Are they really only six weeks apart? I guess they are. It seemed like such a huge gulf when they were newborns, but for all practical purposes they're the same age.

I do find the differences interesting, and I like to try to tease out the effects of birth order, innate temperament, etc. Colin is probably as laid-back as he is in part because he's a second child and has had to conform to Alex's schedule, likes & dislikes, etc. But he's also much more tolerant of novelty than Alex was at his age, and novelty avoidance is a biologically-mediated temperament factor. So that also goes into the equation.

(And BTW, you know that there aren't many kids like Rachel, developmentally, right? She is way advanced.)