My son was three months old when I bought a set of black-and-white contrast cards because an Instagram ad told me they were “clinically proven to accelerate neural development.” I held them six inches from his face at the exact distance the packaging recommended. He stared at them for about four seconds, then sneezed on one.
I kept going. I had a playlist of Mozart (the research on that turned out to be nonsense, by the way). I had high-contrast nursery art. I narrated everything I did like a nature documentary: “Now Daddy is making coffee. Daddy is using the pour-over method because Daddy has opinions about extraction.” I was operating on a belief so deeply embedded I hadn’t even examined it: that the right stimulation, delivered at the right time, in the right way, would make my child measurably smarter.
It’s a belief the entire parenting industry runs on. And the research says it’s mostly wrong.
The Study That Reframes the Conversation
The largest review of twin studies ever conducted was published in Nature Genetics in 2015. Researchers analysed 2,748 publications covering 17,804 traits across fifty years of data. I mentioned this study in my first article because it reframes how we think about parenting in general. But it has something specific to say about intelligence that’s worth sitting with.
For cognitive ability, the heritability estimates are among the highest of any trait studied. Roughly 50% of the variation in IQ between people is attributable to genetic differences. And that number increases with age. A meta-analysis tracking twins from childhood through adulthood found that heritability rises from about 41% at age nine to 66% by seventeen, and reaches 70-80% in adulthood.
Read that trajectory again. As children grow up, leave home, and make their own choices, the influence of their family environment on their measured intelligence decreases, and the genetic component increases.
That is not what I expected to find when I started reading this literature.
What “Heritable” Actually Means (and What It Doesn’t)
Before you close this tab, I need to be precise about what heritability means, because it’s one of the most misunderstood concepts in all of science, and getting it wrong leads to genuinely harmful conclusions.
Heritability is a population statistic. It tells you what proportion of the variation between people in a given population is associated with genetic differences. It does not tell you that any individual’s IQ is “80% genetic.” It does not mean that intelligence is fixed at birth. And it does not mean the environment doesn’t matter.
Here’s an analogy that helped me. Imagine a hundred plants grown in identical soil, with identical water and sunlight. The variation in their heights would be almost entirely genetic, because the environment is the same for all of them. Now imagine those same seeds planted in wildly different conditions — some in rich soil, some in sand, some in darkness. The variation in heights would be heavily environmental. The heritability of height hasn’t changed. The seeds haven’t changed. The environment changed.
This isn’t just a thought experiment. It’s almost exactly what the research shows for intelligence.
The Finding That Changes Everything: Poverty Rewrites the Rules
In 2003, Eric Turkheimer and colleagues published a study in Psychological Science that should be required reading for anyone who talks about IQ and genetics. They studied twins from the National Collaborative Perinatal Project, a dataset that — critically — included a substantial proportion of families near or below the poverty line. Most twin studies draw from middle-class populations, which bakes in a major blind spot.
What they found: in affluent families, IQ heritability was about 72%, and the shared family environment (parenting style, household resources, educational investments) accounted for almost nothing. But in families living in poverty, the pattern reversed. Heritability dropped to around 10%. The shared environment — the stuff parents actually control — accounted for roughly 60% of the variation.
Sixty percent.
Let that recalibrate. In wealthy families, where children’s basic needs are met and opportunities are abundant, genetic potential expresses itself relatively freely and parenting style makes little measurable difference to IQ. In poverty, where environmental constraints are severe, the family environment dominates and genetics barely registers.
I should note: follow-up studies in different countries have produced mixed results. Some Australian and UK samples found a weaker or non-significant interaction, which may reflect the smaller range of socioeconomic deprivation in those populations. The interaction appears strongest where poverty is most severe. But the core finding — that environmental constraints suppress genetic expression — has held up across multiple research designs and is now the standard framework in behavioral genetics.
This is probably the single most important finding in the entire IQ-and-parenting literature, and almost no parenting content mentions it. It means the answer to “does parenting affect IQ?” is: it depends entirely on whether your child’s basic needs are being met. For most families reading a parenting website in Australia, the honest answer is: probably not much.
I want to acknowledge how that might land. If you’ve been investing time and money into enrichment activities, educational toys, and brain-building programs because you believed they’d give your child a cognitive edge — and I was exactly this parent — hearing that the research doesn’t support that belief is uncomfortable. Nobody wants to feel like their effort was misplaced.
