I keep seeing two claims that seem to pull in opposite directions. One is that social interventions cannot meaningfully raise IQ in the long run. The other is that severe deprivation suppresses cognitive development below genetic potential. If deprivation can push IQ down dramatically, why cant enrichment push it up by the same amount in the opposite direction? Is there an asymmetry in how environment affects intelligence and if so why?
The asymmetry is real and important. Environment operates more like a floor than a dial. Severe deprivation suppresses cognitive development below genetic potential and removing those insults produces real gains. But enrichment above an adequate threshold produces rapidly diminishing returns. Once basic developmental needs are met, additional enrichment does not keep pushing scores upward proportionally. The genetic architecture expresses itself most fully in adequate environments, and beyond adequacy more environment buys you progressively less 
Fixing a vitamin deficiency and taking vitamin supplements are not symmetric interventions even though they involve the same nutrient 
The same logic applies to cognitive environments. Removing what is actively harmful produces real gains. Adding more of what is already sufficient produces much less 