Americans' IQ scores are lower in three domains, higher in one

But a new study from Northwestern University has found evidence of a reverse "Flynn effect" in a large U.S. sample between 2006 and 2018 in every category except one. For the reverse Flynn effect, there were consistent negative slopes for three out of the four cognitive domains.

Ability scores of verbal reasoning (logic, vocabulary), matrix reasoning (visual problem solving, analogies), and letter and number series (computational/mathematical) dropped during the study period, but scores of 3D rotation (spatial reasoning) generally increased from 2011 to 2018, the study found. Composite ability scores (single scores derived from multiple pieces of information) were also lower for more recent samples. The differences in scores were present regardless of age, education or gender.

Despite the decline in scores, corresponding study author Elizabeth Dworak said she doesn't want people to read these findings and think, "Americans are getting less intelligent."

"It doesn't mean their mental ability is lower or higher; it's just a difference in scores that are favoring older or newer samples," said Dworak, a research assistant professor of medical social sciences at Northwestern University Feinberg School of Medicine. "It could just be that they're getting worse at taking tests or specifically worse at taking these kinds of tests."

The study was published earlier this month in the journal Intelligence.

The scientists used the Synthetic Aperture Personality Assessment (SAPA) Project, a free survey-based online personality test that provides test-takers feedback on 27 temperament traits (e.g. adaptability, impulsivity, anxiety, humor) and their ability scores. Thestudy examined survey responses from 394,378 Americans between 2006 to 2018 to examine if cognitive ability scores changed within the U.S. in those 13 years.

A smaller subset of participants (303,540) was recruited between 2011 and 2018. The 3D rotation data only exists for those who took the survey between 2011 and 2018.

Why the decline in IQ scores?

While the study didn't examine the reason for the decline in IQ scores, Dworak said there is no shortage of theories in the scientific community, including poor nutrition, worsening health, media exposures and changes to education.

"There's debate about what's causing it, but not every domain is going down; one of them is going up," Dworak said. "If all the scores were going in the same direction, you could make a nice little narrative about it, but that's not the case. We need to do more to dig into it."

To that end, Dworak and her colleagues are currently trying to access a dataset that contains 40 years of data to conduct a follow-up study.

A shift in perceived values in society also might have affected scores, Dworak said.

"If you're thinking about what society cares about and what it's emphasizing and reinforcing every day, there's a possibility of that being reflected in performance on an ability test," Dworak said.

She gave the example that there's been more emphasis on STEM education in recent decades, but does that mean other areas, like abstract reasoning, are receiving less attention in schools?

Another factor could be due to a decline in motivation, Dworak said. Because the SAPA Project is advertised as a personality survey, individuals seeking out the test may be more engaged with sections related to measuring temperament and less engaged with sections that are seemingly unrelated to personality.