I think saying #2s are the piece-movers is something that can be gleaned from the show alone. I mean, we already know that GRRM has a love affair with disabilities, disfigurements, and anybody not "the first son". I bet the only reason Jaime, as the first son, is still alive is because Locke chopped his hand off. If he ran as good as Robb, death would quicken upon him like the wrath of a scorned Shae.