Retroviral genetic engineering once we know what genes control rationality.

Would those genes have time to be expressed later in life?

Also, how heritable is rationality? How do we even measure it, to find out? And if we find some subset of our genes which can be reasonably tweaked to enhance rationality, I'm guessing that they would probably affect people's capability to be rational, in a bunch of indirect ways. There would have to be a memetic component, alongside any genetic tweaks we make. I think.