Beats me. Always has. Some kids choose almost exclusively to play with same
sex children, and extend it into the teens. Others seem sex-blind, including
many, at this moment, that are about the same age as Kate. I can't make hide
nor hair of it; never could. Although when the school was pretty tiny, in
the '70's and early 80's, it was a lot harder for children to only play with
same-sex kids. Sometimes narrowed their choices down to zero.
It is not my opinion that Kate chose her friends on the basis of their
gender. She chose friends with whom she had a lot in common, as do all of
the other kids, and it worked out that way. And there is a tremendous
overlap of course; she spends lots of time with boys. Maybe I will be
smarter in my next life and understand these choices. My own daughter played
mainly (but far from exclusively) with girls when she was small, but as she
saw it she was part of a large group of kids of both sexes, learning to live
together in a natural way, and I bet Kate sees it the same way.