⬇️ Prefer to listen instead? ⬇️
- Intelligence links to starting puberty earlier in both boys and girls.
- Even though their bodies mature early, intelligent people wait a long time to have children.
- System integrity theory connects intelligence to better body function overall.
- Life history theory says more intelligence means people take a slower, more planned way to have children.
- Things like education and having birth control strongly affect how intelligent people decide about having children.
Why do smart people start puberty earlier but wait to have children, or even not have them at all? This is a key question in new findings published in Adaptive Human Behavior and Physiology. Researchers looked at data from thousands of people over many years. They are finding out how intelligence affects when puberty starts, how people act sexually, and when they have kids. This goes against what people expected from how humans evolved.
A Big Study Over Many Years
To understand the link between intelligence and puberty, and how it affects having children, researchers used two big long-term sets of data
- The UK’s National Child Development Study (NCDS). This looked at over 17,000 people born in 1958, who were typical of the country. They were followed until they were adults.
- The U.S. Add Health study. This followed more than 20,000 teenagers starting in 1994 until they were adults.
These studies gathered a lot of data. They looked at mental skills (like IQ scores). But they also looked at signs of growing up, like when puberty started. And they looked at many life habits, from school to having children. Not many studies have this much detail, cover this many things, and last this long. This makes the findings very strong.
Intelligence and the Body: More Connected Than You Might Think
A main idea in the study is the System Integrity Theory. It says intelligence is not just sharp thinking. It shows how well the body’s systems work overall. This theory sees the body like a machine working together well. If one part works best, other parts often work well too.
Starting Puberty Early Shows Systems Work Well
From this view, starting puberty early isn’t just random. It shows the body is growing up well. The researchers saw the same pattern in both sets of data
- Girls with higher intelligence started having periods (menarche) earlier.
- Boys with higher intelligence showed signs of puberty earlier, like their voice getting deeper and growing facial hair.
This finding is strong because it held true even when the researchers looked at body mass index (BMI) and how much schooling people had. This means intelligence itself, not other things, was a key reason for early puberty.
Basically, the smarter someone is, the earlier their body seems ready to have children. This suggests they developed well early on. It might also mean fewer problems with genes or development.
Starting Puberty Early — But Waiting to Have Children
Here is the puzzle: while Intelligent people’s bodies become able to have children earlier, they choose to wait to have them.
For example
- In the British group, the most intelligent women waited almost seven years longer to have children than the least intelligent women.
- In the U.S. Add Health data, similar waiting patterns showed up. Also, intelligent teenagers had fewer sexual partners. And they were less likely to get married or live with someone in their twenties.
So, intelligent people start puberty early. But their choices about sex and having children are more planned and careful. They often pick to wait a lot longer before starting a family. Or sometimes they choose not to have children.
How the System Integrity Theory Works Here
System Integrity Theory explains how intelligence shows how strong the body is, not just the mind. But it also suggests something more complex: smarter bodies can have children sooner, but smarter minds wait to do so.
Many body signs of early puberty usually mean someone is ready to have children. A long time ago in human history, starting puberty earlier may have helped people survive. They could have children sooner. But today, that body readiness is often less important than personal goals, social settings, and making hard choices. These are areas where intelligent people might act differently.
So, this theory goes beyond just a medical idea. It helps us see human behavior more broadly. And it shows how the ways humans have changed over time are now shaping how people live today.
Life History Theory — Intelligence as a Smart Tool
The link between intelligence and having children later also fits well with another key idea: Life History Theory.
Fast vs. Slow Ways of Living
Life History Theory says living things (like people) change how they grow, have children, and try to stay alive based on where they live. Some people use ‘fast’ ways of living. They grow up fast, have kids early, and don’t plan much for the future. People often need to do this in hard places where you don’t know if you will survive.
Intelligent people, though, tend to follow slower ways
- They put more effort into school, building a career, and improving themselves.
- They are more likely to wait for rewards, which is a common sign of high intelligence.
- They choose to have children later, if they have them at all. They like quality over how many children they have.
Looking at it through life history, this is not just a strange choice. It is smart change: using their thinking skills to get the best results in life when they live in a place with many resources and things you can count on.
Evolutionary Novelty Theory — Intelligence Lets People Make New Choices
Another idea that adds more detail is the Evolutionary Novelty Theory. It suggests smart people are more likely to do things differently than how humans typically evolved to act.
