The prenatal influence has been directly tested. They compare pairs of non-identical siblings, some who are twins and some who are not. The same pregnancy pairs

The prenatal influence has been directly tested. They compare pairs of non-identical siblings, some who are twins and some who are not. The same pregnancy pairs are about as similar to each other as the other pairs. So the prenatal environment either has little effect, or it has a significant but highly nonlinear effect. (Once again, this assumes no extreme hardship.)