Retroviral genetic engineering once we know what genes control rationality.
It has the advantage that it would be in people's self-interest to do this. I suspect that some kind of individually beneficial modification is the solution.
Psychosurgery or pharmaceutical intervention to encourage some of the more positive autistic spectrum cognitive traits seems more likely to work than this. We are far from identifying the genetic basis of intelligence or exceptional intelligence, never mind an aspect as specific as rationality.
It's also not clear that it is in someone's self interest to do this. I know you said retroviral genetic engineering, but for now I'll assume that it would only be possible on embryos. In that case, if someone really wanted grand children, it is not clear that making these alterations in her children would be the best way to achieve that goal.
The question that the ideas are supposed to be in response to is:
Here are links to some proposed ideas that you should vote for, assuming you agree with them. You do have to register to vote, but the email confirmation arrives right away and it shouldn't take much more than two minutes of your time altogether. Why should you do this? The top voted ideas from this request for ideas will be seen by some of the top policy recommendation makers in the USA. They probably won't do anything like immediately convene a presidential panel on AGI, but we are letting them know that these things are really important.
Research the primary cause of degenerative diseases: aging / biological senescence
Explore proposals for sustaining the economy despite ubiquitous automation
Establish a Permanent Panel or Program to Address Global Catastrophic Risks, Including AGI
Does anyone have any other ideas? Feel free to submit them directly to ideascale, but it may be a better idea to first post them in the comments of this post for discussion.