Long ago, having sex early and having children often helped people survive. Children could help with work and safety. But today, with birth control, jobs using computers, and longer lives, these old reasons for having children early are not as strong.
Intelligence and How People Live Today
People with higher intelligence are more likely to
- Think about family setups that are not typical (or not having a family at all).
- Put personal happiness, money, travel, or helping others before having children.
- Do things that would have been very rare or risky a long time ago (like not having sex for a long time, trying hard for art or job goals instead of family).
Simply put, intelligence lets people think about ideas that are not simple, are hard to grasp, and are new based on how we evolved. This includes waiting to have children or deciding not to have them.
One Body, Two Paths: Biology vs. How People Act
This study is especially interesting because it shows a difference between when the body is ready and how someone actually acts.
- Body path: Intelligent bodies grow up faster.
- Action path: Intelligent minds move slower toward having children.
This shows a key point: just because the body is ready does not mean the mind agrees. A long time ago, how the body worked told people what to do. But today’s society has given people, especially smart ones, ways to not follow that plan.
Culture, Birth Control, and Today’s World
All these findings happen within a bigger culture. Today’s Western societies offer
- Lots of birth control available.
- Society is okay with people waiting to get married and have children.
- School and job choices that are easy to get for both men and women.
This safety net lets people, especially smart ones, put off having children without a big immediate body cost. But it also brings issues later on, like more people not having children or having fewer children later in life.
Intelligence and Schooling: They Are Closely Linked
An interesting finding from the data: when researchers looked at schooling, the link between intelligence and waiting to have children became weaker.
Why? Because intelligent people tend to stay in school longer. And that on its own helps cause them to wait to have children.
So, schooling acts like two things here
- It shows how smart someone is.
- It changes what people care about most in life. This often makes starting a family less important during the years when people are most able to have children.
This makes it hard for researchers to figure out how much having children is shaped by natural thinking skill. It’s also hard to know how much it is shaped by things like going to college and moving up in a job.
Does This Happen Outside Western Countries?
Both sets of data in the study are from Western countries, the U.K. and the U.S. This makes us ask: would these patterns be the same in other places?
In societies with
- Old ways for men and women,
- Not much access to school or birth control,
- Or strong religious or culture pressure to get married early,
…the link between intelligence and having children may look very different.
How Culture Matters
Intelligent people who have to marry early or have children early because of culture might not be able to act as freely as people in the West. Or, places with strong social support and open views on sex may make smart people even more likely to wait to have children or not have them.
How This Helps Us See Bigger Trends in Having Children
Fewer children are being born around the world
- In countries like Japan, Italy, and South Korea, not enough children are being born to replace the parents.
- People with a lot of schooling who are intelligent are often the ones having the fewest children, or none at all.
This trend could mean big things for how populations look, how economies work, and even how genes are passed on.
If intelligence is tied to having fewer children, what will that mean for how smart people in society are in the future? Could this create a cycle in evolution that prefers slightly different kinds of people or smartness? These are serious questions that researchers are starting to look at.
What Researchers Will Look at Next
This study gives us strong ideas. But it is not the last word. Next, researchers will
- Look at how intelligence affects choices about being a parent, not just if or when people have children.
- Study personality traits connected to intelligence, like being careful, not liking risk, or caring more about the future. And look at how these affect having children.
- Find out about things society and rules do that either help or stop intelligent people from having children.
Understanding these workings helps people who study populations and evolution. And it also helps teachers, family helpers, and those who make rules. They need to get used to changing ideas about having children.
Intelligence, Biology, and How Humans Change — Still Becoming Clear
At first, smart people starting puberty early but waiting or not having children might seem odd. But when you look at it through evolutionary psychology, life history strategy, and how people adapt to today’s society, the puzzle makes full sense.
Smart bodies are ready early. Smart minds decide to wait.
Humans are not just living things. They figure things out, plan, and change. And in this world that is always changing, with chances, stress, and waiting, how we evolved to act is being written again right now.
Jose Yong, one of the study authors, notes that the differences seen between biology and how people act often show bigger truths about being human. It is a mix of instinct, smartness, and new ideas guiding choices about having children today.
Citations
- Yong, J. C., & Kanazawa, S. (2025). Able but unwilling: Intelligence is associated with earlier puberty and yet slower reproduction. Adaptive Human Behavior and Physiology. https://doi.org/10.1007/s40750-025-00258-5
