I see two completely opposite claims about this. One side says IQ is basically set by your genes and stabilizes early in childhood. The other side points to things like the Flynn effect, early interventions, and lead exposure research to argue that IQ is highly malleable. What is the actual scientific consensus here and is there a way to reconcile these two positions or are they genuinely incompatible?
They are not actually incompatible once you separate two different questions. The first is whether IQ scores can change in absolute terms. The answer is clearly yes. Environmental insults like lead exposure, severe malnutrition, untreated hypothyroidism, and chronic stress can meaningfully suppress cognitive development, and removing those insults can produce real score gains. The Flynn effect shows that average scores rose substantially across the 20th century, which proves the phenotype is environmentally sensitive at the population level.
The second question is whether relative standing within a population is malleable, meaning can interventions change where you rank compared to others. That is where the evidence gets much less encouraging. Heritability of IQ rises from around 40 percent in childhood to around 80 percent in adulthood, meaning environmental factors that differentiate people matter less and less as they age. Early intervention gains tend to fade out by adolescence. The rank order of individuals stays remarkably stable once you get past childhood.
So the reconciliation is this: IQ is malleable at the extremes, especially when bad environments are suppressing potential, but it is not meaningfully malleable across the full distribution through enrichment alone.
A useful way to think about it is a rubber band. You can stretch it or compress it with enough environmental force, but it has a resting length it returns to. Removing a thumb on the scale is real and important. Permanently repositioning someone on the distribution through enrichment is a much harder claim and the evidence for it is weak.