There is no formal concept of “de-evolution” in biology. Modern evolutionary theory does not recognize evolution as necessarily progressive or as a process that always increases complexity or “improves” organisms. All genetic changes—whether they lead to greater or lesser complexity, the loss or gain of functions, or adaptations that seem “backwards”—are simply considered evolution[9][11][13][15]. Evolution only means changes in a population’s gene frequencies over generations driven by selection, mutation, drift, and gene flow, resulting in organisms that are better fitted to their current environment[10][12][14][18].
The notion of “de-evolution” or “backward evolution” assumes a linear, goal-oriented view where some traits or forms are intrinsically better, more advanced, or more desirable than others. However, evolutionary biology rejects this perspective. In evolutionary terms, any change that increases fitness—that is, the ability to survive and reproduce in the current context—is adaptive, whether it appears as increased complexity or simplification from a human point of view[9][13][14].
Thus, if a species loses a function or becomes less complex in some ways (for example, losing eyes in cave-dwelling animals or flight in some birds), this is not evolution “in reverse” but simply adaptation through the same evolutionary processes. No hierarchy of “primitive” versus “advanced” is assumed by biologists[9][11][13]. The term “de-evolution” or its concept is not compatible with current scientific understanding of evolution[9][13][15][17].
Intelligence and Reproductive Trends: Implications for Future Populations
Introduction
The relationship between human intelligence and reproductive behavior has long been a subject of interest in psychology, sociology, and demography. Traditional views often suggested that higher intelligence correlated with lower fertility—a pattern sometimes described as “dysgenic fertility.” However, recent large-scale and representative studies paint a more nuanced picture, revealing complex patterns of fertility across intelligence strata that have significant implications for the future composition of human populations.
Current Understanding of Intelligence and Fertility
The correlation between intelligence and fertility varies depending on population, time period, and methods of measurement. Historically, many studies found that individuals with higher IQs tend to have fewer children and delay reproduction, a pattern especially observed in many Western societies during the 20th century. This pattern aligned with socio-economic factors such as longer education, career prioritization, and lifestyle choices associated with higher cognitive abilities.
However, an important recent study from Sweden using comprehensive national data (covering all men born 1951–1967) challenges the simplistic notion of a universal negative relationship between intelligence and fertility. This research found an overall positive association between cognitive ability and completed fertility in men after adjusting for familial, socio-economic, and educational confounders. Specifically, men with very low IQ scores tend to have significantly fewer children, while men with above-average IQs have slightly more children on average[1].
The key findings from this study include:
- Men with very low IQ (below 76) had on average 0.56 fewer children than men with median IQ.
- Men with high IQ (above 126) had about 0.09 more children than median IQ men.
- The positive correlation remained after controlling for education, family background, and genetics.
- Low fertility was concentrated among socially disadvantaged groups, who may face obstacles in realizing fertility preferences.
- The study suggested that children born to men with lower IQs often occur at younger ages, while higher IQ men reproduce later and invest more resources per child.
Another meta-analysis of global studies, though confirming many regional differences, indicates that the relationship between intelligence and fertility is not firmly negative and can vary by country, cohort, and gender[6][7].
Demographic Implications and Future Trends
The implication of these data is important for understanding population changes over time. Although earlier literature warned of “dysgenic” fertility—where lower intelligence groups have higher reproductive rates—in many contemporary high-income countries, this trend is becoming less clear-cut.
- In Nordic countries, the fertility pattern indicates that higher cognitive ability groups maintain or even slightly increase representation in reproduction.
- The lower fertility of socially disadvantaged groups may result from both personal choice and socioeconomic constraints.
Because fertility decisions intertwine with socio-economic status, health, and education, demographic transitions and social policies can shift these trends further.
Fertility Timing and Cognitive Ability
Another crucial dimension concerns timing of reproduction. Lower intelligence is often associated with earlier age at first childbirth, while higher intelligence correlates with delayed reproduction. This delayed reproduction among intelligent individuals aligns with prolonged education and career development.
The effect of delayed reproduction affects generational replacement dynamics: earlier reproduction may increase the relative proportion of faster reproducing traits, but in societies with below replacement fertility levels, common in modern developed nations, delayed reproduction can stabilize or increase the proportion of slower reproducing but higher intelligence groups over time[1].
