A major new study provides evidence that environmental factors, not genetics, are the primary drivers behind the Flynn effect – the steady rise in IQ scores observed over the 20th century.
Researchers analyzed IQ data from over 700,000 Norwegian males and found that changes in IQ occur even between brothers from the same family.
This suggests that whatever is causing IQ to change over time is influencing people after birth, rather than being inherited.
Key Facts:
- IQ scores increased steadily through the 20th century, a phenomenon known as the Flynn effect. Recently this trend has reversed in some countries.
- The causes of the Flynn effect have been heavily debated, with hypotheses ranging from genetic factors to environmental ones.
- This new study uses IQ data on over 700,000 Norwegians to compare IQ changes between and within families.
- IQ gains and declines occurred just as strongly within families as across society. This indicates environmental factors are the main driver, rather than genetic ones.
- The results rule out prominent explanations for IQ declines like dysgenic fertility and immigration. The true causes likely involve changes in health, education, media, nutrition or other environmental factors.
Source: Proc Natl Acad Sci USA
The Flynn Effect Explained
The Flynn effect refers to the steady rise in IQ test scores observed throughout the 20th century.
On average, measured intelligence increased by around 3 points per decade between the 1930s and early 2000s.
This means the average person today would score dramatically higher on an IQ test than their grandparents.
This increase contradicted earlier theories that IQ was declining due to lower-IQ individuals having more children (known as dysgenic fertility).
It also raised the question of what was causing such rapid rises generation after generation.
In recent decades, the Flynn effect has reversed in some Western nations.
Average IQ scores have stagnated or even declined slightly.
This led to speculation that the earlier Flynn effect was just temporarily masking an underlying dysgenic trend due to lower-IQ people having more children.
Disentangling The Causes
To distinguish between the many proposed explanations for the Flynn effect, researchers from Norway categorized them based on whether they could generate IQ changes within families.
Genetic factors like dysgenic fertility or immigration would not cause IQ differences between brothers.
But environmental factors that vary during childhood like nutrition, education quality and media exposure could influence siblings differently.
The key question was whether IQ gains and declines occurred both across society and within families.
If so, that would indicate environmental factors are the main drivers, rather than genetic ones.
Powerful Data From Norway
To answer this question, the researchers analyzed an unprecedented dataset – IQ scores from over 700,000 Norwegian males tracked across decades.
Norway’s comprehensive administrative data enabled the researchers to precisely identify family relationships between brothers.
This allowed comparing IQ changes within families to changes in society overall.
The data covered IQ scores from 1962 to 1991, measured during mandatory military conscription at age 18-19.
During this period, average IQ rose from 1962 to 1975, then declined from 1975 onwards.
The symmetrical rise and fall provided the perfect setup for assessing causes of changing intelligence.
Environmental Factors Drive IQ Changes
The results were clear – IQ gains before 1975 and declines after 1975 occurred just as strongly within families as across society.
This means brothers experienced similar IQ trajectories over time, even though they share genetics and family environment.
This establishes that the large changes in average cohort intelligence reflect environmental factors and not changing composition of parents.
In turn, this rules out several prominent hypotheses for declining IQ like dysgenic fertility.
In other words, whatever is driving the Flynn effect up and down over time is influencing people after they’re born, rather than being passed down genetically at birth.
These findings counter the conclusions of some recent studies suggesting genetics and immigration are the main causes of IQ declines.
They also demonstrate that dysgenic fertility is not occurring to any meaningful degree in Norway during the period studied.
Potential Environmental Factors
While these results clearly implicate environmental factors, the exact causes remain unknown.
The data does not allow distinguishing between exposures at different ages or which types of environmental stimuli matter most.
Nevertheless, the findings are consistent with a number of leading hypotheses around declining IQ:
- Poorer education quality or reduced educational exposure
- Increased media exposure like TV and video games
- Worsening nutrition or health
- Negative spillover effects from increased immigration
The most prominent explanations like dysgenic fertility are ruled out, but many environmental theories could still hold.
Further research is needed to pinpoint which environmental changes are driving IQ lower in developed countries.
Key Implications: IQ Trends & Flynn Effect
This study significantly advances our understanding of what factors shape intelligence over time.
The results lend very strong support to environmental explanations over genetic ones for recent IQ trends.
More broadly, the findings highlight that even our cognitive ability is not fixed – it can fluctuate substantially based on modifiable factors in our environment and experiences.
With the right interventions, IQ could potentially be improved.
While more research is still needed, these discoveries around the environmental malleability of intelligence across birth cohorts provide hopeful news.
They suggest that if we can unravel what changed in recent decades to lower IQ, it may be possible to alter environmental conditions and implement policies that raise it once more.
References
- Study: Flynn effect and its reversal are both environmentally caused
- Authors: Bratsberg & Rogeberg (2018)
