, because at every level of schooling, there is a variety of intellectual ability.

Even among those with comparable levels of schooling, the greater a person's intellectual ability, the higher that person's weekly earnings. Workers with the lowest levels of intellectual ability earn only two-thirds the amount workers at the highest level earn. Because differences in schooling are statistically controlled, the rise in earning must be due to other factors, such as intelligence.

FACT 9: Intelligence is context-dependent

The setting in which we measure intelligence matters. In 1986, a colleague and I published a study of men who frequented the racetracks daily. Some were excellent handicappers, while others were not. What distinguished experts from non-experts was the use of a complex mental algorithm that converted racing data taken from the racing programs sold at the track. The use of the algorithm was unrelated to the men's IQ scores, however. Some experts were dockworkers with IQ scores in the low 80s, but they reasoned far more complexly at the track than all non-experts—even those with IQs in the upper 120s.

In fact, experts were always better at reasoning complexly than non-experts, regardless of their IQ scores. But the same experts who could reason so well at the track were often abysmal at reasoning outside the track—about, say, their retirement pensions or their social relationships.