But there’s a liberation in this, and I’ll get to it.
What Adoption Studies Show (and Where the Limits Are)
If family environment really doesn’t matter much for IQ in adequate-resource households, you’d expect adoption studies to confirm that. They do, with an important caveat.
Children adopted from severely deprived environments — orphanages, extreme neglect — into stable families show substantial IQ gains. A meta-analysis of 75 studies covering 3,800 children from 19 countries found that children raised in institutional care scored roughly 16 IQ points lower than adopted peers who grew up in families. Sixteen points is enormous. That’s the difference between the 50th percentile and the 16th.
So adoption into a functioning family rescues children from environmental deprivation. That’s unambiguous good news about the power of the environment.
But here’s the subtler finding: among children adopted into families that range from adequate to excellent, the differences in adoptive family quality produce much smaller effects on IQ. And those effects diminish with age. By adulthood, adopted siblings raised in the same household show almost no more IQ similarity than strangers.
The pattern is consistent across decades of research. The environment sets a floor. Below that floor, outcomes are devastating and intervention is critical. Above it, additional enrichment produces diminishing returns on measured intelligence. The difference between a good-enough home and a meticulously optimised one, in terms of IQ, is close to zero.
The Head Start Puzzle: Why IQ Gains Fade But Life Gets Better
This is where the story gets genuinely interesting, and where the research challenges simplistic narratives on both sides.
The Perry Preschool Project, launched in the 1960s, was a randomised controlled trial that gave high-quality early childhood education to low-income children in Michigan. The initial results were exciting: children in the program showed meaningful IQ gains.
Then those gains faded within a few years.
This is the “cognitive fadeout” finding, and it’s been replicated across multiple early childhood programs, including Head Start. IQ boosts from enrichment programs tend to wash out by mid-childhood. If you’re keeping score from a pure IQ perspective, this looks like evidence that environmental interventions don’t work.
Except the Perry Preschool kids were tracked through age 40. And something remarkable happened. Despite no lasting IQ advantage, the children who received the program had higher earnings, better health outcomes, lower rates of criminal justice involvement, more stable families, and better executive functioning as adults. The benefits even extended to their own children.
The program didn’t make them “smarter” by IQ standards. It gave them something else: better self-regulation, stronger motivation, more effective habits. The skills that actually determine how well you navigate adult life aren’t captured by an IQ test. They’re captured by how you show up, day after day.
This should sound familiar if you read the first article in this series. The things that produce good outcomes for children are habits, culture, and environment. Not cognitive enhancement programs.
The Flynn Effect: Proof That IQ Is Culturally Malleable
One more piece of the puzzle. Over the 20th century, IQ scores rose by roughly 3 points per decade across most developed nations. This is the Flynn Effect, named after the researcher James Flynn, and it represents gains far too rapid to be genetic in origin. Genes don’t change that fast.
The likely drivers: better nutrition, more cognitively demanding modern environments, changes in how people think rather than how much raw intelligence they possess. Flynn himself argued that populations weren’t actually getting smarter — they were getting better at the kind of abstract reasoning that IQ tests measure, because modern life demands more of it.
What’s interesting for parents: the Flynn Effect has recently stalled or reversed in several countries, including Scandinavian nations and the US. Whatever environmental factors were driving the gains appear to have plateaued. This suggests there’s a ceiling to environmental influence on IQ scores, at least within the range of conditions that exist in developed nations.
The Flynn Effect proves the environment matters for IQ at a population level. It also suggests that once basic environmental conditions are met, additional improvements produce diminishing returns. Sound familiar?
So What Actually Matters for Your Child’s Cognitive Development?
If you’ve read this far, you might be wondering what the point is. If IQ is largely heritable in adequate-resource households, and enrichment programs don’t produce lasting cognitive gains, should you just give up?
No. But you should redirect your energy.
The research consistently points to a distinction between measured intelligence (IQ) and the cognitive skills that actually determine life outcomes. Executive function — the ability to plan, focus, regulate impulses, and adapt to new situations — is more responsive to environmental influence than IQ, and more predictive of real-world success than IQ once you account for a threshold level of cognitive ability.
