I thought about posting this in the novel thread, but it relates to the movie too, so I'll post it here:

I had always wondered what the impetus was behind Puzo's decision to write the Corleone brothers as he did...why Sonny was the hothead, why Fredo was written as the weakest, and Michael was essentially the "golden child," who Vito had such high hopes for? Recently, I was reading some studies about how birth order can affect children's personaliies. Some firstborns take on the role as leaders and are far less agreeable, which I think describes Sonny since he was protective over his brothers, especially in the novel and he had a much more fiery temperanment than Michael and Fredo. Fredo seemed to suffer from "Middle Child Syndrome" in that he was just another son and was never deemed strong or smart enough to handle the family's imporant operations. And Michael, who was the last son, was the one destined for greatness in Vito's eyes, especially when he realized that Sonny wasn't going to be college bound.

It now makes me wonder if Puzo did any psychological research into birth order of children or he just arbitrarly assigned personalities to each of the Corleone boys.