[gwern]

> You would only be able to select from those 5 backgrounds, however, and the predicted heights for those embryos will be drawn from a normal distribution; you can only "design" from what you've sampled.

Yes, and this is cheap and does lead to gains. (Specifically: around 1 inch.)

> but this would be very expensive and basically impossible without synthetic embryos.

First, this is not remotely what you said. Second, it's not true: you only need more eggs and they don't have to be 'synthetic' whatever that means, and there's a lot of work on inducing egg development from stem cells so in another 10 years it may well be possible for parents to do massive selection like that. Third, you don't need millions of embryos if you just want a very tall person, as the advantage is cumulative over generations (which is the critical insight behind Iterated Embryo Selection: it's much more efficient to take a few hundred embryos through multiple generations of selection than it is to try to brute force a single selective step). That's omitting any gains from CRISPR or genome synthesis or other methods not yet thought of. Fourth, why would any parent want that in the first place as that's into the realm of potential healthy problems (even if we're assuming only male embryos) and beyond the useful level of height advantages, especially when they could be instead spending that count of embryos to maximize a weighted sum of all health and other complex traits? (Remember, just because everyone talks about doing embryo selection on a single trait at a time doesn't make that remotely a good idea; there are big gains to selecting on many traits simultaneously.)