Aline commented on the previous entry that she believes the difference between men and woman are hard-wired. I have found some evidence for that with small children--at my daughters' preschool playground, it did seem that the most persistently and noticeably wild and energetic were more often boys than girls.
I also am perhaps imagining it, but I think baby boys seem more frustrated at their inability to move around, and cheer up quite a bit once they're able to crawl around.
My favorite story that relates to this was when I was pregnant with our second daughter. We were at a family gathering with my two sisters, who had each a boy and a girl. The grownups were lingering over tea and noticed that all the children had dispersed.
"Where are the kids?"
"The girls are upstairs, playing with dolls," came one reply.
"Then where are the boys?"
"Outside. They found a dead rat in the gutter. They're poking it with a stick."
My husband and I looked at each other smugly. "We're having another girl," we said.