What builds executive function? Not flash cards. Not contrast cards. Not Mozart.
Autonomy. Practice making decisions. Responsibility for real tasks. The experience of managing frustration and trying again. An environment where a child can choose, fail, and learn from the consequence without a parent swooping in to manage the outcome.
A two-year-old who sets the table every night is building executive function. A three-year-old who gets dressed by themselves — slowly, with mismatched socks — is building executive function. A child who has to wait for something they want, without a screen to fill the gap, is building executive function.
This is, not coincidentally, what Montessori environments are designed to produce. Not higher IQ scores. Better self-regulation, concentration, and the habit of purposeful, independent activity. The 2023 meta-analyses I cited in the first article found positive effects across executive function, social skills, creativity, and motor development. These are the outcomes that map to Perry Preschool’s long-term benefits. They’re the outcomes that matter.
The Reframe
Here’s where I landed after reading through this literature, and it’s the same place Article 1 brought me, just from a different angle.
You cannot meaningfully raise your child’s IQ through parenting choices, assuming their basic needs are met. That is what decades of twin studies, adoption studies, and intervention research consistently show. The contrast cards don’t work. The Baby Einstein DVDs (remember those?) didn’t work either — Disney offered refunds in 2009 after the research made that clear.
But you can build the cognitive habits that determine what your child does with the intelligence they were born with. You can create an environment that develops concentration, self-regulation, persistence, and the capacity to work through difficult things without someone rescuing them. You can raise a child who is resourceful, autonomous, and intrinsically motivated.
That’s not a consolation prize. Those skills are more predictive of life outcomes than IQ once you’re above a basic threshold. And unlike IQ, they’re directly responsive to the environment you create.
Stop trying to make your child smarter. Start giving them the habits and the freedom to use what they’ve already got.
This is part of the Little Groundwork protocol series — an evidence-based framework for what actually matters in parenting. Next: The Case Against Helicopter Parenting.
References
Polderman, T. J. C., Benyamin, B., de Leeuw, C. A., Sullivan, P. F., van Bochoven, A., Visscher, P. M., & Posthuma, D. (2015). Meta-analysis of the heritability of human traits based on fifty years of twin studies. Nature Genetics, 47(7), 702–709. https://doi.org/10.1038/ng.3285
Briley, D. A., & Tucker-Drob, E. M. (2013). Explaining the increasing heritability of cognitive ability across development: A meta-analysis of longitudinal twin and adoption studies. Psychological Science, 24(9), 1704–1713. The “Wilson Effect” meta-analysis of 11,000+ twin pairs showing heritability increases linearly with age.
Turkheimer, E., Haley, A., Waldron, M., D’Onofrio, B., & Gottesman, I. I. (2003). Socioeconomic status modifies heritability of IQ in young children. Psychological Science, 14(6), 623–628. https://pubmed.ncbi.nlm.nih.gov/14629696/ (The one that should have changed the entire conversation about IQ and parenting. It mostly didn’t.)
van IJzendoorn, M. H., Bakermans-Kranenburg, M. J., Duschinsky, R., Fox, N. A., Goldman, P. S., Gunnar, M. R., … & Sonuga-Barke, E. J. S. (2020). Institutionalisation and deinstitutionalisation of children: A systematic review and meta-analysis. The Lancet Psychiatry, 7(8), 703–720. The 75-study, 19-country meta-analysis on institutional care and IQ.
Schweinhart, L. J., Montie, J., Xiang, Z., Barnett, W. S., Belfield, C. R., & Nores, M. (2005). Lifetime effects: The HighScope Perry Preschool study through age 40. HighScope Press. IQ gains faded. Everything that actually matters didn’t.
Bratsberg, B., & Rogeberg, O. (2018). Flynn effect and its reversal are both environmentally caused. Proceedings of the National Academy of Sciences, 115(26), 6674–6678. Evidence of Flynn Effect reversal in Scandinavia.
Demangeon, A., Claudel-Valentin, S., Aubry, A., & Tazouti, Y. (2023). A meta-analysis of the effects of Montessori education on five fields of development and learning in preschool and school-age children. Contemporary Educational Psychology, 73, 102182. https://doi.org/10.1016/j.cedpsych.2023.102182