Factors That Can Affect Intelligence and Its Measurement


By Catherine A. SandersonAmherst College

Are there group differences in intelligence? On the one hand, there are pretty consistent group differences in scores on intelligence tests, and group differences on intelligence tests are found in many cultures. In the United States, whites outscore Blacks; in Israel, Israeli Jews outscore Israeli Arabs; in Japan, most Japanese nationals have outscored the Burakumin.

Online Intelligence test on a laptop screen
Intelligence test scores are also influenced by group differences. (Image: Rawf8/Shutterstock)

Culturally-free Intelligence Tests

Here’s one example of a culturally-free intelligence test: Water lilies double in an area every 24 hours. At the beginning of the summer, there is one water lily on a lake. It takes 60 days for the lake to become covered with water lilies. On what day is the lake half covered? Did you get it right?

Figuring out the answer doesn’t require any math; just insight. The lake is totally covered with water lilies on day 60, and every day the water lilies double. So, the day before day 6059it is half covered.

But many intelligence tests are not, in fact, culturally free. Here are a few sample questions from a test developed by the United States government to evaluate the intelligence of both immigrants and World War I army recruits.

The Pierce-Arrow is made in: Flint, Buffalo, Detroit, Toledo.
The Brooklyn Nationals are called the: Giants, Orioles, Superbas, Indians.

So, how did you do? I’m going to guess not very well. But the bigger question here is, of course, how well these items actually measure what we consider intelligence. Instead, they seem to assess cultural familiarity with products, sports teams, and so on. 

It’s not so surprising then that many immigrants who took such tests were labeled “imbeciles” or “feeble-minded”, which in turn became a basis for turning away immigrants. It’s virtually impossible to create a culturally-free test of intelligence.

Brain SizeIntelligence Link

A human brain
According to some studies, the bigger the size of the brain, the more intelligent the person is. (Image: Patrice6000/Shutterstock)

There’s been strong interest in figuring out more objective ways to assess intelligence. One old idea has been to measure brain size. After all, humans have larger brains than most animals, and humans are smarter than animals. Most research historically has pointed to no association between brain size and intelligence. 

But a 2019 study testing more than 13,000 people found a slight positive association. On average, people with larger brains do tend to perform a little better on intelligence tests including assessments of logic, memory, and reaction time than people with smaller brains.

These results held true even when researchers took into account other factors linked with intelligence, such as age, sex, socioeconomic status, and height. But as the study authors themselves note, their finding isn’t as impressive as you might suppose; brain size accounted for only about 2% of the overall intelligence test performance.

This article comes directly from content in the video series Introduction to PsychologyWatch it now, on Wondrium.

The Influence of Nature and Nurture 

Another question of great interest is the extent to which intelligence test is influenced by nature versus nurture. And the short, hard-won answer is “yes” and “yes”. First, considerable evidence suggests that people who share the same genes do share comparable mental abilities. 

And yet, that’s definitely not the whole story. The environment also has a major effect on intelligence in really interesting and important ways. For example, we know that by the time kids are four years old, those who are in poor families may have heard millions fewer spoken words, than those who are in middle or upper-class families.

And regardless of class, families vary considerably in the amount of helpful parentese they use, and the number of two-way conversational turns, or interactions, they provide.

Environmental Factors

Environmental factors have a tremendously strong influence on group differences in intelligence. For example, cross-cultural studies demonstrate that Asian students outperform American students on math tests. Does this mean people who are Asian are genetically smarter? Probably not. 

Woman's hands resting on a table next to an incomplete jigsaw puzzle
Even environmental factors influence people’s intelligence. (Image: Orawan Pattarawimonchai/Shutterstock)

Asian students attend school about 30% more each year and spend a greater percentage of their time in and out of school studying math. So, they actually spend many more hours each year learning and practicing math which probably pays off.

Similarly, parents certainly encourage their children to pursue some interests or activities more than others, either directly or indirectly. Parents may send their sons to math camp or buy them particular types of spatial-enhancing toys (blocks, Legos, train sets), but encourage their daughters to work on writing, reading, or artistic endeavors.

How Stereotypes Affect Intelligence Tests

The term “stereotype threat” refers to how one’s awareness of a negative stereotype about members of a person’s group leads members of that group to experience anxiety, which in turn impairs performance.

One of the first demonstrations of stereotype threat was published in 1995, when researchers at Stanford University brought in both African American and white college students and asked them to take a test of verbal skills, a test in which there is a stereotype that African Americans do less well than whites. 

To test whether inducing this stereotype would influence performance, half of the students were told that the test they would be taking evaluated “intellectual ability.” The other half of the students were simply told that they would be taking a “problem-solving task unrelated to ability.”

Although all students took the same test, African American students who were specifically told that the test measured intellectual ability performed substantially worse than whites. In contrast, there were no differences in test scores between African Americans and whites for those who were not deliberately reminded of the negative stereotype about their ethnic group’s verbal abilities.

Subsequent research has revealed that stereotypes about group performance on tests can have positive as well as negative effects. For example, Asian American women who are asked to think about their Asian identity before taking a math test do even better on this test, whereas those who are asked to think about their female identity do worse. Such studies illustrate that group differences in intelligence may reflect testing conditions and cultural cues, not inherent genetic differences.

Common Questions about the Factors That Can Affect Intelligence and Its Measurement

Q: Why was the intelligence test developed by the United States government hard for the immigrants to pass?

It was because the intelligence test was not culturally-free, meaning that questions were good for assessing cultural familiarity with sports teams and products.

Q: What is the association between brain size and intelligence?

According to a 2019 study, people with larger brains perform much better on intelligence tests than people with small ones.

Q: What is a stereotype threat?

Stereotype threat is the social perception about the group, and it clearly seems to contribute to group differences in intelligence. In other words, when someone is aware of the negative stereotypes of a group, the members of that group become nervous, and their anxiety leads to poor performance on tests.

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