Cautions and Complexities
It is important to note:
- Trends vary widely by country, gender, and cohort; some populations still exhibit traditional negative correlations of intelligence and fertility.
- Fertility decisions are influenced by cultural, economic, and policy factors, which can shift rapidly.
- Intelligence is multifaceted and measured variably in studies—historical biases and methodological limitations exist.
- Most research focuses on populations of European descent or high-income nations, limiting global generalizability.
If Current Trends Continue: What Could the Future Hold?
Projections based on older models suggested a potential future where populations become dominated by lower intelligence groups due to differential fertility—a scenario sometimes interpreted as a decline in genotypic IQ by the year 2100 or shortly thereafter. However, the latest evidence from representative studies complicates this narrative, showing potentially stabilizing or reversing fertility trends in some societies.
Still, ongoing monitoring of demographic patterns combined with social interventions in education, health, and family support remain crucial to shaping equitable and sustainable population futures.
Summary
Recent large-scale research indicates a positive or neutral relationship between intelligence and fertility in some high-income countries, contrary to older assumptions. Fertility tends to be lower among very low IQ groups, possibly due to social disadvantages and reproductive constraints. Timing of reproduction differs by intelligence, with higher IQ associated with later parenthood. Predictions of “dysgenic fertility” leading to population cognitive decline have become less certain, reflecting complex socio-demographic dynamics. Population intelligence composition depends on many factors including education, socioeconomic opportunity, fertility timing, and health policy.
References
1. Z. C. Mills et al., “Cognitive ability and fertility among Swedish men born 1951–1967,” Proceedings of the Royal Society B, 2019.
2. G. Lynn & J. Harvey, “Fertility and IQ: A review of the data and implications,” Intelligence, 36(3), 2013.
3. Wikipedia contributors, “Fertility and intelligence,” Wikipedia, June 2008.
4. A. Kanazawa, “Intelligence and fertility: A cross-national study,” Personality and Individual Differences, 2007.
5. Retherford & Sewell, “How Intelligence Affects Fertility 30 Years On,” Twin Research and Human Genetics, 2019.
6. OpenPsych, “International meta-analysis of differential fertility for intelligence,” 2020.
7. L. Nilsen et al., “A systematic review of the relationship between cognitive ability and fertility,” Journal of Biosocial Science, 2018.
Read More
[1] https://royalsocietypublishing.org/doi/10.1098/rspb.2019.0359
[2] https://pubmed.ncbi.nlm.nih.gov/12267266/
[3] https://en.wikipedia.org/wiki/Fertility_and_intelligence
[4] https://www.sciencedirect.com/science/article/abs/pii/S0160289607000244
[5] https://www.cambridge.org/core/journals/twin-research-and-human-genetics/article/how-intelligence-affects-fertility-30-years-on-retherford-and-sewell-revisited-with-polygenic-scores-and-numbers-of-grandchildren/AB8EF68EE05C8DFD0A1C424B4FF7BC1F
[6] https://openpsych.net/files/submissions/9_International_meta-analysis_of_differential_fertility_for_intelligence.pdf
[7] https://www.sciencedirect.com/science/article/abs/pii/S0191886918302903
[8] https://pubmed.ncbi.nlm.nih.gov/25131282/
[9] https://consensus.app/questions/evolution-and-devolution-in-biology/
[10] https://www.reddit.com/r/evolution/comments/1ie5pu2/is_evolution_always_progressive/
[11] https://handwiki.org/wiki/Biology:Devolution
[12] https://biology.stackexchange.com/questions/57356/why-should-evolution-not-be-equated-with-progress
[13] https://en.wikipedia.org/wiki/Devolution_(biology)
[14] https://news.harvard.edu/gazette/story/2013/04/not-as-evolved-as-we-think/
[15] https://www.basicknowledge101.com/pdf/maintain/Devolution%20(biology).pdf
[16] https://ncse.ngo/misconception-monday-myth-de-evolution-part-1
[17] https://www.bionity.com/en/encyclopedia/Biological_devolution.html
[18] https://pmc.ncbi.nlm.nih.gov/articles/PMC2753274/