Leaving aside heritability, the environment in which the child will grow up is more or less set in concrete by the time they are born. This includes the parents socio-economic level, but more importantly to that, their attitudes towards relationships, learning, critical thought, reading, victim vs control personality and plenty more. A house without books doesn't fill with books when the child arrives.
Pretty much by the time the first cry is made, a huge chunk of their future is already written. The margin is small, but, having said that, small changes at the margin can make a big difference in individuals.